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.
A fellow bonsai enthusiast at my bonsai club asked for advice from club members on their choice of tool materials, which prompted me to look into the physical differences between the materials on offer.
The bonsai tools most people have include a range of branch, root and knob cutters, pruning shears/secateurs, scissors and pruning saws. A quick surf of bonsai stores online show that these tend to be made from steel of one form or another. Steel is an alloy made from iron and carbon. Its properties are adjusted by steelmakers by changing the level of carbon in the steel, by heat treating it or by adding other materials to the alloy such as nickel or manganese.ref Many different types of steel are made with different combinations of these factors and the manufacture of steel tools can be an artisanal enterprise, with specific alloys and processes resulting in tools with different properties.
The Japanese are known for their hard steel called tamahagane which was used for forging Samurai swordsref1. Iron-rich sand from Shimane was smelted with charcoal in a furnace, resulting in a high carbon steel which was then folded over and over to create a very strong, sharp sword.ref Japan still has a massive steel industry and they are 3rd largest producer in the world after India and China, they have a reputation for ‘higher quality’ or at least higher carbon steels than other countries.ref
The properties that are important for bonsai cutting tools include (1) strength, (2) the ability to hold an edge (stay sharp), (3) resisting deformation under stress (ie. not bending), (4) longevity, (5) rust resistance and (6) price to manufacture/buy. Hardness is the term used by steel manufacturers to encompass 1-3 of the above properties. In general a harder material is stronger, less able to be deformed by stress and more resistant to abrasion.ref Ease of sharpening is inversely associated with hardness, as resistance to deformation or abrasion means resistance to sharpening. The downside of hardness is that this type of steel may be more brittle, as it would break or snap instead of deforming under stress. Knives made from very hard steel can chip or break more easily.
So for a bonsai tool which is strong, holds its edge and stays true, you are looking for a high hardness number (in the knife industry the Rockwell scale of steel hardness is used – HRC). However the harder it gets, the more difficult it will be to sharpen.
The types of steel you might encounter when buying a bonsai tool include:
High carbon steel – higher carbon content is the main way that steel is made harder. Exactly how hard depends on how much carbon has been added. High carbon is usually 0.6% carbon content or more and a HRC rating of >60.
White steel #1 is 63+ HRC containing 1.25-1.35% carbonref (officially ‘high carbon’ steel) also known as Shirogami in Japanref
White steel #2 61-62 HRC, with 1.05-1.15% carbonref (officially ‘high carbon’ steel)
White steel #3 60 HRC with 0.80-0.90% carbonref (officially ‘high carbon’ steel)
Blue steel #1 & #2 also known as Aogami in Japan, are white steel #1 & #2 with added chrome and tungsten, and HRC 58-60 & 63-65 respectivelyref The chrome gives a stainless steel component with rust resistance.
Super blue steel is blue steel with added molybdenum and vanadium, HRC 63-64ref
‘Powdered’ steel can be extremely hard at 64-68 HRC – a product known as ZDP-189 contains 3% carbon and 20% chromium so is extremely hard and rust resistance. But I can’t find any bonsai tools which use ZDP-189, it’s mainly used for knives.
Medium carbon steel has between 0.3-0.6% carbonref and has HRC <60
‘Black carbon’steel isn’t a type of steel, it’s a brand name used by Chinese manufacturer Ryuga or a description of a colour (eg. a black coloured carbon steel tool)
Low carbon steel has 0.3% carbon or lessref and you ideally don’t want tools made of this
Stainless steel is a steel with at least 11% chromium content, which reduces its vulnerability to rust. It’s not necessarily the case that stainless steel is less hard than carbon steel – they can both be manufactured to different hardness levels. Studies performed on steel knife blades by the Cutlery and Allied Trades Research Association (CATRA) in Sheffield, England found that at hardness 61 stainless steel had slightly superior cutting performance over carbon steel.ref Ryuga’s stainless steel is HRC 55+-2 which qualifies as medium carbon steel.ref
Looking at what is available out there on the internet, it’s hard to see any data on hardness or carbon content in any of the bonsai tool descriptions. Caveat emptor applies then, since it could be any old steel composition that you are buying. It could easily be low carbon steel, which will lose the edge quickly and not be very strong, and rust easily!
Given this I would personally prefer to buy a tool which either has a specified hardness rating, or is labelled with the type of steel it’s made from so you can work out its hardness. Anything labelled high carbon, white or blue steel should work well. If you’re buying a high carbon steel tool, you do need to prevent rust, so make sure it’s cleaned and oiled after use, and stored out of the damp.
If you prefer to have some rust resistance, stainless steel is a perfectly good option and again look for something with a specified hardness in the high carbon range. Tian Bonsai has a 62 level hardness stainless steel range. Kiku appears to have 62-63 hardness stainless steel in their Gold range. Kaneshin have a range of blue steel tools (mostly scissors), and UK-based Niwaki have secateurs made by Tobisho from blue steel #2 and some blue steel scissors. I’d also consider the slightly more affordable ARS – a Japanese brand with a European distributor who produce high carbon steel secateurs and scissors and are available online.
Be prepared to fork out some significant cash for your high carbon and/or stainless steel tools, they are what one might call ‘investment pieces’. But since steel production contributes 8% of global carbon dioxide emissions, buying once and using forever is probably the best strategy.
Update: if you’re wondering what you can use to clean your tools, you can check the compatibility of carbon steel with various substances here, and stainless steel here. With carbon steel avoid anything acidic, according to these charts it can be damaged by buttermilk, gelatin, malic acid (apples), citric acid (lemons), lactic acid (milk), mustard, salt water, tomato juice, water, petroleum jelly, wine and whiskey! Stainless steel is much less sensitive.
While many of us might simply use the hosepipe to water our bonsai, there are actually a range of options for recycling or collecting water for this purpose. I’ve looked into some of these below to understand how suitable water from different sources is for watering your trees.
Dehumidifier water – good unless you have a dessicant humidifier and toxic air
Dehumidifiers remove water from the air in a few different ways. One common type is a compressor dehumidifer. This type pulls air through a filter and over cooled metal coils which cause water in the air to condense onto the coils and drip into a reservoir.ref Since this is effectively distilling the water, it should be relatively pure. Where contamination could come into play is if the coils or the water reservoir are not kept clean, but for bonsai tree watering, this shouldn’t be an issue.ref You should be cleaning the reservoir anyway to avoid Legionnaire’s disease (see below). The water from a dehumidifier won’t have any minerals or nutrients in it (unlike rainwater), so fertiliser would be needed.
A dessicant-type dehumidifier pulls water through a dessicant material – such as zeoliteref (if you’re wondering where you’ve heard that name before, it’s used as a bonsai soil additiveref). Water is absorbed in the dessicant then this material is heated and the water drips out into the reservoir. It’s not exactly the same as distilling because the water is in contact with the dessicant as it is condensed and could hold dissolved compounds. Researching the properties of zeolite will take you down an entirely new internet rabbit hole (including 1.24M research paper results on Google scholar). But this substance is known for extracting heavy metals and other contaminants from liquidsref1 , ref2. So in theory the zeolite could hold other molecules which could be released into the water as it condenses. This might be an issue if you are using dehumidifier water from a location with particularly toxic or polluted air. If not, the risk to your bonsai should be fairly low. As above cleaning the water reservoir and dessicant regularly is important.
Tumble dryer water – good if condensing, less good if venting
Tumble dryers also work in slightly different ways but the main mechanism for extracting water is that warm air evaporates water from the clothes, then this air either passes over a condenser or is vented outside, in both cases water condenses from the air as it cools.
With a condensing dryer, the process is another form of distillation so should be relatively pure water and fine for use on bonsai.
If you are using a vented machine however there may be microplastics, particulates and lint from the drying clothes coming through the venting pipe. In 2021 tumble dryers were found to be a leading source of microfibre air pollution.ref You might not think this would affect your tree very much, but it has been found that nanoplastics and microplastics can enter plants through their roots, carrying a range of toxic substances including pesticides, polybrominated diphenyl ethers (PBDEs), endocrine-disrupting chemicals (EDCs), polycyclic aromatic hydrocarbons (PAHs), phthalates, and bisphenol-A. PLA microplastics have also been shown to negatively impact arbuscular mycorrhizal fungi diversity and community structure.ref If you want to avoid microplastics in your pots and tree roots you probably don’t want to use vented tumble dryer water.
Air conditioner water – good
Air conditioners work similarly to dehumidifiers and dryers – they pull air through a mechanism which cools it, and in doing so water is condensed from the air. So it’s fine to use for watering (although as above will contain no nutrients).
Boiler condensates – not good
I’m not sure how many people would be trying to use boiler condensates for watering but just in case you think of it, boilers which use fossil fuels produce condensates containing carbonic acid, sulphuric acid and nitric acid, all of which reduce the pH of the water coming from the system.ref Although slightly acidic water (in the range 5.5-6.5) has been show to optimise plant growthref, boiler condensates can be as low as 3 which is toxic to plants.ref
Rainwater – good – maybe the best
Rainwater is a different proposition to the previous water sources. Whilst it is distilled from air just like the others, it has to fall through the atmosphere to reach the ground. As it does this, rain absorbs compounds present in the air in particulates and gases. It also runs off roof surfaces, down drains and pipes and into storage tanks. So rainwater collects contaminants along the way. As atmospheric carbon dioxide is one of the molecules rain collects, it has an average pH value of about 5.6 (just at the lower end of preferred plant water pH which is 5.5-6.5)ref
The chemical composition of rain varies geographically even before it hits the ground. For example in Samoa the rainwater composition is highly influenced by marine sources, which makes sense since it’s an island.ref Locales near the ocean have rainwater with a similar (diluted) composition to sea waterref but those inland vary depending on natural and manmade influences such as industry, topography and weather. Raindrops have also been found to contain airborne bacteria and fungi.ref
Rainwater in the UK is monitored for ions of sodium, calcium, magnesium, potassium, phosphate, nitrate, ammonium, sulphate, sulphur dioxide and chloride, and for acidity/pH and conductivity.ref The latest measurement at my nearest monitoring station showed the following contents from a 3mm sample – nutrients are there but nowhere near the same levels as a seaweed fertiliser.
The other factor to consider is where you store your rainwater. Outdoor water reservoirs are usually colonised by bacteria, fungi and other organisms, and have rotting plant matter, bird faeces, dead insects and other detritus. This may also be a good source of fertiliser, depending on how concentrated your detritus gets, or a source of toxic microbes and algae, particularly if there isn’t much flow and replenishment of the water.
But overall rainwater is a good choice for watering bonsai. It has a favourable pH for plants, and is a mild fertiliser containing a range of macro and micro nutrients. As it doesn’t have carbonates like groundwater, using rainwater can help you avoid limescale marks on leaves & pots if you live in a hard water area. Just try to avoid leaving it standing or stagnant for long periods particularly if the temperature is above 20oC (see Legionnaire’s disease below).
Aquarium water – maybe depending on your tank
The subject of aquarium water almost warrants its own post. Anyone wanting to better understand the chemical and biological parameters inside a planted aquarium should read the brilliant book “Ecology of the Planted Aquarium: A Practical Manual and Scientific Treatise” by Diana Walstad.
The water in your aquarium is likely to be completely fine for bonsai if it’s fine for fish. They have high standards – and can’t handle large pH ranges, excessive levels of nitrites, ammonia or excessive nutrients like heavy metals. In fact this water can be an excellent source of nitrogen since fish in aquariums excrete ammonia, which is the main source of nitrogen for most fertilizers. Usually when doing a water change on an aquarium the levels of rotting organic matter have accumulated and ammonia levels are at their highest – one of the main reasons for doing water changes is to reduce them. Planted aquariums which use soil as a substrate and have fish (and fish food as an input) also contain other macro and micronutrients, to the extent that fertiliser isn’t necessary. So aquarium water could be a good addition to your bonsai, as a fertiliser.
On the other hand, if you have certain plants in the aquarium, some are known to be ‘allelopathic’ – that is they produce compounds to inhibit other organisms. For example the water lily Nuphar lutea kills duckweed and lettuce seedlings (Walstad, 2012). Aquatic algae and bacteria can also behave allelopathically, to the extent that Diana Walstad keeps her prized plants in their own substrate and even in separate tanks to stop them being killed by competitive organisms. So there is a risk with water from planted aquariums or aquariums with algae that there may be allelopathic compounds which damage your tree. It’s hard to make a recommendation since it’s impossible to know without testing whether toxic compounds are in your aquarium water. If you have no algae or plants, then the water is probably fine.
Greywater – not unless you treat it
Grey water is waste water from your bath, shower, washing machine, dishwasher and sink.ref It can contain detergents, oil, dirt, organic matter like food, skin particles, microplastics, bacteria, fungi or anything that you wash off yourself, your dishes or your clothes. As such greywater isn’t suitable for watering your bonsai. Some larger properties like hotels recycle their greywater, but it’s treated first using UV light, chemicals or serious filtration systems.ref So unless you have access to a high quality greywater filtration system, do not use this to water your trees.
Blackwater – no
The things you learn about when researching bonsai websites. We won’t go there – just – no.
Seawater – not unless you have a desalination plant
Unless you are growing kelp forests, do not water your bonsai with sea water. Excess salt is extremely detrimental to plants, and can kill them.ref
Lakes, streams, rivers, boreholes, wells or ponds – it depends
Natural water sources such as these are not treated and can contain all sorts of things aside from water, but this really depends on what runs into them, what happens and lives on and in them. Some lakes & streams are very clean, others have industrial runoff, pesticides, sewage overflowref, algal blooms from excess fertiliser runoff and worse. What you want to avoid with a water source is dissolved contaminants which might harm your tree – usually these will derive from human activity going on upstream. So before using such a water source, it would be wise to investigate what might be entering it and perhaps to invest in some testing.
It’s thought that water obtained from aquifers underground (eg. via wells or bore holes) is of higher quality if it comes from a deeper confined aquifer than from a shallow one.ref Shallow aquifers are more open to contamination from pollutants.
pH is a consideration when using a natural water source. According to the Kentucky Geological Survey: “Streams and lakes in wet climates such as Kentucky typically have pH values between 6.5 and 8.0. Soil water in contact with decaying organic material can have pH values as low as 4.0, and the pH of water that has reacted with iron sulfide minerals in coal or shale can be even lower. In the absence of coal or iron sulfide minerals, the pH of groundwater typically ranges from about 6.0 to 8.5, depending on the type of soil and rock contacted. Reactions between groundwater and sandstones result in pH values between about 6.5 and 7.5, whereas groundwater flowing through limestone strata can have values as high as 8.5.”ref Since you want to keep water pH between 5.5 and 6.5 for plant watering, it would be a good idea to test the pH of any natural water source you are using.
A note on Legionnaire’s disease
Legionnaire’s disease is a potentially fatal form of pneumonia caused by inhaling mist or droplets of water containing Legionella bacteriaref. This bacteria can grow in any non-sterile waterref, feeds on algae, rust, scale or sludge, and thrives in temperatures between 20oC and 45oC.ref So it could be present in some of the above sources, particularly when water is left standing. It’s worth being more careful when temperatures are above 20oC as you really don’t want to get this disease, which may already be a risk to bonsai practitioners since another source of Legionnaire’s is compost & potting soil.ref In warmer weather it may be safer to use tap water for bonsai.
So to wrap all of this up from a bonsai perspective – feel free to use rainwater or water which comes from the distillation process of condensing humidifiers, tumble dryers or air conditioners. Be careful though if this water has been standing for a long time in temperatures above 20oC. Feel relatively free to use aquarium water (but keep an eye on your trees to monitor for allelopathic effects) or dessicant-type dehumidifiers. Test the water if you’re using an untreated natural water source like a river, lake or well. Avoid using water from vented tumble dryers. And definitely do not use sea water, boiler condensate, untreated grey water or black water. Happy watering!
We recently had guest speaker Amelia Williams talk to members of Twickenham bonsai club about tropical bonsai. She has moved entirely to keeping tropicals & sub-tropicals, to the extent that her back garden is full of her ex-non-tropical bonsai trees which are now planted as a foliage bed! This post references some of Amelia’s talk, if you would like to read more she has written several articles for Swindon Bonsai Club.
So what is a tropical tree? At a basic level it’s a tree which lives in the tropics. The tropics are the region of the Earth between the Tropics of Cancer and Capricornref, shown on the map below. The Tropic of Cancer is a line at 23.4° N where the sun is overhead during the northern summer solstice, and the Tropic of Capricorn is the line at 23.4° S when the sun is overhead during the southern summer solstice; the sun is never fully overhead at locations outside of these lines.ref In the middle of this region is the Equator where there is always 12 hours of daylight.
The first thing you will notice from the map is that the tropics and the subtropics cover a large proportion of land mass on Earth (36%ref), and aside from being where 40% of the world’s population and over 50% of its children live, it’s also home to 80% of the planet’s terrestrial species and over 95% of its corals and mangroves.ref
It’s estimated that over 40,000 tree species live in the tropics, compared to only 124 in temperate Europe.ref These species have distinct ranges – that is, species in the Americas are different to those in Africa which are different to those in the Indo-Pacific. So tropical tree species offer bonsai enthusiasts a huge opportunity to diversify our collections and explore trees we may never have known about before. Add in the sub-tropics where trees can handle a wider temperature variation, and the selection becomes absolutely enormous.
Tropical trees live fast and die young, growing twice as fast as trees from temperate and boreal regions and living on average 40% shorter lives (around 200 years). Research shows a relationship between high temperatures, fast growth rates and shorter lifespan for trees which is most evident in the tropics. It’s particularly the case where annual mean temperatures exceed 25.4°C and trees die much earlier than in cooler places. Below is a map showing the longevity of 3,343 tree populations across 438 species worldwide – the darker dots are locations of the oldest trees, none of which fall into the tropical region.ref
https://www.pnas.org/doi/full/10.1073/pnas.2003873117 (note: each dot reflects the average data for 3 or more trees in a location)
How did the researchers work out the age of all these trees? They used tree rings – the science known as dendrochronology. Despite what you might have heard, tropical trees do produce annual growth rings, not based on seasonal growth like in temperate areas, but instead on limiting environmental factors such as water shortage during the dry season or root anoxia in flooded forests during the wet season.ref
The good news is that tropical trees grow quickly, which is awesome for bonsai and helps us get nicely shaped trees faster, just as long as we provide the tree with the environment it needs. So what is that environment?
A defining attribute of the tropical environment is its weather. The temperature in the tropics doesn’t vary much, ranging between 25oC and 28oC all year roundref. It never gets cold in this region and it certainly never gets frost. As you get closer to the Equator, the annual cycle of the Earth’s angle of rotation has a smaller and smaller effect, so these locations don’t change a lot in terms of their distance to the sun. Since the distance to the sun doesn’t vary that much, neither does the temperature.
The same applies to daylight. This never varies from 12 hours at the Equator, and even on the edges of the tropics the daylight period in winter is only 3 hours less than in summer (see the daylight chart for Alice Springs, on the Tropic of Capricorn). Compare this to London where the difference is 8 hours between summer and winter.
The amount of sunlight a tropical tree actually receives of course depends on its habitat and position within that habitat. In Costa Rica a study found that understory plants in a tropical forest received only 1-2% of the total light available, and that up to 77% of the light they did receive was from ‘sunflecks’ (spots of light which make their way to the forest floor through gaps in leaves).ref1, ref2 So whilst trees which occupy the forest canopy or live in a wet-dry tropical desert environment may require intense light for 12 hours a day, there are plenty of tropical species which can thrive in shady positions as well. As with any plant care, it’s all about understanding where your tree naturally thrives and trying to emulate that environment.
As well as constant temperatures, tropical plants receive a lot of rainfall. Two thirds of annual global rainfall occurs in the tropics and sub-tropicsref with different patterns in different zones. While the Earth’s rotation has less of an effect on temperature in this region, it has a greater effect on weather systems, which occur more spontaneously in the tropics than elsewhere.ref
The equatorial zone has high monthly precipitation (60mm or more) and annual precipitation of 2m or more. In this zone are many of the tropical rainforests, where there are often dry, humid mornings and rain in the afternoon. Outside of this there are ‘Dry and Wet’ regions with lower rainfall and distinct dry and wet seasons which depend on position relative to the Equator. Finally some areas of the sub-tropics are categorised as monsoon zones, where there is higher temperature variation (for example going down to 13oC in Chittagong in January), but also periods of dry and periods of significant rainfall (known as the monsoon).ref Your tropical tree will be expecting a lot of water at some point in the season!
What all of this means is that to keep tropical bonsai in non-tropical areas we need to create a suitable environment, with four main attributes: (1) a stable, high temperature, no drafts or strong temperature variations and definitely no frost, (2) a consistent level of light between 10.5-13.5 hours (with intensity depending on the species) (3) high humidity and (4) a good watering regime. For anyone who doesn’t live in a tropical location, this means keeping them indoors, in a house or heated conservatory, near a window with some sunlight and away from drying drafts or wind.
In their natural home many tropical species will be used to temperatures above 18oC and up to 28oC, but Amelia Williams recommends no less than 12oC and for Ficus it should be above 15.5oC. This is the lower limit for temperature for your tree, ideally it should be higher, so room temperature of around 20oC with maybe some heat from the sun during the day should work well.
If your tree is in a conservatory or even in a window, it might get quite hot during the summer. Medium heat stress in trees is thought to be transitory and doesn’t result in long-term damage (although it does increase net energy use), but “long or exceedance of heat tolerance thresholds leads to irreversible damage to the photosynthetic biochemistry and leaf tissues”.ref One study found in Phaseolus vulgaris that very hot conditions (over 40oC) photosynthesis declined rapidly and the cost of respiration exceeded the energy from photosynthesis at around 43oC. Damage to the leaf and death of cells and chloroplasts was visible from 48oC.ref This means that a tree which is extremely heat stressed may need to regrow leaves to recover. However even in this situation they need to have enough water. Trees under heat stress which have enough water keep their stomata closed and minimise water loss, but when they don’t they actually open up their stomata. This causes more water loss through transpiration and the possible death of the tree.ref So it’s super important at high temperatures to keep your trees well watered.
Sunlight can be a difficult commodity to provide your trees all year round, but a protected window position (and occasional rotation of the pot) will give it the best light, which can be augmented by artificial light if you want to more accurately emulate a tropical photoperiod. Don’t worry if you have no natural light though, it is possible to use entirely artificial light, your trees just won’t be as vigorous. A study on houseplants in Uzbekistan found that those with no natural sunlight required artificial light of a minimum of 2000 lux per day and ideally 5000 lux.ref
Taking your tree outside in the summer is an option, but be careful taking one from behind the protection of UV-filtered glass straight into a hot sun. Model plant Arabidopsis was found to take 8 days to synthesise and accumulate maximal levels of sinapate esters in its leaves, which act as a UV sunscreen.ref Instead give the tree some shade or protection for a period while it acclimatises (this actually applies to any plant you keep inside for part of the year).
Humidity can be provided by regular spraying (or misting if you have the facility), but Amelia Williams uses humidity trays. These are trays or dishes filled with a layer of pumice which the bonsai pot sits upon. Water is added to the tray and evaporates up to the foliage, the roots also detect the humidity below (and can sneak out of the holes in the pot in search of this water).
As a watering regime, in the UK once a week is enough in winter, twice a week in spring/autumn and daily in summer, but as with all bonsai watering, this needs to be considered based on each tree’s needs. Water well in hot or dry weather to minimise heat stress.
Other tips from Amelia when keeping tropical bonsai – as with all bonsai use soils with good aeration and differing particle grades (see Bonsai growing medium), repot when actively growing, and avoid ‘cold’ soil substrates or substrates which don’t warm up easily or quickly (such as grit).
Choosing which tropical or sub-tropical bonsai you are going to start with is probably the hardest part, there are just so many options!
On the angiosperm (flowering tree) side of the fence, there are many, many options. Acacia, Diospyros, Eucalyptus,Ficus, Adansonia (Baobab), Bougainvillea – the list goes on – but if you are in the UK or a place with similar weather, Amelia Williams’ articles on Swindon Bonsai’s website give you lots of great suggestions – in this one she has a list. A quick tour of the websites of bonsai sellers located in the tropics yields a lot of the same species we see in Europe – Japanese pines and junipers. But there is inspiration out there – here are some specific African bonsai styles, tropical bonsai from Indonesia by Gede Merta, a bonsai farm in China and one in Florida (strictly Florida is sub-tropical so this owner takes his trees inside during cold weather). You don’t even need to stick to trees, as there are also tropical non-trees which look amazing as bonsai – such as these ‘Rambo form’ Adeniums from Thailand (note that Adeniums have toxic sap which was used to create poisoned arrow headsref).
If you’re a conifer fan there are fewer options, since angiosperms (flowering plants) have dominated in the tropics since the Cretaceous period. One exception is the Podocarpaceae family which thrives in nutrient-poor soils in these environments, including bogs.ref Tropical species are found in the Podocarpus genus including Podocarpus macrophyllus or Buddhist Pine. They are also found in the Dacrycarpus genus for example Dacrycarpus dacridioides is apparently a ‘popular bonsai subject’ according to this website, although it’s not mentioned anywhere else I can find online. Other tropical gymnosperms include some members of Araucariaceae such as Agathis or Kauri trees, Araucaria cunninghamii the Hoop Pine and the sub-tropical Araucaria heterophylla Norfolk Island Pine. Two pine species can also be found in the tropics, Pinus merkusii from South-East Asia and Pinus hondurensis from Mexico.
The main constraint you will have in adding tropical or sub-tropical trees to your bonsai collection is probably going to be accessing plants or seeds, so let your imagination fly and see what works for you (I admit to being side-tracked while writing this post by this UK Adenium seller and now await my first packet of seeds!)
Sometimes in a bonsai context it’s said that specific branches are connected to specific roots – often in discussions about pruning and carving. For example it may be suggested that pruning a specific branch will kill an associated root, or vice versa.
As I’ve learned over the last 6 months researching this site, when it comes to trees – ‘it depends’.
The effect of pruning branches or roots on the rest of the tree comes down to its ‘plumbing’ – that is, the way in which the xylem (water) sap and the phloem (sugar) sap flow around the tree. That plumbing is laid down as new shoots and other organs develop – each new organ has a connection to a vascular bundle with xylem and phloem ‘pipes’. These pipes (in reality different types of cells which connect to each other), then connect to the vascular system in a branch, then in the trunk, then to the roots.
Trees can have what is called ‘sectorial’ growth in one or both of these systems. Phloem appears to be more sectorial than xylem – there is less of it, it only runs around the outside of the trunk in a thin layer and it has fewer connections between cells than xylem. Since roots are dependent on phloem from leaves, this would suggest that roots might be more likely to die from a branch being cut than the other way around.
Xylem is a different system, and the way the xylem cells of a particular species are structured determines how sectorial that species is – trees with more connections between their xylem cells are less so (because water has more routes it can travel to reach an organ).ref
If you’ve read the post on xylem, you’ll know that all gymnosperms/conifers (and some angiosperms) accumulate water-conducting xylem rings over time and have many layers conducting at once. This type of wood is called diffuse porous. Some angiosperms have a different strategy – they regrow their conducting xylem every year and only use that one outer layer for water transport. These trees are called ring porous.
It may then be obvious to you that ring porous species are more sectored than diffuse porous species. This was confirmed in one study using dye injections into xylem vessels – diffuse porous Acer saccharum, Betula papyrifera, and Liriodendron tulipifera had dye show up in more leaves than ring porous species Castanea dentata, Fraxinus americana, and Quercus rubra.ref This is presumably because in diffuse porous trees there are more water conducting cells and more options for water to travel – it is less likely to get cut off.
Trees which have more isolated root – leaf paths include Quercus, Fraxinus, Prunus, Ulmus, Cotoneaster, Crataegus, Sorbus, Populus, Salix, some Acer and Olive.ref1ref2 If you prune their roots, there is a higher risk of removing a xylem sap flow path to certain leaves and vice versa. Interestingly, if you look at anecdotal reports of ‘summer branch drop’ where trees drop their branches for no obvious reason, the species most susceptible to it appear to be these trees – Quercus, Fraxinus, Populus, Salix and Ulmus are all known for this phenomenon.ref This implies that a sector has died – perhaps due to embolism (air gaps in the xylem cells) – and the branch has dropped off as a result. The same could happen to your bonsai trees of this species, either by root pruning or by underwatering. Fellow bonsai enthusiasts have reported this in Salix (willow) and Morus (mulberry – also a ring porous species). The upside of this behaviour is the survival of the tree – since the death of one part of it doesn’t cause the death of the rest of it.
Trees which are more integrated include all gymnosperms/conifers and these have more uniform water distribution.ref Therefore they should be less susceptible to losing sectors due to root pruning or uneven watering. But once you’ve reached the point where they aren’t getting enough water overall (due to overly aggressive root pruning) or energy overall (due to overly aggressive leaf pruning), the tree is more likely to die since it is less able to keep one part alive separately to the others.
Note that trees may also drop branches for ‘economic’ reasons, when they don’t get enough return on investment from that branch, but that’s a post for another day.
Most bonsai enthusiasts will have come across the term ‘live veins’ in the context of bonsai. Live veins are areas of living bark surrounded by deadwood. They are often seen on juniper bonsai, where a section of bark twists around the tree in a dramatic contrast to the white deadwood (Sierra juniper are particularly amazing). But how does this actually work and is it a ‘vein’?
The bark layer on a tree contains the phloem, which is responsible for transporting photosynthates (sugars) and other molecules around the tree – it sits just at the base of the bark next to the sapwood. As new plant organs develop, a connected line of phloem cells is created so that sugars can be transported from these organs (if they are leaves) or to them (if they are sugar consuming organs like roots).
Phloem cells in the leaves connect to phloem cells in the branch, then to phloem cells in the trunk. They are long tubular cells with the main connection point for sap flow at the end of the tube, and minor connections in the sides. Below is an vertical image of sieve tubes of Cercidiphyllum japonicum (Katsura tree) – you can see the sieve tubes in blue, and their connections at a diagonal in dark blue. The brown cells are companion cells which help the sieve tubes to function. In this example there are some connections between the sides of tubes, but most of the connections are end to end.
Phloem & sugars preferentially flow along this natural end-to-end route. One research study looked at what happens when you block a phloem path by girdling. It was observed that initially sugar flow to roots from that branch stopped, then resumed partially by finding another route (probably laterally through the sides of the phloem cells), then the tree grew new phloem and resumed sap flow.ref
So what does this mean for bonsai? Basically – live veins (or more accurately, ‘live strips’) of bark can supply sugars to roots as long as they have phloem connections to sugar producers (leaves) and to sugar consumers (roots). What is really important is that we work with the orientation of the phloem cells when creating deadwood. Cutting across the grain of the phloem would sever the sap connections and be a form of ringbarking. Instead leaving a strip which goes along the grain of the phloem will provide a leaf to root connection. The phloem tubes will always be aligned lengthwise along a branch or on the trunk – ie. heading down to the roots. The variation you might see is that some phloem & bark spirals around the trunk and some goes straight down. This should be obvious from the bark pattern.
It’s also important to ensure there is enough foliage at the top of the live vein to meet the needs of the tree (or scope to grow more foliage). It’s useful to know that sugar from a leaf is prioritised for use local to that leaf. Leaves provide sugars for the developing shoot apex nearest to them, and flowers or fruit on the same branch; so from an energy perspective, as soon as it can be, a branch is self-sustaining. Lower leaves on a branch typically are the ones exporting sugars to the roots.ref This might be useful when thinking about deadwood creation and possible options for live veins.
If you are aggressive with your live vein creation, and remove a lot of bark, it’s likely that some roots will die. One way to minimise this is to maintain a reasonable bark/phloem coverage around the base of the tree, and to start the deadwood further up.
One final word – there isn’t really any such thing as ‘finding’ a live vein. All phloem/bark is live until you create deadwood above or on it. It’s more about creating the deadwood and leaving the live vein (or ‘live strip’) behind.
At this time of year (December) I like to grow from seed, just to give me something green to watch.
Ginkgos & Dawn Redwood growing on a desk in my study (Dec 2022)
To many people the very idea of growing an actual tree from seed to the point that it can be used for bonsai seems completely ridiculous. If, like me, you’ve come to the hobby as a ‘mature adult’ (ahem), you might be doing calculations to work out if anything you grow from seed now will ever make it to a decent size in your lifetime.
But you don’t actually need that long to grow some species to a size that’s suitable for bonsai, and if you are a fan of mame (very small bonsai) it’s eminently achievable. You will need at least 5 years up your sleeve realistically but it’s a nice project to have on the boil while you’re working away at your desk dreaming of retirement. If you choose a so-called pioneer species (trees which establish first in clearings), it will grow very quickly. Such species in Europe include birch, alder, willow, poplar and rowanref, in the Pacific north-west these would include Douglas fir, western hemlock, western redcedar and Pacific silver firref. From personal experience, larch grow extremely quickly as well, particularly if they’re in the ground. More info on thickening the trunk of a tree as quickly as possible is in Thickening the Trunk.
This cedar was started from seed about 4 years ago – it’s about 15cm high now. It’s not going to win any bonsai competitions but I like having it in my garden and the nebari is gnarly. Cedar have quite dense short-needled foliage so they don’t need too much work to look decent.
Another way to use seed-grown trees is in small bonsai forests – when they are seedlings they can be positioned very close together in groups without disturbing their roots. When they grow their roots intertwine and their trunks become close which makes for a nice aesthetic. Japanese maple and Ginkgo are good for this.
The advantage of growing from seed is obviously that you can influence the shape of the tree from the very beginning – and in the case of coniferous species which don’t readily backbud (particularly Pinus), this helps get the branch structure right and allows the introduction of trunk movement. The key is not putting the tree in a bonsai pot until its trunk is at the desired thickness as this will basically stop the tree growing. Grow it on in a normal pot or in the ground until it gets the trunk size you want, shape the trunk as you go, then proceed as per the instructions for shaping bonsai.
A key point to understand when growing from seed is that seeds come from sexual reproduction between male and female gametes (in plants defining this is more complicated than in animals due to the two generations involved in reproduction but that’s a subject for another post). Sexual reproduction mixes up the genes of both parents in the offspring (actually it mixes up the genes of all four grandparentsref), which means that you cannot grow named varieties from seed – as named varieties always have specific genetics.
You probably don’t want to grow from seed if your goal is to have flowering or fruiting trees, or trees with cones. Flowers only develop when a plant reaches sexual maturity which can take decades in some species. This means seed growing realistically is best for foliage trees. Clonal propagation is better for flowering trees (cuttings or micropropagation) since it uses the parent plant (including its age) as the starting point.
The other thing I have noticed is that seed packets and advice online appears to vary widely and can’t all be correct. There are many factors which have evolved to help seeds be more successful in germinating and it’s beyond the scope of this post to cover them all. But whenever trying to germinate a seed, always take a look at Google Scholar to see if any research has been done on the seed in question. One also needs to consider where the tree lives – for example Swamp or Bald Cypress lives in warm, humid areas of the Americas where the temperature doesn’t go below 5 degrees C – why on earth would they need cold stratification for several months? And in fact – they don’t.ref
Many seeds exhibit dormancy once they have dried out, this is a state of suspended animation which “improves survival by optimizing the distribution of germination over time”.ref The plant growth regulator abscisic acid (“ABA”) is involved in initiating and maintaining this state – according to one study, it prevents seeds from absorbing water which is a key mechanism required to break dormancy.ref The seed coat is known to be a site of ABA synthesisref and the removal of it can enable germination to proceed, in the presence of water (I recently did this with some wisteria seeds and they immediately germinated).
Some hard-coated seeds use their coat as the barrier to water entry, and can be helped along by nicking the coat with a knife, sanding down the end of it or otherwise allowing water to enter (known as scarification). For really hard seeds, acid is recommended! In this study seeds soaked in concentrated sulphuric acid for 3h showed the highest germination (but 4h was too long).ref On the other hand, gibberellic acid is also known to induce germination. It is used in in-vitro micropropagation to break seed dormancy so a seed can be used as an explant (the material from which to propagate new plants) (Johnson, 2020). Gibberellic acid can be purchased from suppliers to the hydroponics/plant micropropagation trade. Some seeds have embryonic dormancy (ie. they are prevented from germinating due to substances occurring within the embryo or seed itself)ref – this is harder to get around, without destroying the seed.
It’s also useful to find out if your particular seed requires light – in one study all 8 taxa included had significantly improved germination when exposed to white light, and in some red light was sufficient.ref
The next section is not based on science at all, but reflects my personal experience of growing trees from seed:
Ginkgo – a very easy tree to grow! The hardest part is finding a female tree – they are unpopular because of the stink that the ginkgo fruit creates as it rots. The fruit ripens all summer and then falls from the tree apparently ready to germinate – although the later they fall, the better they will germinateref. I put them in the fridge for a couple of months and then plant them in some potting compost, this results in quite a high germination rate. I’ve had similar success just putting them into a pot over the winter and they eventually emerged in the late spring.
Cedar (cedrus) – also very easy once you’ve managed to get plump seeds from a cone. Find a semi-falling-apart cone and extract the seeds – you’ll see which ones look healthy and which don’t. Put them in the fridge in a bag with a bit of moist paper towel, wait a month or so and they will start germinating. Then you can pot them into little pots to grow on. My interest in cedar came from seeing this amazing video by George Omi, wiring a blue atlas cedar as he was taught by his father in the 1950s: https://georgeomi.wordpress.com/2016/10/26/my-bonsai-video/
Oak – oak trees are the easiest plant to grow, they have all the food they need in their acorn, they tolerate heat, drought, frost and being treated badly and they backbud fantastically when cut back. I make oak groups planted on mounds & rocks which are really easy and look nice even after just a couple of years. To germinate – take your acorn, stick it in a pot over winter, keep giving it some water occasionally and hey presto, an oak tree will grow.
Horse Chestnut – the same as oak. Stick in a pot, wait. A tree will emerge. A bit more tricky to make good bonsai but I have a couple of nice mame.
Crab Apple– a common choice for bonsai but note that seeds will reflect genetic variation and will probably be different from the tree that the seed came from. Extract the seeds, pop them in the fridge and plant out a month or so later.
Japanese Maple– also easy to grow. Gather fresh seeds from the tree in autumn, put them in the fridge in a bag with some moisture for 3-4 months. They should start to germinate in the fridge and at this point bring them out, plant them up and let them grow. Note as per above you cannot be guaranteed of similar characteristics as the parent tree due to the seed being a mix of it’s grandparents’ genes.
Dawn Redwood – I’ve grown dawn redwood straight from a fresh seed pod I collected in Richmond Park in London – no refrigeration was required (although the germination rate was quite low).
Hinoki Cypress – if you’re lucky enough to find one with a seed cone, give it a shot, I found they germinated surprisingly easily.
Anyway, if you have the winter blues, get some green in your life and grow some trees from seed!
Francis Hallé is a venerable French plant biologist who is famous for his work defining tree architectures along with his colleagues Oldeman and Tomlinson.
Based on his book In Praise of Plants, he is also a philosopher, artist and poet of the like you don’t see often when reading science books intended for a general audience. I confess I love his quirky book. It has beautiful illustrations, quotes poetry and takes some whacky diversions, but most of all it has some of the deepest insights you will find about the nature of plants and their lives.
Because aside from the philosophy there are also amazing scientific facts which change the way you understand plants. Aiming to avoid the mistakes of all those before him in using animal models to explain plants, Hallé lists the crucial differences.
They get energy from the sun. They have plastic development due to persistent meristems at shoot, root & stem. They have no ‘vital organs’ simply growing another if one is lost. They don’t excrete but contain toxins inside the vacuole. They don’t have an immune system. They have around 30 types of cell, many of which can regenerate the entire plant, versus the specialised 200+ cell types in animals. They are fixed but not immobile, growing to meet their needs. Their cells are all connected sharing the same cytoplasm. Parts of plants can die without killing the whole. They produce their own pharmaceuticals. They change the chemistry of their environment by exuding chemicals from roots & leaves. They reproduce in two genetically distinct stages – a haploid generation followed by a diploid generation. They have flexible genomes which can change the number of chromosomes even during the lifetime of the plant. They can tolerate cell mutations and have genetic diversity within one plant such as different branches in a tree’s canopy with different genomes!
This book has really opened my eyes to the massive differences between animals and plants, and shown me how easy it is to fall into a zoocentric frame of mind. Already a fan of trees, I’m now even more in awe of the plant world.
Although a little expensive (£20 on Amazon) it’s definitely worth a read if you have a curious mind and a philosophical soul. If you speak French you can download the original version for Kindle for £8.99 (“Eloge de la plante. Pour une nouvelle biologie (French Edition)”).
Weirdly the definition of bark seems to be variable depending on what book or article you read. As my main reference for this post I’m using Romero’s “Bark: Structure and Functional Ecology” accessible via a free account on JSTOR here.
According to Romero, bark is all the tissues on a tree outside the vascular cambium – that is everything from (and including) the phloem outwards. The inner bark is simply the phloem (both the conducting layer and the non-conducting layer). The outer bark collectively is known as the ‘ritidome’. This is where a diagram is needed! This is the best one I could find (from the University of Vigo website).
The ritidome contains another meristem within the tree – the cork cambium. The cork cambium (called the phellogen) works similarly to the vascular cambium – it has a layer of stem cells which create layers of differentiated cells. In the top diagram there is one phellogen, a pale beige line. On its left is the phelloderm – this layer is not always created depending on the species but if present it contains living cells. On its right is the phellem, or cork, this is the thickest layer and these cells become suberised/lignified (impregnated with suberin or lignin) so they become the corky bark texture we are familiar with. All of these ‘phello’ layers together are known as the periderm. A microscope image of these layers in an old Pinus sylvestris are shown in the image below:
What’s interesting is that multiple periderms can develop over the life of the tree. A new periderm will develop on the inside of the old one, pushing that periderm layer to the outside. These aren’t always continuous either, and are affected by the structure of rays and growth rings within the phloem, which is why old bark has more character. Periderms can be shed, or retained, depending on the species. The pattern of a tree’s bark is genetically determined by the structure of the phellem cells which are produced, and by the location of successive periderms. Smooth bark can come from a single periderm and continuous shedding, while rough bark is created when the periderm has structural fractures or constraints – for example due to the development of rays (radial lines of cells in the phloem).
See this image of old bark from ‘The Plant Stem – A Microscopic Aspect’ by Schweingruber & Börner. It shows how the bark splits apart as the xylem and sapwood layers expands from the inside of the tree.
Bark is made up of quite different materials from the wood or foliage of the tree, with considerably more mineral compounds (such as ash). Both the inner and outer bark contain so-called ‘extractives’ (organic substances which can be dissolved in solvents, such as polyphenols, alcohols, resin acids, vitamins, alkaloids, pigments including flavonoids, terpenes, steroids and essential oils) as well as suberin, lignin and cellulose. Bark chemistry in general is poisonous and indigestible, representing a good barrier to herbivores or insects. As the inner bark is living tissue, it can produce its own metabolites as a defence mechanism, whereas outer bark is dead tissue and relies on its physical structure and the substances impregnated into its cells to repel invaders.
Bark helps trees reduce water loss, prevents pathogens entering and provides a protective layer to protect the living tissue underneath from mechanical and heat/cold damage. It provides a flexible covering for the tree which can absorb the stresses of bending and twisting, and prevent cracking of the trunk.
As bonsai enthusiasts, bark is a key part of the look of our trees and we want to encourage interesting bark with good character. Since the cells of bark are renewing from the inside, the only way to modify the appearance and texture of bark in a natural way is to manipulate the periderm – as mentioned above, this causes fractures and divergence of the growth habit of the phellem. Harry Harrington has a video showing exactly this on a young black pine – he wires the tree so that the wire interrupts the shape of the periderm and forces the phellem to grow in a twisted habit.
https://www.youtube.com/watch?v=f0fe6v7X0MQ
I’m a bit nervous by the suggestion to leave the wire in as this seems like it would then cut through the phloem and ultimately the xylem. Whilst the twisted shape should leave continuous conducting cells for both, I’d be concerned at how much water and photosynthate conducting would be reduced. If possible to remove the wire I think I would.
