Category Archives: Creating New Bonsai

Bonsai Pinetum Species List

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 (Sequoiadendron giganteum), 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 (Taiwania cryptomerioides), 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 (Pinus sylvestris/thunbergii/parviflora), Abies koreana (Korean fir), Cedrus atlantica (Atlantic cedar), Engelmann spruce (Picea engelmannii), European Larch (Larix decidua)

Extensive (9 genera plus extras): as above plus Eastern white pine (Pinus strobus), Mountain Hemlock (Tsuga mertensiana), 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 (Dacrydium cupressinum)

SCIADOPITYACEAE

Japanese Umbrella Pine (Sciadopitys verticillata)

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)

Creating a Bonsai Pinetum

Family 1: Araucariaceae

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 redwoods^ref, 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.

Family 2: Cupressaceae

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).

Expanding to ten specimens across ten genera would allow the addition of five other reasonably easy to procure and grow species: Giant Redwood (Sequoiadendron giganteum), 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 (Taiwania cryptomerioides), 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.^ref Cathaya 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 Keteleeria evelyniana 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.

Family 4: Podocarpaceae

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 habitats^ref, 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 stems^ref. 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 Dacrydium cupressinum, 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.

Podocarpus salignus (willow-leaved pine)

Family 5: Sciadopityaceae

Sciadopityaceae has only one genus (Sciadopitys) and within that only one species, the Japanese umbrella pine or Sciadopitys verticillata. 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.

*Sciadopitys_verticillata* at Pinetum Blijdestein, The Netherlands

Family 8: Taxaceae

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 colouration^ref. 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.

Japanese Yew at Matthei Botanical Gardens (*Taxus cuspidata*)

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.

Thanks to the University of Oxford Department of Plant Sciences Conifer Database and the fantastic Gymnosperm database for source material for this article.

Bonsai Tree Growth Stages

Most bonsai trees progress through stages of development, each with a different objective. In general the progression is thicken trunk -> achieve branch & root structure -> achieve branch, foliage & root ramification -> reduce leaf size -> evolve as branches grow/fall off. The faster we can move through the first few development stages, the faster we will have beautiful, well-proportioned bonsai – harnessing the tree’s natural growth is a way to speed this up. We also want to avoid doing things which slow down a tree’s growth during these phases, as this will mean it takes longer to get the tree we want. Read about how trees grow before starting at #1 below. Also consider what do old trees look like?

1. Trunk

Some bonsai enthusiasts collect mature trees for bonsai specifically so they can start with a thick trunk, following a collection process which minimises damage to the tree. The alternative is growing your tree’s trunk. Once a tree has its roots and foliage reduced in size in a bonsai pot, it won’t generate the energy needed to make significant sapwood additions and its girth will only increase by small increments every year. So you really need to be happy with the trunk size first before you stick it in a tiny pot. But – how big should a bonsai tree’s trunk be?

2A. Branch Structure & Overall Shape

Arranging the branches is what gives you the canopy and overall foliage shape that you’re after and the first step in this process is growing (or developing) the branches you want in the positions they are needed. Growing a branch starts with a new bud, which, unless it’s a flower bud, becomes an extending shoot and eventually a new branch. So firstly you need to work out where new buds will grow on your tree and then deal with the extending shoots as needed to get the required internode length.

You may need to remove some buds and shoots if they don’t help achieve the shape you are looking for – this should be done as soon as possible to avoid wasting the tree’s finite energy reserves. You have a trade-off to make here because leaving more foliage on the tree will provide more energy overall which contributes to its health and ability to recover from interference. However, growing areas of the tree which won’t be part of the future design is a waste of energy. You don’t want to remove so much of the tree’s foliage that it struggles to stay alive or develop the areas that you do want to grow out.

When you are creating your branch structure, often you will need to reposition branches – this is done with a wide range of different tools and techniques. A more advanced technique for adding new branch structure is grafting.

Sometimes the trunk itself or larger branches need a rework, to make them more interesting or to make them look more like old trees – for example adding deadwood or hollowing out the trunk. Usually this is achieved through carving.

2B. Creating a Strong Root System

The trunk thickening and branch structure phases both work best when the tree has lots of energy and so letting it grow in the ground or in a decent sized pot during these phases will get you there quickest. This also allows the roots to keep growing, but you want to understand about the role of roots, and root structure & architecture even if you still have your bonsai in a training pot. Particularly in this case, knowing about how to foster the the rhizosphere will help your tree stay vigorous. To maximise the roots’ exposure to nutrients and water you want to encourage Ramification of Roots (lateral root development).

Eventually it’s time to move the tree into a bonsai pot. This requires cutting back the roots, but as long as the roots are balanced with the foliage in terms of biomass, the tree should be OK. Root growth is usually prioritised outside of times of stem/foliage growth, and above 6-9 degrees C. So repotting might be best conducted at times that meet this criteria. Your growing substrate/medium is an important consideration.

3. Ramifying Branches & Foliage

Ramification is when branches subdivide and branch, giving the impression of age and a full canopy – and a well-ramified tree is a bonsai enthusiast’s goal. There are some techniques for increasing the ramification of branches and foliage. But not as many as there are for root ramification.

This stage also involves ongoing branch selection and reshaping (see 2A above). Another consideration is whether to keep or remove flower buds.

4. Reducing Leaf Size

An end stage in the journey to bonsai perfection is leaf size reduction. In nature, leaf sizes reduce relative to the biomass of the tree as it ages but since bonsai are small this effect doesn’t translate since the biomass never gets large enough. The tried and tested method for reducing deciduous tree leaf size is actually to practice one of the various methods of defoliation. A couple of others are covered here in reducing leaf size.

When to conduct these various activities depends on when the tree can best recover from them – which is a function of the Tree Phenology (or Seasonal Cycles).

5. Evolving Branches

Trees are not static organisms – they obviously continue to grow which is what we harness in the above steps. Part of this is that eventually branches may become too large for the design, or they may fall off (Peter Warren notes that Mulberry are known for this). As bonsai artists we want to have this in mind so that branches are being developed which can take their place in the future. This is an ongoing version of step 2A.

What is a Bonsai?

I’ll admit, there isn’t a lot of science to this post. Bonsai itself isn’t much of a topic for research and it gets used as a name for a type of decision tree which confuses searches.

To answer this question I will borrow Colin Tudge’s definition of a tree: “‘Tree’ is not a distinct category, like ‘dog’ or ‘horse’. It is just a way of being a plant.”

Similarly, “Bonsai is not a distinct category, like ‘lichen’ or ‘flowers’. It is just a way of being a tree.” The bonsai way of being a tree involves living in a very small pot – in a space that’s much much smaller than nature ever intended – and, that’s pretty much it. Everything else is aesthetics!

Buds

Thickening the Trunk

The first quality of a good bonsai is a thick trunk with movement and mature bark. So what actually contributes to the growth of a tree trunk?

Two processes are involved. The first is the creation of new sapwood. Sapwood is the living wood towards the outside of a trunk which conducts water (Ennos, 2016). Sapwood formed in spring is called ‘earlywood’ and is optimized for water & nutrient transport to help the tree with its growth spurt. Latewood is designed for structural support and carbon storage.

Water and nutrients are conducted from the roots through xylem vessels. The mechanism by which they work is explained in xylem but for the purpose of this section it’s important to understand that the reason why trees add new xylem vessels is because as it adds biomass – new branches and leaves – more water is required. So – the more biomass is added in a given growing period – the more water is needed – the more xylem vessels are added to the trunk. Xylem vessels also become non-functional for reasons explained in embolisms, so trees need to replace them as well as adding to them due to new growth.

New sapwood (with xylem vessels) is added around the previous sapwood, encircling the tree. How much of the girth of a tree increases each year is determined by the tree’s food supply (Trouet, 2020); this is a combination of the amount of rainfall and the energy from the sun during that year.

This study^ref found that “low precipitation at the start or during the growing season was found to be a significant factor limiting radial growth” for a range of urban trees in the UK. According to Trouet, “alternating wet & dry years create wide and narrow rings respectively.” So low water levels lead to small rings and high water levels lead to large ones. The earlywood creates a larger ring than the latewood, since the xylem vessels are larger in earlywood (for water transport) and smaller in latewood (for structural strength) (Ennos, 2016).

What this means for bonsai is that watering your tree well is important while developing its trunk, whilst ensuring you have a well-drained growing medium to avoid creating anoxic conditions (lacking oxygen). If your medium is well-drained and you water thoroughly throughout the tree’s growing season (but particularly during earlywood development), you’ll boost your tree’s girth by creating wide ‘good times’ sapwood rings.

The other factor mentioned is energy from the sun. Energy from the sun is used by the tree in photosynthesis, which converts energy into a form that the tree can use to respire and grow. If there is more sun, more energy is available and the tree is able to create more xylem, buds, leaves and biomass. This isn’t a straightforward linear relationship however, as photosynthesis reaches a saturation point based on a number of limiting factors (more in the post about photosynthesis).

The key point here is that reducing the ability of the tree to capture and convert energy will affect its growth. If you reduce the foliage on your tree or cut it back in spring, you reduce its biomass, it can’t generate as much energy, and doesn’t need as much water, so doesn’t add as many xylem vessels as it would have nor as wide a ring of sapwood. This reduces the trunk thickening you can achieve in a given time period.

It’s worth noting that the roots of a tree need to be capable of delivering the amount of water that its foliage and branches require. Optimising trunk thickness requires a dense canopy of leaves and branches, matched by roots capable of delivering the amount of water that they need. This is why many bonsai enthusiasts will start a tree off in the ground or in a large pot, allowing growth to drive the trunk size until it’s at the level required.

Attempting to restrict the roots and size of the tree too early (e.g. by putting it in a bonsai pot) will restrict trunk growth by reducing the water available to the tree and reducing the energy it can create by reducing its foliage.

Like people, trees are genetically programmed to have different maximum heights and lifespans. Some trees are slow-growing (such as Yew) and some are fast (such as Eucalyptus) so to an extent the amount of trunk thickening that is possible also depends on the species of tree.

Trees grow most vigorously when they are free from environmental stressors – such as drought, extreme cold, loss of leaves due to high winds, attack by insects or animals. A stressed tree will grow a narrow ring. BUT stress in the form of wind can foster positive qualities in a trunk. Ennos (2016) says that trees exposed to high winds without a prevailing wind direction grow shorter, with thicker trunks & roots, and adjust their wood cells to spiral around the tree creating a twisting effect. It’s not just the trunk that is affected – apparently this results in smaller leaves and shoots as well. Get your bonsai a wind tunnel!

Another way to thicken a trunk is to grow a ground-level branch, as layers of xylem will be added around this branch as well as the truck, or to have a multi-stem tree, which operates on the same principle. You want to avoid having one too much above the ground though, as it might cause the dreaded reverse taper.

I mentioned two processes involved in secondary thickening – the second process is the effect of an increasing bark layer. In most cases this will be dwarfed by sapwood increases but nevertheless biomass is added as bark via the cork cambium, another secondary meristem on trees. Some trees which retain multiple periderms (layers of cork with their meristems) can develop very thick bark which does contribute to the overall trunk girth as well.