Living in London means that even though spring has blossomed forth, there is still a chance of frost all the way through to the end of May. And many of us will have learned the hard way that frost and/or sub-zero temperatures can seriously damage our trees.
One of the key risks of frost and extreme cold is the problem of ice. Ice formation pulls water from plant cells, causing dehydration, which can be just as lethal as dehydration from underwatering.ref Ice masses which form and thaw can deform and damage cell membranes, as well as buds and other plant organs, and ice in trees’ xylem vessels can cause embolisms (air bubbles) to form, cutting off the water flow above the bubble.
Another problem is that low temperatures make photosynthesis dangerous due to an excess of energy which ends up as damaging reactive oxygen species (“ROS”). This is why some trees convert green chloroplasts to bronze chromoplasts in cold weather – read more here, and others shed their leaves altogether to eliminate the problem.
Plants which are ‘hardy’ or ‘frost-hardy’ have mechanisms to resist the effect of ice – either by ‘avoidance’ such as supercooling (lowering a liquid to below freezing point without freezing), or ‘tolerance’ – using biochemical changes and physical adaptations allowing the tolerance of ice in their tissues.ref Some pines have evolved specific mesophyll cells which allow the ice into the intracellular spaces without deforming the key structures within the needles – a physical adaptation. Oaks and ring porous trees regrow their conducting xylem every year, since embolisms during winter make last seasons’ xylem ineffective.
Crucially, since many cold-hardiness mechanisms rely on biochemical changes within the plant, they require time, so that the relevant proteins, enzymes and other metabolites can be synthesised in sufficient quantities to have the desired effect in plant cells. This process is called ‘cold acclimation’. Smith et al (2010) give the example of rye. 50% of non-acclimated rye plants will die at -6oC, but after spending 2 days at 4oC, they can go down to -21oC before 50% of plants will die. The cold acclimation process is what’s known as an ‘epigenetic’ process – where environmental triggers such as shortening days and reducing temperatures turn on genes – in this case known as COR (cold-regulated genes).ref These then produce the proteins which are used to create the cold-hardy biochemical changes.
There is an equal and opposite process known as cold deacclimation. Smith et al (2010) say that above a temperature of 10oC cold hardiness is rapidly lost, which is why a spring frost at the end of May can be so damaging.
To complicate matters, shoot hardiness is not the same thing as root hardiness.ref Studies have shown that roots are less hardy than shoots, even when exposed to identical temperature acclimation treatments.ref For example, the stems of Pyracantha are hardy to -25.6oC, whereas mature roots are hardy to -18.8oC and young roots are hardy to -6.1oC.ref This is an important insight for bonsai because the scale of our trees and the fact they are suspended in pots means their young roots are particularly vulnerable. A symptom of winter damage to young roots is when a tree flushes later than normal, and has retarded growth during the season.ref It may not kill the tree, but it will certainly give it a handicap for the next season.
So what does it all mean for bonsai?
For me the main risk boils down to root damage. Those lovely fine roots we work hard to encourage during the growing season are more vulnerable to frost than any other part of the tree, and unfortunately due to the size, shape and positioning of most bonsai pots, they are very exposed to cold temperatures. In one study it was found that container-grown trees were subjected to temperatures down to -15oC when the night-time temperature reached -30oC, whereas the soil only went down to -6oC.ref And since surfaces cool faster than air when it’s cold, resting up against a cold pot surface is the last place a young root wants to be! When the temperature gets really cold, mature roots can be damaged as well, which could be fatal to the tree.
Sure – leaves can be damaged by frost as well – mainly when deciduous trees have leafed out expecting above-zero temperatures and then a frost comes along. Most will deal with this and should regrow their leaves – there will be an energy penalty which will reduce the overall energy they have to devote to the growing season, and they won’t look great in the meantime, but it shouldn’t be fatal if the stems and roots are still healthy. But certainly to keep your trees looking and growing their best, you want to avoid deciduous trees leaves being exposed to frost if at all possible (see below for some strategies).
To be able to manage and prevent frost damage, we need to know when a frost will happen. This starts with monitoring the temperature during winter. Importantly, when you hear or see a temperature forecast for a location, it is a forecast for the air temperature. The temperature of the ground is often several degrees lower. The UK Met Office says “As a general rule of thumb, if the air temperature is forecast to fall between 0 °C and 4 °C on a night with little or no cloud and light winds, then you need to bear in mind there may be a frost outside in the morning. The closer it is to zero, the greater the chance of seeing frost. If the air temperature is forecast to be below zero, then the risk of seeing frost is much higher.”ref This goes for surfaces like pots as well. Wind and cloud cover reduce the chance of frost at a given temperature.
The actual temperature which will damage or kill roots is species-specific, so there’s no hard and fast rule but knowing where a tree comes from should give you an indication. In general the species common in the boreal forests such as many Pinaceae and Betula pendula will cope best with freezing temperatures. Some ornamental tree ‘killing temperatures’ are provided in this list.
So what should you do to help your trees defend themselves against frost damage?
The first thing to do is to let them be exposed to the cold over time, before it freezes. Give them time to activate their COR genes and establish cold acclimation. This includes not putting them in a polytunnel or shed or wrapping them up until they’ve had some time in colder temperatures (but not freezing).
It’s also useful to have actual temperature data from the location where your pots are going to be over winter. This can be achieved using a digital thermometer/hygrometer which records the temperature and humidity at regular intervals – there are many wireless/bluetooth-enabled options now which are very reasonably priced (I just bought 3 for £35, one for my garden bonsai bench, one for my allotment bed and one for my allotment greenhouse). This will enable you to monitor what the temperature does relative to the forecast so you can better predict the forecast in your actual location – and you can see which locations might provide better winter protection. If you see the temperature approaching zero you can act to protect your trees.
Then, adopting a form of overwintering system could be beneficial. Somewhat counter-intuitively, this involves watering everything well, without overwatering.ref The reason for this is to avoid dehydration. Then providing a physical barrier to the cold, effectively a ‘tree duvet’. This can move the frost surface away from the pot edges. You could use an insulating blanket of some kind, but do some research – one study found that ‘microfoam thermoblanket’ made a difference, but clear, black or white poly did not.ref The covering would need to have insulating properties, and not get saturated with water such that it would then freeze anyway. You could also move your trees into a polytunnel, garden shed or greenhouse, with the walls and air inside acting as a form of insulation. If you use a digital thermometer you can monitor these to ensure they aren’t getting too cold – note that polytunnels have been found to get cold enough to damage roots.ref
The absolute best is to provide some form of heat so that the temperature can’t go below a certain point – in practice this could be moving your pots closer to your house, using a (lightly) heated propagation bed, or putting them in a (lightly) heated greenhouse or outhouse. You don’t want them to think it’s spring so there shouldn’t be too much heat, just enough to keep the temperature above freezing. Be aware that in a warmer environment such as a shed your trees will deacclimate earlier as well, so make sure they don’t go back outside into a frost as they may have lost their cold acclimation.
Spring has sprung (finally) in the UK and now is the time that many trees flower, so they can pollinate the next generation with enough time for seeds to develop before winter. Plant reproduction is quite a complicated multi-stage process, but for the purposes of this post it’s enough to know that pollen is the vehicle for the male gamete (the plant equivalent of sperm). It is produced by anthers in angiosperms (flowering plants) and by male cones in gymnosperms (including conifers).
It turns out that pollen production can happen in different places. In some trees, both male and female parts are within the same flower (known as ‘perfect’ or ‘complete’ flowers). In others, they are separate flowers or cones on different parts of the same branch, shoot or tree – known as monoecious trees. In yet others they are separate flowers or cones on single-sex trees – known as dioecious.ref
Whilst female flowers/cones and the fruit associated with them are usually obvious, the pollen-bearing male flowers/cones are often hard to spot and sometimes people don’t even realise they are there (it was only about 2 years ago that I realised that oak trees even had flowers!). After releasing their pollen, male reproductive parts usually drop off and disappear from view, while the female flower/fruit/cone persists as the seeds develop. So it’s definitely worth appreciating male tree flowers when you see them, as they don’t hang around for long.
In angiosperms, many male tree flowers take the form of catkins. A catkin is an “elongated cluster of single-sex flowers bearing scaly bracts and usually lacking petals.”ref Male catkins appear on oak, chestnut, alder, birch, hazel, poplar, aspen, hornbeam, walnut and willow – a good overview of these is on the UK Tree Guide website and some examples are shown below. There are female catkins as well, but these don’t contain pollen, which is one way to tell them apart.
Some trees have flowers that look more like what we learn about at school, so-called ‘perfect’ flowers, with both male and female parts. Examples of trees with this type of flower structure include maples, hawthorn, lime, horse chestnut and many fruit trees including apple, cherry, pear and plum (see below).
Gymnosperms do not have flowers at all, instead they have male and female cones (also called strobili). 98% of gymnosperms use the wind for pollination, with the male pollen cones releasing their pollen into the wind to find its way to females.ref Because this is a bit of a hit and miss approach, gymnosperms produce a *lot* of pollen – one study estimated Juniper pollen production to be up to 532 billion pollen grains per tree!ref These vast quantities of pollen create pollen clouds, which you can see in a video on youtube here.
Another nifty thing is that female conifer cones produce a ‘pollination drop’ – this is a drop of liquid which sits on the surface of the cone to catch wind-borne pollen. As soon as pollination takes place, the drop is quickly retracted back into the cone.ref And you’ll notice that often male pollen cones are positioned at the top of a tree and in open positions along the stem (ie. not covered by leaves), to give their pollen the best chance of going far and wide.ref
Male cones on gymnosperms are smaller and less conspicuous than females, but, I think they’re still beautiful and worthy of our appreciation. Here are some examples of gymnosperm male cones:
Conifer species which have leaflets have a sort of tree-within-a-tree approach – such as the cypress below. – it has browny-pink male pollen cones on the ends of the shoots towards the back of the leaflet, and green developing female cones on the ends of the shoots at the tip of the leaflet.
There are more great images including scanning electron microscope images of gymnosperm pollen in this research paper.
But what if the trees in your bonsai collection don’t have any flowers or cones at all? Unfortunately this is a sign that they haven’t yet reached their reproductive phase. It’s hypothesised that a plant’s transition to its reproductive phase happens after a certain number of cell divisions have taken place.ref If you keep pruning the new growth off, your tree may never reach a reproductive phase, since it may never achieve the number of cell divisions required. The only way around this is to let your tree grow, use a mature tree to begin with, or use grafted material which comes from mature trees.
In the meantime, make sure to take a good look at your trees and the trees around you, and appreciate the underrated male tree flowers when they make an appearance.
Below is a shopping list if you’re wanting to create a bonsai pinetum. You may want to explore alternative species to include, if so the Gymnosperm database is a fantastic resource.
ARAUCARIACEAE
Monkey puzzle tree (Araucaria araucana) and/or Wollemi pine (Wollemia nobilis and/or Kauri (Agathis australis)
CUPRESSACEAE
Basic (5 genera): Hinoki cypress (Chamaecyparis obtusa), Dawn redwood (Metasequoia glyptostroboides), Japanese cedar/sugi (Cryptomeria japonica), Sabina Juniper (Juniperus sabina), and Thuja (eg. Thuja occidentalis)
Intermediate (10 genera): as above plus Giant redwood (Sequoiadendrongiganteum), Coast redwood ( Sequoia sempervirens), Swamp cypress (Taxodium distichum), Italian/Mediterranean cypress (Cupressus sempervirens) and an Oriental arborvitae (Platycladus orientalis).
Extensive (17 genera): as above plus Chinese Coffin Tree (Taiwaniacryptomerioides), Tasmanian Cedar/Pencil Pine or King Billy Pine (Athrotaxis cuppresoides/Athrotaxis selaginoides), Rottnest Island Pine or Oyster Bay Pine (Callitris preisii, Callitris rhomboidea), Chinese Fir (Cunninghamnia lanceolata) , Chilean Cedar(Austrocedrus chilensis), Incense Cedar (Calocedrus decurrens) and Chinese Swamp Cypress (Glyptostrobus pensilis)
Complete (25 genera): as above plus Diselma, Fitzroya, Libocedrus, Microbiota, Papuacedrus, Tetraclinis, Thujopsis and Widdringtonia.
PINACEAE
Basic (5 genera): Scot’s or Japanese Black or Japanese White Pine (Pinussylvestris/thunbergii/parviflora), Abies koreana (Korean fir), Cedrus atlantica (Atlantic cedar), Engelmann spruce (Picea engelmannii), European Larch (Larixdecidua)
Extensive (9 genera plus extras): as above plus Eastern white pine (Pinus strobus), Mountain Hemlock (Tsugamertensiana), Golden larch (Pseudolarix amabilis), Douglas fir (Pseudotsuga menziesii) and Yunnan youshan (Keteleeria evelyniana). Consider also Pinyon pine (Pinus monophylla)
Complete: all 11 genera: as above plus Cathay silver fir (Cathaya argyrophylla) and Bristlecone hemlock (Nothotsuga longibracteata)
PODOCARPACEAE
Basic: Buddhist Pine (Podocarpus macrophylla)
Extended: as above plus Celery Top Pine (Phyllocladus alpinus or Phyllocladus asplectiifolius), Chilean plum yew (Prumnopitys andina),
+ if you are a true collector and willing to track down seeds or specimens in collections near you, try for a Kahikatea (Dacrycarpus dacrydioides), Creeping Strawberry Pine (Microcachrys tetragona) and Rimu (Dacrydiumcupressinum)
SCIADOPITYACEAE
Japanese Umbrella Pine (Sciadopitysverticillata)
TAXACEAE
Basic: Common Yew (Taxus baccata) or Japanese Yew (Taxus cuspidata),
Intermediate: as above plus Japanese Plum Yew (Cephalotaxus harringtonii)
Extended: as above plus White Berry Yew (Pseudotaxus chienii) and Japanese Nutmeg Yew (Torreya nucifera)
+ for collectors Stinking Cedar (Torreya taxifolia)
A pinetum is an arboretum, or collection of trees, dedicated to conifers. There is a fabulous pinetum at RHS Wisley in the UK (my ‘About Me’ pic was taken there), and the UK National Pinetum at Bedgebury Forest has a collection of 12,000 specimen trees.
Whilst very few of us have the space to create a full-sized pinetum, the wonderful thing about bonsai is that you can create your own miniature version. There are only six conifer families, and within those, 68 genera, some of which would be impossible or at least extremely difficult to procure. So you could have a very respectable and representative bonsai pinetum with around 50 trees. A mame-sized bonsai pinetum might even fit on a single table!
Only a small number of conifer species are common bonsai subjects, so embarking on this project would require some creativity – there won’t be online tutorials or examples for many of these species. Some may be completely hopeless for bonsai (most of the Araucariaceae family for example), others may require conditions that you just can’t provide, but along the way I’m sure you would find a few that make excellent bonsai and give you something unique and different for your collection.
If you want to jump straight to the shopping list here it is, otherwise read on to find out about the trees in the list and where they fit in the different conifer families. For beginners to taxonomy, you start with a family, then a genus (or genera if there is more than one genus), then a species. So for example for Scot’s Pine Pinus sylvestris, Pinaceae is the family, Pinus is the genus, Sylvestris is the species.
You have a few different options for your representative tree/s from Araucariaceae as it has three genera (agathia, araucaria & wollemia) which cover a range of different forms. The most well-known in Europe would be the monkey puzzle tree (Araucaria araucana) but you could also include a Wollemi pine (Wollemia nobilis) which are available to buy, albeit at a cost. Neither of these are the easiest of bonsai subjects as they have a very regimented architecture with whorled branches, however they do have the advantage of being frost hardy. Wollemia nobilis also does backbud, and grows as a multi-stem.
An alternative could be an Agathis, also known as a Kauri tree. The New Zealand Agathis australis is the third largest known conifer after the giant and coast redwoodsref, depending on where it has been grown it may or may not be frost hardy. The New Zealand Bonsai Association has a Kauri forest on their native species web page.
If you live in the southern hemisphere, many of these options will be easier to find and will be happy outside.
The Cupressaceae conifer family is a lot easier to cover in your pinetum as it contains 25 genera (listed below) and 152 species.ref You can read about the leaves of many Cupressaceae species in my post on conifer scale leaves. It would be appropriate to include several members of this large family, as many are known as bonsai subjects anyway. An easy selection of five from different genera could include a Hinoki cypress (Chamaecyparis obtusa), a Dawn Redwood (Metasequoia glyptostroboides), a Japanese Cedar/Sugi (Cryptomeria japonica), a Sabina Juniper (Juniperus sabina), and a Thuja (also called arborvitae or cedars, although they are not true cedars).
Hinoki Cypress at RHS WisleyDawn Redwood ForestSugi/Cryptomeria Japonica at RHS WisleyTerry’s Sabina JuniperThuja occidentalis
Expanding to ten specimens across ten genera would allow the addition of five other reasonably easy to procure and grow species: Giant Redwood (Sequoiadendrongiganteum), Coast Redwood ( Sequoia sempervirens), Swamp Cypress (Taxodium distichum), Italian/Mediterranean Cypress (Cupressus sempervirens) and an Oriental Arborvitae (Platycladus orientalis).
If you’re a purist and want to include more genera from Cupressaceae,some excellent options would be the Chinese Coffin Tree (Taiwaniacryptomerioides), Tasmanian Cedar/Pencil Pine or King Billy Pine (Athrotaxis cuppresoides/Athrotaxis selaginoides), one of the Australian Callitris species such as Rottnest Island Pine or Oyster Bay Pine (Callitris preisii, Callitris rhomboidea), Chinese Fir (Cunninghamnia lanceolata) , Chilean Cedar (Austrocedrus chilensis), Incense Cedar (Calocedrus decurrens) and the Chinese Swamp Cypress (Glyptostrobus pensilis). You may need to grow these from seed, depending on where you live.
To fully represent Cupressaceae you’d also need to add the remaining eight genera, Diselma, Fitzroya, Libocedrus, Microbiota, Papuacedrus, Tetraclinis, Thujopsis and Widdringtonia. Many of these are specific to small or remote locations and/or endangered, but you may come across them while travelling, or while visiting full-sized pinetums or botanic gardens.
Family 3: Pinaceae
This family is a stalwart of the bonsai hobby, containing a massive (for conifers) 11 genera and 232 different species, including the eponymous pines, which account for more than half of these. You can read about the leaves of most Pinaceae in my post on conifer needle leaves. For your first pinetum Pinaceae, consider the UK native Scot’s Pine (Pinus sylvestris), Japanese black pine (Pinus thunbergii) or Japanese white pine (Pinus parviflora) – and perhaps a Pinus strobus which is a separate subgenus within Pinus.
For something unique, you could also include the only single-needled pine, Single-leaf piñon or Pinus monophylla. It’s actually a pine which has fused needles (five of them) which appear as one, and this results in its needles being very fat.
A top 5 selection from Pinaceae would also include species from the fir (Abies), true cedar (Cedrus), spruce (Picea) and larch (Larix) genera, all of which have species which are relatively easy to source and grow, at least in Europe (and most are extremely frost hardy). Let’s make it top 6 to include another well-known genus, the hemlock (Tsuga).
The 7th and 8th Pinaceae specimens could be a beautiful Golden larch (Pseudolarix amabilis the only species in its genus), and the classic Pacific north-west representative the Douglas fir (Pseudotsuga menziesii). The Douglas fir is not actually a fir, it’s a ‘false hemlock’, one of seven species of Pseudotsuga.
Most of these trees are well-known, can be procured either as plants or seeds, and could be relatively easily added to your collection. But the Pinaceae family also includes some extremely rare genera. Cathaya has only one species, Cathaya argyrophylla, or Cathay silver fir, which has a similar history to the famous Dawn redwood – thought to be a fossil but then ‘discovered’ in a small living stand in China in 1946.refCathaya was also discovered in China in 1938, but the discovery was not recognised as a new species until the 1950s.ref It is endangered with less than 1000 mature individuals in its native habitat.ref
Bristlecone hemlock or Nothotsuga is also a genus with only one species (Nothotsuga longibracteata) which comes from China, where it is near-threatened and “populations are highly fragmented, with some consisting of just a few scattered individuals”.ref Unfortunately Cathaya and Nothotsuga are probably out of reach for your pinetum unless you have access to seeds via a botanic garden, or live in China.
The 11th and final genus in Pinaceae is also relatively unknown outside of its native region of China, Taiwan & Vietnam – Keteleeria or ‘Yunnan youshan’ has three species, of which Keteleeriaevelyniana can be found as seed. So with Keteleeria evelyniana you can still have an unusual Pinaceae in your collection without having to raid your nearest full-sized pinetum.
Podocarpaceae is another large family within the conifers, with 172 species across 20 genera – so it is larger than Cupressaceae in terms of size but less well known in Europe and North America.ref This may be because podocarps are mainly found in tropical and subtropical mountain habitatsref, which has resulted in their leaves being quite different to other conifers (read more in my post on conifer flat leaves). It also results in trees from Podocarpaceae being a little harder to obtain in Europe.
There are three main groups within Podocarpaceae which could be represented in your pinetum – these are known as the ‘prumnopityoid clade’, the ‘dacrydioid clade’ and the ‘podocarpoid clade’.
The first group includes Phyllocladus – the so-called celery pines which come from Australasia. A somewhat hardy species includes Phyllocladus alpinus which I note can be purchased in the UK from Bluebell Nurseries, and seeds for other species such as Phyllocladus aspleniifolius are also available online. Depending on where you live these will need protection from hard frosts. Phylloclade species don’t have true leaves, aside from very small, almost invisible ones when they are seedlings. Instead they have ‘phylloclades’ which are photosynthetic flattened stemsref. So, of course you must have one of these interesting plants in your collection!
Also in the first group is the Prumnopitys genus from Polynesia and South America. Prumnopitys andina is known as the Chilean plum yew, and was the International Dendrology Society’s Tree of the Year in 2017. They produced a comprehensive report about this tree which you can read online, and which lists locations where they have been planted outside of Chile. It appears to be seasonally available in a small number of plant nurseries in the UK. I would have included a photo but no decent creative commons images were available, so I’ll add one when I get the chance to find a Chilean plum yew for myself.
The second group (the dacrydioid clade) includes the Dacrycarpus genus, of which Dacrycarpus dacrydioides is a species known and loved in New Zealand as ‘Kahikatea’ from the Maori. It’s the tallest tree in New Zealand, and apparently has a hardy form which can be bought from this provider in the UK. Below is one as a bonsai from the New Zealand bonsai association:
Also in this image is another member of the dacrydioid clade – which happens to be (according to Farjon) the smallest known conifer in the world. Microcachrys tetragona, or the Strawberry Pine, is from Tasmania. Bonsai enthusiast Diana Jones explains in the Newsletter of the Australian Plants as Bonsai Study Group: “Growing on the top of Mt. Wellington is a large tree, about 10m in diameter, called Microcachrys tetragona or creeping strawberry pine. Few people notice it because it is only about 5cm high, being negatively geotropic. This causes a few problems when making it into an attractive bonsai, because in a pot, it just flops.” Nevertheless I think her specimen has a certain charm, and she is definitely one tree ahead of me in the bonsai pinetum stakes.
And one can’t leave this group without mentioning Dacrydiumcupressinum, also from New Zealand and called variously red pine, red spruce or ‘Rimu’. My favourite podcaster has covered this species on his blog In Defense of Plants where he says the fleshy cones of this tree are an essential part of the diet of the endangered kākāpō bird. Availability of the ‘fruit’ (not really since it’s a conifer – it’s a female cone) triggers breeding for the kākāpō so it’s seen as critical to their survival.ref The tree itself is a wonderful tree with long dangly stems so I’d love to have one even without a kākāpō.
But if you cannot find Kahikatea, Tasmanian creeping strawberry pine or a kākāpō-infested Rimu, the third group in Podocarpaceae (the Podocarpoid clade) is going to be much easier to represent in your pinetum, because it contains the oft-seen bonsai species the Buddhist pine, or Podocarpus macrophyllus (also called Kusamaki in Japanese). If you are going to have at least one specimen Podocarp, this is likely to be the easiest one to obtain. I have a couple in my London garden (in the ground) but after the last brutal winter I think they would prefer to be indoors, and they are sold as indoor bonsai in the UK.
There are various other Podocarpus species available at nurseries such as Podocarpus salignus or willow-leaved podocarp – it’s probably a good idea to do a local search to see which species are available in your area.
Sciadopityaceae has only one genus (Sciadopitys) and within that only one species, the Japanese umbrella pine or Sciadopitysverticillata. This tree is an endemic Japanese evergreen conifer, with relatively slow growth rates, used in gardens and construction in Japan.ref1,ref2
Unfortunately since this is the only representative of one of the six conifer families, you really do need one in your collection if you are to truly represent all the conifers. This tree isn’t very easy to propagate, and is also quite expensive to buy (at least in the UK), not only that, it doesn’t appear very commonly as a bonsai. I haven’t had any luck growing it from seed (and they were expensive) so I think your best best is keeping an eye on nurseries and waiting until you see one at a reasonable price.
Finally we come to the yews, the Taxaceae family which has six genera and 28 species. Many will be familiar with its most prominent genus, Taxus, containing the Common yew Taxus baccata. According to the gymnosperm database, most members of Taxus look pretty much the same, so to save yourself money and time, I’d suggest simply finding the yew that is easily available in your area. For me Taxus baccata are a dime a dozen, you also see Japanese Yew Taxus cuspidata used in bonsai.
I believe you should also have a Japanese plum yew in your collection. Cephalotaxus is a genus with 11 species, which used to be considered its own family, but DNA testing revealed it really belonged in Taxaceae.ref This tree has yew-like leaves and small plum-like fleshy cones which start green and then move through red and dark purple colourationref. It’s not commonly seen as a bonsai but it’s not difficult to propagate and I have seen them available at plant nurseries in the UK. One would assume adopting a similar styling approach to yew would work with this tree.
The remaining genera in Taxaceae are a lot rarer and more difficult to include in any non-tropical pinetum. The New Caledonia Yew Austrataxus spicata is the only southern hemisphere Taxaceae, and thrives in the very unusual habitat of the island, based on ultramafic rock containing chrome and nickel and not much else in the way of nutrients.ref
The Catkin Yew Amentotaxus is a threatened genus with six species found in China, India, Laos, Vietnam and Taiwan.ref One nursery in the UK sells Amentotaxus argotaenia var. argotaenia as a pot-grown specimen to be taken indoors during winter (but when I checked they were out of stock).
The White Berry Yew, Pseudotaxus chienii, is the only species in the Pseudotaxus genus, and is also native to China.ref It has white arils instead of the red arils of Taxus baccata. Cited as rare, it is nevertheless available from some suppliers.
The final genus of Taxaceae and of this pinetum article, is a really interesting one called Torreya which I think deserves a place representing this family alongside the more familiar trees mentioned above.
Torreya nucifera was the International Dendrology Society’s Tree of the Year (2019) and has a full report write-up available online here. This tree is known as the Nutmeg Yew, or in Japanese ‘kaja’ and ‘kaya’ and oil from its seeds (not nuts!) have been used for tempura cooking oil. As a widely cultivated tree, there is availability of Torreya from plant nurseries (eg. here) although the article I linked to says they do not like cool summers so a protected position or a pot may be needed.
Another option from Torreya if you can find one, and have the budget, is the alarmingly named Stinking Cedar or Torreya taxifolia. This is a rare species native to Florida USA, now protected in the Torreya State Park in Florida. One can be acquired for £50 per 2L pot in the UK here.
The end (of this post, but the start of your conifer collection?)
So there you have it, a set of suggestions for creating your own mini-pinetum using bonsai trees across the six conifer families. If you decide to take on the challenge, I’d love to see your efforts – tag me on Facebook (Bonsai-Science) or twitter (@BonsaiScience). Here’s another link to the shopping list, to get you started.
A pathogen is a microorganism such as a virus, bacterium, oomycete (water mould) or fungus which causes disease and/or death.ref Examples you might have heard of include Dutch Elm disease (caused by the fungus Ophiostoma ulmi), Horse Chestnut bleeding canker disease (caused by the bacterium Pseudomonas syringaev.aesculi), Ash dieback (caused by the fungus Hymenoscyphus fraxineus), Sudden Oak Death (caused by the oomycete Phytophthora ramorum) and mosaic viruses in vegetables.
The groups of pathogens causing forest tree diseases and their prevalence is shown in the chart below. 85% of these are fungi and the remainder are divided among bacteria, viruses, nemotodes (microscopic worms) and oomycetes. Oomycetes appear similar to fungi but are actually not genetically similar, and are classed in the Chromista kingdom.ref
Link to source article here; original sources Butin, 1995; Capretti and Ragazzi, 2010; Manion, 1991; Tainter and Baker, 1996
Note that as they are not microorganisms, insects are not classified as pathogens, although they can also cause significant damage and death to trees. Insect damage is a subject for another post, but for this one, their main role is in introducing pathogens to trees by delivering microbes on them into wounds they create (this is called being a pathogen ‘vector’).
Every living thing is in a battle for survival so that its genes can be passed on to future generations. This means that both trees and pathogens are constantly evolving to outsmart each other. But fungi and bacteria can also be beneficial and even necessary for trees, so you can’t just try to kill off every fungus or bacteria as this will destroy the ones that trees actually want and need.
The other thing to keep in mind is that plants have a completely different ‘lifestyle’ to animals in the sense that they are plastic and can grow new tissues, synthesise new compounds and respond to pathogens in a very different way to humans. As noted elsewhere, in response to wounds, trees ‘don’t heal they seal’ and they don’t have an immune system in the same way as animals.
But, there are three ways to help your trees avoid damage from pathogens. The first is to prevent them from being exposed to pathogens in the first place. The second is to remove any pathogens which do take hold. The third is to bolster your trees’ defences so they can fight off the pathogen and its effects.
How can you prevent exposure to pathogens? A tree’s first line of defence is its bark, leaf cuticle, and the pectin and lignin in cell walls, which are physical barriers which prevent pathogens from entering its cells.ref Since they can’t run away, this is the main way that trees avoid exposure. But in bonsai we do a heck of a lot of pruning, which unfortunately breaks the physical barrier, leaving the tree vulnerable.
To minimise the chance of nasty microbes attacking your tree as a result of pruning there are some steps you can take. Firstly, practice excellent hygiene and make sure your pruning tools are disinfected regularly, particularly when moving between trees. But be careful that the disinfecting method you use doesn’t damage your tools (carbon steel is particularly vulnerable). Soap and water can work (but dry off the water), or you could use an antimicrobial oil such as tea tree oil (from Melaleuca alternifolia) or oregano oil (from Origanum vulgare) – not only are these antimicrobialref1,ref2 but they also protect steel from corrosion.ref1,ref2
Another approach is to prune during the wintertime. The latter helps because in general most living things are more active in warmer weather – including pathogens. Pruning in winter reduces the likelihood that a pathogen will enter a wound before the tree can seal it off.ref Also many fungi and oomycetes prefer a moist environment, so when pruning try to avoid leaving the wound wet. Angling pruning wounds towards the sun can also be beneficial, since sunlight has disinfecting as well as drying properties (although probably not so much during the wintertime in higher latitudes).ref
Contrary to some advice, wound sealants have not been shown to reduce bacterial or fungal infection on tree wounds.ref This is because a tree wound is not sterile so any sealant can seal pathogens in as well as out. One wonders whether applying an antimicrobial oil to a wound might work, but I cannot find any studies looking into this. Recently I tried applying raw linseed oil to the cut ends of various crabapple branches I was trying to propagate, but these ended up with large communities of mould on them regardless (possibly because I was using high humidity which is perfect for fungal growth).
One interesting pathogen avoidance method is to decouple the seasonal timing between the host plant and the pathogen vector (vector means the delivery method of the pathogen, often an insect). One study found that Dutch Elm disease was avoided by trees which flushed early, since they were not as susceptible to infection after this point.
The second way to help your trees avoid pathogens is to remove them once there. This is tricky since by definition a pathogen is microscopic and impossible to see with the naked eye. You can’t go and squash every bacterium on your tree! It is possible to kill pathogens using antibiotic and/or antifungal substances, but effectiveness varies depending on what you need to remove and usually involves unpleasant and toxic chemicals (such as glyphosate) which also kill good microbes.
Biological control is an alternative to chemicals, this means finding another organism which feeds on or somehow damages the pathogen in question. An example of biological control is the introduction of the Myxoma virus into Australia to control the rabbit population. In trees, the fungus Trichoderma is used as a biological control agent, and it is found in bacterial inoculants for plants such as this one. Species of Trichoderma have been found to be effective against Armillaria root rot (also known as the dreaded honey fungus) and pine pitch canker.ref
The final way to help your trees is to bolster the natural processes they use to resist the negative effects of pathogens. Plants produce a huge range of substances which have defensive effects, and can detect pathogens with surprising speed and specificity. When a pathogen is detected by a plant, it first activates a specific ‘pattern-triggered immunity’ response which is believed to be sufficient to defend against a wide range of pathogens.ref This is known as ‘basal resistance’. A second line of defence detects the so-called ‘effectors’ – substances that pathogens create to avoid the pattern-triggered response. This second ‘effector-primed immune response’ causes cell death at the site, limiting the spread of the pathogenref and is known as the ‘hypersensitive response’. Plants also synthesise a wide range of defensive compounds such as resins in conifers, terpenoids or essential oils, saponins and flavonoids (of which 9,000 are known).ref1,ref2 These help them deter pathogens by making their cells poisonous or unpalatable.
So ensuring your plant is not stressed by lack of water, light or nutrients is one way to help it have the resources to defend itself. Another way is to provide it with beneficial microbes such as mycorrhizal fungi and beneficial bacteria. One study found that providing the bacteria Bacilluscereus to tomato plants enhanced their resistance to pathogens by activating the plant growth hormones salicylic and jasmonic acids. Cultivating a healthy rhizosphere (root microbiome) which supports your tree’s health can be achieved by using a well-aerated soil mix and by not constantly repotting. Repotting risks losing the microbiome which a tree has built up over time, for this reason I always try to add back in some of the previous soil when I repot.
So what should you be doing as a bonsai enthusiast to avoid any nasty pathogens ruining your great work? I’d suggest three things. The first is good pruning practices – minimising wounds, avoiding pruning wounds becoming wet or humid, and vigilance in disinfecting tools and pots. The second is to keep your trees vigorous and healthy – particularly before doing large-scale pruning or defoliation. Give your trees lots of water, nutrients and sunlight to help them bolster their defences. And finally help your trees out with the addition of beneficial fungi & bacteria; products containing these can be found online.
Flat leaves are found across the conifer families including Taxus, Cephalotaxus, and even in Pinus (check out Pinus krempfii) as well as many of the Araucariaceae and Podocarpaceae families. Commonly known trees with this leaf shape include all varieties of yew, swamp cypress, dawn redwood and coast redwood. Here are some examples:
Taxus baccata (Yew)Wollemia nobilis (Wollemi Pine)Cephalotaxaceae (Plum Yew)
Conifer flat leaves are relatively inefficient from a photosynthesis point of view, since water and photosynthates have to travel further to reach the vascular bundle/s.ref As a result, they have better photosynthetic efficiency in humid, low light environmentsref and are more common where it is wet or tropical. In these areas their greater leaf surface can help them attract the energy they need without drying out due to having more stomatal openings on their surface.
The primarily southern hemisphere family Podocarpaceae is super interesting and not one that I have worked with much from a bonsai perspective, although buddhist pine (Podocarpus macrophyllus) is a species commonly found as an indoor bonsai in the UK. (It is hardy down to just below -10°C so could be an outdoor bonsai as well). Podocarpaceae are interesting from a foliage perspective, as they have evolved a wide range of different leaf shapes which in some cases have become similar to angiosperms and take the forms of flattened leaves with modifications that allow them to grow larger. Below are some examples from which it’s clear that this family has found a workaround for the venation constraints of others in the conifers.
One study mapped the structure of Podocarpaceae leaves and created a cross-sectional image for each sub-family, their results are shown below (apologies for the poor resolution but the original paper wasn’t great to start with). You can see that although most species only have one vascular bundle, they also have various mechanisms to get substances to and from it – including the orange ‘organised accessory transfusion’ cells and ‘pitted thick-wall mesophyll’ cells. Interestingly leaf (f) from the image above – the largest leaf – is also shown below and it’s the only one to have multiple vascular bundles (N and O) which is similar to angiosperm leaves.
Whilst the majority of Podocarpaceae are flat-leaved, quite a few members of Araucariaceae have flat leaves as well. The famous Wollemi Pine or ‘pinosaurus’ Wollemia nobilis has flat leaves, as does the equally well-known monkey puzzle tree Araucaria araucana and various others in the family. These trees don’t tend to be common bonsai subjects as their leaves are quite large and observing the Wollemi pine in my garden, they don’t have much variation in their form so wouldn’t be that easy to style.
Also unusual among conifers is the Phyllocladus genus. Plants in Phyllocladus don’t have true leaves at all – any leaves they develop are non-synthetic and ephemeral – ie. they quickly drop off. Instead Phyllocladus use their stems, which have developed a leaf-like flattened form, to photosynthesise.ref These are called phylloclades – see below for some examples from New Zealand:
I know they look identical to leaves – and they contain all the same components including vascular bundles and photosynthesising palisade and mesophyll cells. The nuances of why they are not leaves but flattened branches instead are based on the way they develop and branch, and the relationships between organs – if you want to read the details refer to this excellent paper. But sometimes they may get lumped in with flat leaved conifers.
Which brings me to Taxus or yew. This is a very common bonsai subject, with its own family, Taxaceae. Yew leaves are famous for two things. Firstly, along with every other part of the yew except for its aril (the red ‘berry’), yew leaves are renowned for being extremely toxic. They contain ‘taxine alkaloids’ of which only a small amount is needed to bring on “dizziness, nausea, vomiting, diffuse abdominal pain, cardiac arrest, respiratory paralysis and death” in all animals including humansref. Secondly, they are one of the few conifers which are known to do just fine in shady positions, although they are also happy in full sun as their leaves adjust to different light levels.ref
Taxus have a unique stomatal feature called ‘papillose’ cellsref which can only be described as ‘pimply’ (see below) – basically the entire surface of the leaf has tiny protrusions as part of the cuticle structure. The presence of these pimples is one of the ways of identifying a Taxus leaf. I couldn’t find any clear explanation as to the role of these papillae, except perhaps to provide some level of water-repellence.ref Taxus are known to harbour endophytes (microbes) in their leaves which help combat pathogensref so it could be that the nooks and crannies created by papillae are a nice home for endophytes.
Anyway, what of all this is relevant for bonsai? It sort of depends on which flat-leaved species that you have. Many will be suited to humid and/or low-light environments, so keep an eye on your watering and don’t let them dry out – they will probably appreciate a misting every now and then. Full sun may not be necessary – or may even be over the top depending on your location – but as always find out what a given species needs as there are lots of quirky members of this club with their own unique requirements.
I’ll be honest, I find conifer needles to be quite weird. As someone raised in the southern hemisphere, true pine trees were something we had at Christmas and that was about it (this is true because the most common Christmas tree found in Australia is Pinus radiata). But actually, needles really are just plain old leaves. They contain all the same components as ‘normal’ leaves, like an epidermis, photosynthesising mesophyll cells, green chlorophyll pigment, xylem/phloem, stomata and so on. They just happen to have pushed the ‘leaf’ form to the extreme, ending up extremely long, thin and tough.
Within conifers, what are *called* needle leaves are found across most of the families, including Araucariaceae (Norfolk Island pine, Cook pine, Hoop pine), Cupressaceae (juniper, Thuja, Cryptomeria japonica), Pinaceae (pines) and Podocarpaceae (Platycladus).
At this point though I think it’s important to point out that the Pinaceae family is the least similar to all the other conifer families, having diverged from them very early in evolutionary history. Look at the ‘family tree’ of conifers below and you can see that Pinaceae including pine, cedars, larch, fir and spruce, has been evolving separately for the longest of any conifer family. This means that ‘needle leaves’ in Pinaceae are not the same thing at all as needle leaves in other families, and they probably need to be treated as two separate sub-categories.
This is why it can be confusing to understand what people are talking about when they refer to needle leaves in conifers. Needle leaves in Pinaceae are obvious – they are long, thin, spiky, tough, 3-dimensional and in mature foliage form in clusters known as fascicles which are actually short shoots (see my post on shoots for more on these). What you and I would call pine/fir/spruce needles – as per these examples:
PineCedarSpruce
Needle leaves in other families are a bit more ambiguous. Sometimes they are referred to as ‘awl-shaped’ or ‘sabre-shaped’ and often they have a needly element but also a scale-leafy element. Needle leaves in these families, such as Juniperus and Cupressus, often mature into scale leaves. Here are a few examples of non-Pinaceae needle leaves:
As is obvious, these are different to Pinaceae needles and it’s not just the leaf shape and configuration – one major difference is that these species usually have a photosynthetic stem which pines definitely do not have. You can sort of see how these leaves could change to become scale leaves by the way they are attached to the stem in alternating pairs – if they just shrink and get closer to the stem you could see a mature scale leaf emerge.
For the sake of the rest of this post, I’m going to focus on Pinaceae needle leaves, since these are persistent needles whereas most of the needle leaves on other families are juvenile (although there are some exceptions).
One of the key attributes of Pinaceae needle leaves is that they have a 3-dimensional profile – usually with a quadrangular, triangular or semicircular cross-section, as shown in the images below (1,2,3 & 5 needle pines):
https://www.flickr.com/photos/146824358@N03
Anatomically, Pinaceae needles are like other conifer leaves in only having one or two vascular bundles for water & sugar transport. Their stomata are arranged more or less evenly around the needle and appear in lines. But needles are unique in having an unusual type of mesophyll cell (a cell used for photosynthesis). Instead of cylindrical palisade cells lined up under the epidermis like other plants, needles have frilly looking ‘arm palisade parenchyma’ (see below). These have very lignified (woody) cell walls which intrude into the cells and it is this feature which is thought to provide needles with their extreme cold resistance.
arm palisade cells (pine needle)Pine needle with arm palisade cells
Pinaceae needles are actually *the* most frost-hardy leaves of all. Mountain pine needles survive temperatures down to −93°C and can still perform gas exchange (oxygen for cell respiration) even when their needles are frozen.ref They are also highly resistant to herbivory, due to being tough, spiky, full of toxic resins and not very nutritious. And they live anywhere from 2 to 45 years (Bristlecone pine has the record).ref The needle is one tough leaf!
Pine foliage is also heteroblastic – which means it has one type of foliage during its juvenile phase, which lasts 1-3 years, and a different type during its mature phase. Juvenile needles don’t appear on fascicles, instead developing directly on the stem, and their profile is more flattened – although it’s not super noticeable until you know to look.ref Mature foliage develops in fascicles (bundles) and is the familiar three dimensional profile – they fall off as a group when the fascicle falls off. See examples below – Pinus cembroides juvenile on the left and adult on the right of both frames.
Juvenile and mature needles have different properties with juveniles having 3x the photosynthetic capability of mature needles. This is believed to be able to provide such species with a ‘fast return’ strategy whilst establishing and a ‘slow return stress-resistant’ strategy when older.ref
Surprisingly needle leaves actually have better photosynthetic performance than other conifer leaves – despite that fact that Pinaceae have lost key genes required for photosynthesis in other plants.ref Their improved performance is believed to be because water has to travel further than in non-needle leaves, but there is likely also a genetic factor which hasn’t been discovered yet.ref Needle leaves take a couple of years to reach their full photosynthetic capacity, but once they do, from then on it reduces with age. One study found that for spruce leaves this declines linearly from the 2 year point, reducing to 30% or less of the maximum photosynthetic capacity by the 6 year point.ref
When photosynthesising, trees need to deal with transpiration, which is the evaporation of water from the stomata in their leaves and is the main driver of their water requirements. Needle-leaved trees have a massive advantage in this domain, as many species have wax deposits in their stomata which reduce transpiration. It has been found that wax deposits in Sitka spruce stomata reduce transpiration by two thirds but photosynthesis by only one third.ref Unfortunately these waxes are degraded by pollution, so needle leaved trees can dry out in high pollution areas.ref
Since needle leaves can generate more energy from the light they have, and from a wider range of sun angles, they can survive in poorer light conditions than flattened-leaved species. Along with their cold resistance and ability to minimise transpiration, this is why the boreal forest comprises mainly Pinus, Abies and Picea species, which all have needle leaves.ref
Finally a note on buds and how needle leaves develop on a tree. Leaf buds on mature needle-leaved species form as part of a shoot rather than individually, and these are usually determinate, which means that everything is formed inside the bud. So before bud break the leaf primordia (baby needles) are sitting inside the bud. Below are some cross-sections of long shoot pine buds (which you might know as candles) and a spruce bud (on the right). The brackets show where short shoot buds are located, and within these the baby leaves are waiting to emerge with the shoot.
Contrary to some advice, needles do all their extension in their first year of growth. After this they replace or add to their phloem annually, but not their xylem.ref This means there may be some thickening of needles, but no lengthening after the first year. They may be shorter with increased dryness and poverty of the soil.ref Eventually needles fall off along with their fascicle and the other needles in their group. However, pines have been shown to retain their needles up to twice as long if they have been defoliated (eg. by insects).ref
What’s the impact of this all for the bonsai enthusiast? Firstly your needle-leaved species are going to be tough, they will cope with reduced water, poor soil, wind, rain and freezing temperatures. They definitely do not want to be inside.
Secondly as per conifer leaves in general, needle leaves are not as plastic or regenerative as angiosperms – they are part of a shoot and form in the bud, so there aren’t as many styling options as you find with angiosperms or even flat or scale-leaved conifers. And they have a more relaxed time frame than angiosperms do – needles may stick around for a long time – usually this will be 2-4 years in most species and low-medium elevations but can be a lot higher. So your styling decisions can’t be completely redone on an annual basis – a better approach for needle-leaves is a gradual evolution towards a vision. These trees suit bonsai practitioners with patience and a slow, thoughtful approach.
Bonsai folk like small leaves and in the case of needles, short needles. This is achieved in one of two ways. The first way is that long shoots (candles) are completely broken off early in the growing season. This forces the tree to activate dormant foliage buds at the base of the shoot, which don’t have the time or resources to develop full length needles. I’m not sure whether breaking the part of the candle off (as is also advocated) would also reduce needle size since this retains the active short shoots at the base of the candle to still develop. This practice might have a slightly different effect of creating more short shoots with more needles, so giving denser foliage, rather than shorter needles. The second way to reduce needle size is to starve the tree of water and nutrients, but I’d say manipulating candles would be better for the health of the tree. Obviously to add branches to a tree you need to leave the long shoots in place to develop, as these are what create the long-term framework.
To finish off, I just have to share one of the brilliant images created by Gerhard Vicek who does microscope cross-sections of plants – below is a cross-section of a Cedrus atlantica needle. The beautiful staining he does of his samples makes the different cell types really clear. There are epidermal cells (in red) on the outside, below these the unusual frilly arm palisade cells (in green). Then you have the transfusion cells in a brown & white ring, which move water to the outside of the needle and sugars to the centre. In the centre you can see two vascular bundles with the tiny xylem & phloem cells. Truly his images are art for the bonsai science nerd!
Please visit Gerhard Vicek’s’s website for more great microscope images of trees: foto-vision.at
Scale leaves are a curious form of conifer leaves which cover up the stem in interlocking patterns. I believe they are called ‘scale’ because they look a bit like fish scales in the way they overlap but I have not found an authoritative source which confirms this. Scale leaves appear primarily in the Cupressaceae family – including junipers, various cypresses, Arborvitae/Thuja, redwoods & Callitris, as well as in the Podocarpaceae family including Dacrydium and Acmopyle. Sometimes the scale leaf form is the mature foliage, while the juvenile foliage takes a needle form (see my post on conifer needle leaves).
Some examples of scale foliage leaflets are below:
Juniper with strobiliCypressThuja occidentalis
You’ll note that I called them ‘leaflets’. The actual leaves are the individual scales that you see in the images, which all combine to create a larger leaflet which is actually a short shoot. The leaves are wrapped around and connected to the stem of the shoot underneath.
Scale leaves are usually in opposite pairs, and depending on the species can have main or ‘facial’ scales and lateral scales with slightly different anatomy. Below is a scanning electron microscope image of Thuja occidentalis leaves which demonstrates these two scale types.
Probably the most distinctive attribute of scale-leaved species is the leaf pattern. Each species has a distinct cross-sectional profile, with different leaf shapes and configurations, these are what ultimately create the three-dimensional shape of the leaflet.
Great work was done on this by some Iranian researchers, who created the following cross-section drawings which I have matched to images of the species in each drawing. These show how the scale leaves attach to each stem, the positions of vascular bundles, resin ducts (large holes) and stomata (which are mostly present in the grooves indicated by ‘S’).
As can be seen in the drawings, scale leaves are very simple, usually with a single vascular bundle (other than Juniperus excelsa above which has none), palisade and spongy mesophyll cells for photosynthesis and the darker transfusion cells which move water to and from the stem.ref
In terms of their performance, there actually isn’t a lot of information out there comparing scale leaves with needle or flat leaves. One study found that Thuja leaves were about on par with pine needles in terms of photosynthesisref and another found that juvenile needle leaves of Juniperus sabina outperformed its scale leavesref. Many studies seem to conflate needle and scale conifer leaves and talk about both of them having strong performance in high sun, low humidity situations. There must be some benefit, because quite a few species ‘graduate’ to scale foliage as they age, but I haven’t found any research explaining what that benefit may be.
One weird and wonderful variation of the scale leaf is the unusual ‘axial’ leaf of the coast redwood. Most people associate the coast redwood (Sequoia sempervirens) with a flat-leaved leaflet as shown on the left – and in fact this type of leaf makes up 95% of the leaf surface area of these trees. But 5% is made up of the axial-leaved ‘twiglet’ on the right. These leaves are optimised to absorb water, having much less waxy coating than the flat leaves and contribute up to 30% of the water requirement of the tree. Which can be high, given the size of a coast redwood!
Since scale leaves are associated with older trees, they are preferred for bonsai. If your tree is still in its juvenile phase, you need to let it grow as it’s believed that the trigger for changing phase to mature foliage is the number of meristem cell divisions.ref Pruning the leader on these trees will keep them permanently in a juvenile state, so let the tree grow until it develops mature foliage, then you will need to use all branches & foliage *after* this point to style your tree.
Also remember that scale leaves usually appear on short shoots, which abscise (fall off) as a unit. But don’t worry because usually there will be a bud waiting at the base of the short shoot to replace the one which fell.
Finally if you look at how scale leaves connect to the stem, I believe that the technique of pinching leaves (and stem) off instead of pruning with secateurs or scissors would leave less dead material on the tree. Cutting straight through a stem is always going to sever one or more scale leaves and cause them to die and go brown. The pinching technique is when you use your fingertips, and pull the stem gently so it breaks at a natural breaking point between leaves.
You may have noticed that certain conifer species’ leaves go a different colour over the winter without dying. This phenomenon is most associated with Buxus (box) and Cryptomeria japonica (Japanese cedar) which take on a range of bronze, reddy-brown colours, as shown in the examples below:
Cryptomeria japonica spiralis (from my garden, with frost)Cryptomeria japonica (https://www.flickr.com/photos/oregonstateuniversity/46192395845)Cryptomeria japonica
Although it may look a bit alarming, these leaves will go green again as the temperatures warm up. The reason for the colour change is that in these species, sun-exposed leaves at low temperatures shift their pigmentation to protect themselves from excess energy.
During the winter, evergreen trees face the challenge that the sun can be relatively strong, but the temperatures low. Many of the biological processes needed to convert light into sugars rely on enzymes, which don’t work efficiently at very low temperatures. Although it’s possible for plants to acclimate to lower temperaturesref, beyond a certain point the rate of photosynthesis starts to outpace the downstream processes, and excess energy and reactive oxygen species (“ROS”) accumulate. This can permanently damage Photosystem II (part of the photosynthetic apparatus in plants), so a mechanism is needed to avoid that.
The solution adopted by Cryptomeria japonica is to transform some of its chloroplasts into chromoplasts.ref
The part of the plant cell which photosynthesises is the chloroplast, which is a self-contained organelle with its own DNA known as a ‘plastid’ (and also, in conifers, paternally inherited). Normally it synthesises the green pigment chlorophyll. But plastids can also be ‘chromoplasts’, instead of synthesising chlorophyll, they synthesise coloured carotenoidsref which aside from being different colours, also act as anti-oxidants, removing dangerous excess oxygen levels from plant tissues. The pigments found in winter Cryptomeria foliage include Rhodoxanthin (found in Taxus baccata arils), Zeaxanthin (found in marigolds) and Antheraxanthin (found in dandelions).ref These are all carotenoids which are known to safely dissipate excess energy during photosynthesis.ref
Red Taxus arils coloured by Rhodoxanthin (image: Kew Gardens)Marigolds coloured by Zeaxanthin (image: Wikipedia)A dandelion coloured by Antheraxanthin (image: Wikipedia)
So during winter when temperatures fall, green chloroplasts are converted into coloured chromoplastsref, and vice versaref when the temperature rises. The effect of this temporary transformation is to reduce photosynthesis and to increase the plant’s anti-oxidant levels, so that excess energy is dissipated safely and the photosynthetic apparatus is not damaged.
Aside from being observed in Cryptomeriarefand Buxusref, there is also anecdotal evidence that the same process happens in Juniperusref and Thuja (see below for one of my Thuja bonsai – the white bits are frost and the foliage is a distinctly reddy-brown colour). Interestingly these are all ‘flattened’ forms of conifer leaves, and colour-changing behaviour is rarely observed in needle-type leaves, which may instead deal with the problem of winter sun by plugging up their stomata to reduce the rate of photosynthesis. One exception is Pinus contorta ‘Chief Joseph’ which has golden needles in the winter and green ones in the summer.
The conversion of chloroplasts to chromoplasts is also responsible for the colouring of fruit and flowers, but it’s *not* what makes leaves coloured in the autumn (that would be ‘gerontoplasts’ref).
It seems that the conifer species which can do this are all members of Cupressaceae. So if you see pine needles going brown, that is probably the needles dying (which they do naturally after a certain period of time) unless it is a rare form. But if your Cryptomeria, Juniperus or Thuja develops a winter ‘blush’, don’t worry, it will probably come back all green when the temperature warms up.
I’ve been planning a post on this subject for a while because conifers have always been a bit scary to me from a bonsai point of view – they don’t seem as forgiving or obvious in terms of their growth behaviour. This was one of those subjects which ended up being a lot more interesting and complex than I was expecting – once I hit 3000 words for this post I realised I needed to separate things out! So below is a *summary* overview of conifer leaves, and detail on the three different types of conifer leaves are in separate posts: conifer needle leaves, conifer scale leavesand conifer flat leaves.
But let’s start from the start. What are conifers? Strictly speaking they are any of the species in the family Pinopsida also known as Pinales or Pinophyta (for a reminder review the previous post on The kingdom Plantae and where trees fit in), that is to say, the Pinophytes.Pinophytes are cone-bearing plants, hence the name conifers. They include six different families:
Araucariaceae (including monkey puzzles and the Wollemi pine)
Podocarpaceae (mainly southern hemisphere evergreens including Buddhist Pine), including Phyllocladaceae (celery pines from New Zealand)
Sciadopityaceae (Japanese umbrella pine is the only member in this family)
Taxaceae (yews) including Cephalotaxaceae (Japanese plum yew)
So why do these families have different leaves to those of angiosperms/flowering plants? It’s because gymnosperms (including conifers) and angiosperms diverged in their evolutionary paths 350 million years agoref and as a result they have evolved with key genetic differences. These are exposed in leaves in five key areas:
Venation – the structure of the vascular system which transports water through the leaf and products of photosynthesis back into the tree (ie. its ‘veins’) (and thus determines the possible leaf shapes)
Stomata– the distribution, density and effectiveness of the pores on the leaf which allow air in and water vapour/oxygen out
The photosynthetic apparatus – how the cells in the leaf are arranged to perform photosynthesis and which reactions are used
Heteroblasty – the phenomenon of ‘extreme variation in leaf morphology during plant development’ or in other words, leaves being completely different on young plants versus old plants of the same species (trees which have different juvenile and mature foliage) – although this also exists in angiosperms the versions in conifers are unique genetically
Resin canals – the ducts in conifer leaves & stems containing secondary metabolites
Starting with venation, the vascular system of conifers (which performs water & sugar transport) has only one single vein or two parallel veins per leaf, running up its centre. This is shown in the examples of conifer leaf cross-sections below – purple shows the xylem (water transport) and the blue shows the phloem (sugar sap transport). (2), (10) and (14) have two parallel sets of veins and (5) & (12) have a single, larger vein.
By contrast the vascular system in the leaf of a flowering plant is much more sophisticated with many different vein patterns across speciesref and the average vein length per area in an angiosperm leaf is 2 to 5 times higher than in conifer leaves.ref Some examples of angiosperm leaf venation are below – you can see veins branching and extending to every part of the leaf and this is one of the advantages that allow angiosperms to create larger leaves (hence the name ‘broadleaf’).
The vascular structure of conifer leaves limits how much water can be delivered to their outer edges. From the vascular bundle/s, ‘transfusion tissues’ or specialised cells conduct water and photosynthates to and from the margins.ref Their conducting capacity is limited, which in turn limits how wide a leaf can become. In layman’s terms, because conifer leaves have basic water piping, they can’t grow too wide – which affects the size and shape that conifer leaves can take.
Conifer leaf shape categorisation is inconsistent across the literature, and you may see different descriptions such as awl-shaped, sabre-shaped or even intermediate (a catch-all for anything which doesn’t fit). A reasonable set of descriptions has been created by Paul Fantz at the North Carolina State University. But at the end of the day most conifer leaves fit into one of three types – flat, scale or needle leaves. A nice study was done in Iran which produced line drawings of the main three types of conifer leaf, which you can see below (and here). On the top is a flat leaf of Taxus baccata (yew), on the bottom left is a scale leaf (and stem) of Cupressus sempervirens (italian yew) and on the bottom right is a needle leaf of Juniperus communis (common juniper). Due to their shapes, each type of leaf is a little bit different in terms of how they perform in a given environment, and you can learn more about this in my posts about each type: flat, needle, scale. The fact that the same tree produces foliage of more than one type is covered below in the section about heteroblasty.
Now let’s consider the stomata on conifer leaves (to learn or remind yourself about stomata you can read my stomata post). Whilst conifers have the same basic structure for their stomata, with one guard cell on either side, they differ from angiosperms in their arrangement and effectiveness.
Conifer stomata develop at the base of each leaf, meaning that they grow out in longitudinal bands as the leaf emerges, whereas angiosperm stomata develop at multiple points on a leaf, resulting in more variation in their patternsref. In needle species they are arranged around all sides of the leaf (with a few exceptions), in scale leaf species they appear in the grooves between scales and stem, and in flattened leaf species they appear mainly on the bottom of the leaf. Below is an image of the stomata from a Picea species, showing them arranged in lines:
Stomata in conifers have a couple of other characteristics – often they are ‘sunken’ or set into the layers of the leaf, as well as filled with wax plugs.ref This massively reduces the gas exchange capacity of the leaves – one study found that gas exchange was only 35% compared to species without wax plugs. Their conclusion was although this blocks the stomata and reduces photosynthesis, it may have been an advantage during wetter periods of earth’s history by keeping the pores free of water. The wax plugs also prevent fungal intrusion – which is more of a risk for conifers with long-lived leaves. Finally a less open stomata also reduces water loss. This allows conifers to survive in drier areas and to stay alive for longer with minimal water – hence they are now found in more extreme environments where angiosperms can’t survive. Below is a sunken stomata from a Tsuga canadiensis on the left and a Cryptomeria japonica stomata full of wax on the right.
Next we need to look at one of the most important attributes of a leaf – its photosynthetic apparatus and performance.
Whilst conifer leaves photosynthesise about 30% less effectively than angiosperm leavesref, they live and photosynthesise on average 50% longer when compared to angiosperm evergreens – and obviously much longer (around 300%) when compared to deciduous angiosperm leaves.ref So overall conifers need to invest less resources to generate their energy, since each leaf works for longer periods. Where angiosperm leaves have a ‘live fast, die young’ lifestyle, conifer leaves are more ‘slow and steady wins the race’.
One surprising fact I came across while researching this post was that conifer seedlings can actually grow in the dark. They are able to synthesise chlorophyll and create the photosynthetic apparatus without light, and these are ready to work as soon as the plant is illuminated- although the amount of chlorophyll produced is lower than if the seedling has been illuminated.ref This makes sense since seedlings may often germinate in low light conditions on a forest floor.
Like angiosperms, conifers can have different shade and sun leaves (this is known as ‘heterophylly’). In Abies alba (silver fir) sun leaves are on average longer, have thicker cuticles, more photosynthesising palisade mesophyll cells, fewer spongy mesophyll cells and more stomata than shade leaves, as well as significantly higher photosynthetic performance.ref By contrast shade leaves contain 3 times more chlorophyll content and 2.5 times more carotenoids than sun leaves. Even the arrangement of sun and shade leaves look quite different – see the image below showing sun leaves on the left and the shade leaves on the right.ref
Another factor which determines the photosynthetic performance of a leaf is its age. Except for the few deciduous conifers, conifer leaves can last anywhere from one to 45 years, although the latter is unusually long. The data is scattered across many papers but to provide some examples, the majority of pine needles live for 2-8 yearsref , the scale leaves on Thuja plicata live on average 8 yearsref, and flat yew leaves also live up to 8 years.ref Needle leaves live longer at higher elevations and with poorer conditions in general (such as lack of water).ref
Which brings me to the topic of heteroblasty, or trees which have obviously different juvenile and mature leaves. It’s a well noted phenomenon in bonsai circles that certain junipers have needle leaves when young and scale leaves when older. It turns out that heteroblasty is observed in Cupressaceaeref, Pinaceaeref and Podocarpaceaeref and results from what is called a ‘phase change’ in the shoot apical meristem. This is when the growing tips change to produce different organs – so instead of producing buds that become juvenile leaves, they produce buds which become mature leaves – and eventually buds which become reproductive organs as well. This phase change is relatively stable, so once a meristem produces mature foliage, it will continue to do so. It is also position specific – so the lower branches may retain juvenile foliage even when the rest of the tree has mature foliage.ref
One explanation for heteroblasty is that it’s a useful way for plants to deter herbivores or other environmental hazards that exist for smaller, younger plants. New Zealand has a high number of heteroblastic plants (200 species), and academics have proposed that the unusual branching form in juvenile trees which is specific to the area has specifically developed to deter large ratite birds like emus and moa.ref 10 such species were found which changed their leaves and branches once they surpassed 3m in height (the maximum bite-height of the ratites). However since there are no more moas, it’s hard to prove the theory, which is apparently hotly debated.ref
Phase changes are controlled by genes and plant growth regulators, which change their expression when a meristem has undergone a certain number of cell divisions.ref This was demonstrated by showing that mature flowering meristems, when rooted as cuttings, also flowered and so retained their mature state. This is why position matters when it comes to heteroblasty and only meristems which have reached the mature phase will produce mature foliage.
Since phase change to a juvenile state is desirable for plant cloning, there are studies which have considered how to maintain juvenility or reverse it in mature plants. One method for delaying phase change is to ‘hedge’ – what you and I would call pruning – presumably because this removes the apical meristem programmed for the new phase and reverts to meristems lower down the plant which haven’t changed phase. Another is to apply stress to a plant by starving it, dehydrating it or exposing it to heavy metals.ref
The final and fifth familiar attribute of conifer leaves that differs from angiosperms is that they are almost all resinous. Conifer resins are mostly terpenes made up of linked isoprene elements (C5H8) and are conducted through leaves (as well as some cones and wood) through resin canals. 30,000+ different terpene structures produced by conifers have been identified – some of which are used to produce various products including turpentine, printing inks, soap, plastic, fireworks, and tar. The effect of resinous leaves is to deter insects (Farjon, 2008) and microbes.ref Resin doesn’t feature too much in bonsai (other than when you’re cleaning your branch cutters), but the resin does provide a defensive benefit to your trees which is probably better than many of the chemicals that are sold for the purpose.
Anyway what does it all mean for bonsai? (Thank god I hear you say – it only took her 2000 words!!)
Well let’s start by acknowledging that conifer leaves are quite different from those of angiosperms. Their vascular system dictates that the leaves take one of the three forms – needle, scale or flattened, and aside from the few deciduous conifers, in general their leaves are designed to stay on the tree for much longer than most angiosperms. This means you’re not going to get the same level of leaf turnover on your coniferous bonsai as you would with your angiosperms, and your styling decisions need to be more carefully made and executed. It is going to take longer to fix a mistake on a conifer.
Similarly, their photosynthetic rate is not as high as an angiosperm, so in many cases a conifer is not going to be able to achieve the same growth rates as an angiosperm unless they have a lot of light, although there are some more fast-growing species. As per the previous point, conifers are less forgiving of poor styling decision.
Depending on its leaf type, your different conifers will prefer different conditions (full sun for needle, humid and less sunny for flattened), but you should also be thinking about how to cultivate the types of leaves you want to see on your tree. Sun needles appear denser and better for bonsai, so shading a fir or a pine is probably not a great idea. Similarly making use of short shoots with their increased leaf numbers is important (see my post on shoots).
Species which display needle-scale leaf heteroblasty are a special case as usually you want them to take on mature scale foliage which is preferred in bonsai. To do this, lower, older branches (with the juvenile form) will eventually need to be removed, and you shouldn’t prune the apical stem of these species until they have reached the mature foliage phase. Or sidestep the juvenile phase altogether by taking cuttings of mature foliage which should stay mature unless they are seriously stressed.
A final point would be to say that although conifers all fall under Pinopsida (etc) they have a much longer evolutionary path than angiosperms and more divergence between them, so lumping them all together into one post is not really comparing apples with apples (hehe). So have a look at the other posts which spawned from this one to dive into a bit more detail: conifer needle leaves, conifer scale leaves and conifer flat leaves.
