Author Archives: Bonsai Nerd

Carving Trunks and Branches

Most bonsai enthusiasts I know love a bit of Dremel action – a great way to add interest to a tree and to make it look like an old tree is to create deadwood like jin, shari & uro, or more informal natural-looking deadwood forms. For the art and craft of this you should look elsewhere – Will Baddeley at Wildwood Bonsai runs workshops on carving (and has a good example on a Prunus mume on his website). Let’s be clear that any carving you do to a tree is creating or shaping deadwood. You can’t carve a live branch and have it stay alive, at least not the part you carved.

But what does this physically do to a tree? That very much depends on where you are creating deadwood. If the part of tree you are carving is already dead, then carving it will not affect the tree (although, you then have deadwood to manage, see the end of this post). If you are carving live wood, there are some considerations.

Firstly, go back and remind yourself how xylem and phloem work. They transport water, nutrients and dissolved substances like plant growth regulators around the tree. Leaves ‘load’ photosynthates (sugars) into the phloem and roots load water into the xylem. The point here is that movement through these vascular tissues is required in order for water, photosynthates and nutrients to travel around the tree. If you remove these tissues by carving, it will affect at least some parts of the tree.

Xylem and phloem vessels are not usually just one layer wide and they don’t flow end to end like a pipe – there is movement between adjacent vessels and different ways for xylem and phloem sap to flow if areas are damaged. But if you carve away the entire phloem layer – which most likely you will when creating deadwood – that path for phloem sap is closed. Similarly for xylem – if you carve away active xylem vessels then water can’t flow that way any more. You need to understand what the effect of this will be for branches and foliage which you want to keep. If an area of the tree has its water source shut off – it will die. If an area of root has its photosynthate source cut off the same will happen.

Whilst its not true that trees have ‘veins’ exactly since they have multiple connected cells (more like a bundle) and not just one vessel like a vein, the bonsai parlance which refers to ‘live veins’ is approximately correct. If you can imagine a vascular bundle passing between leaves, trunk and roots, you’ll be able to work out what consequences any carving will have.

Apart from anything else, carving live wood will result in a wound, you can read about how trees deal with these in Repairing (?) damage.

Once you have deadwood, what does it mean for your tree? Well, dead wood which is exposed to the environment decomposes over time, through the action of so-called sapotrophic organisms (those that feed off dead organic matter). The decomposition of deadwood worldwide is a critical component of the global ecosystem, releasing nutrients and carbon into the soil and atmosphere.^ref In the forest, fungi, bacteria, invertebrates (like beetles) and nemotodes are the organisms which decompose dead wood. Basidiomycota is the only type of fungi which is know to degrade lignin, a major component of wood^ref (the dreaded Honey Fungus or Armillaria is a member of the Basidiomycota family). Below is the mix of fungi and bacteria involved in decomposition of a European beech (Fagus sylvatica)-dominated temperate forest.^ref

The rate of decay of deadwood in the forest is determined by environmental and genetic factors. Gymnosperms (conifers) resist decomposition due to the volatile compounds in their wood.^ref Angiosperms which have distinct heartwood, including oak, take longer to decay for a similar reason – heartwood often contains substances which deter fungi and bacteria.^ref A fun fact is that plants don’t excrete like animals do. Instead they store away toxic compounds in their vacuoles (fluid-filled spaces within plant cells which occupy up to 90% of the cell volume) (Hallé). Just some of the compounds stored in vacuoles include pigments in flower petals, latex, digitalis in foxglove, resins, alkaloids such as opium and the chemicals in garlic.^ref So these compounds can have the effect of slowing down decomposition, by being extremely unpalatable to microbes.

From a bonsai point of view, you want to avoid your deadwood being colonised by sapotrophic organisms – or at least you want to slow this process down as much as possible.

One approach for keeping your deadwood fungi/bacteria load down is to apply some ‘extractives’ to the wood – extractives are volatile compounds found in heartwood and bark, which have anti-bacterial/fungal properties. There’s quite a good thesis online which identifies many extractives from a range of different trees – you could try turpentine for example, which is extracted from pine tree resin. I’d avoid putting this into the soil though, for fear of harming beneficial microbes in the rhizosphere.

One of the main accelerators of decay in young stumps is moisture content.^ref This is a another key control you have to minimise decay in deadwood on your bonsai – keep it dry – or in scientific terms reduce its ‘wettability’. This can be achieved by applying something like linseed oil. Other substances I have heard applied to deadwood are superglue (it reduces wettability by creating an impermeable layer on the wood), and wood preservatives but most of these have chemicals I wouldn’t want washing into my bonsai soil.

So in summary – before you carve, work out what you’ll be doing to the phloem and xylem flows to avoid damaging areas of the tree which you want to keep alive, and once you have deadwood, keep it dry and repellent to microorganisms. And in order to help with wound healing, carving in warmer weather when the tree is in active growth gives it the best chance of defending against pathogens which try to enter via the wound.

Adding New Branches or Roots by Grafting

Grafting is the practice of splicing one plant onto another, so that they fuse together to become one plant – the new plant is known as a ‘chimera’^ref. Most of the grafting you’ll see out in the horticultural industry is putting a different root stock together with a named variety above ground, as a form of clonal propagation of the above ground plant.

But the same principle is used in bonsai to add new branches or roots to an existing tree. According to Garner (see references) two main forms of grafting exist – approach grafting and scion grafting. Approach grafting is when two plants (or two parts from the same plant) are held together for long enough that they fuse – but neither are detached from their parent until the union is made. Scion grafting is when the stem to be added is removed from its donor plant before the union takes. Bottle grafting appears to be halfway between the two, where the scion (the stem being added) is sustained by standing it in a bottle of water until the union is made.

I am certainly not the person to be informing you about the techniques for good grafting (check out The Grafter’s Handbook by Garner for that), but I do want to look into the science behind grafting, how and why it works, and what you can do to make it more successful. First some terminology – the plant which is being grafted onto is the ‘stock’, and the piece of plant which is being grafted onto the stock is the ‘scion’.^ref

The basic idea behind grafting is that the vascular systems (the xylem & phloem) of both plants become connected – this is needed so the scion can obtain the water and nutrients it needs to survive, having been separated from its parent plant.

The first requirement for this is to have genetically compatible plants. If you are using the one plant to graft to itself, obviously this will be compatible. If you are using two plants of the same species, known as ‘homografts’, they will be compatible also. Otherwise rootstock and scion belonging to the same botanical species are nearly always compatible, rootstock and scion belonging to different species of the same genus are usually compatible, intrafamilial (within the same family) grafts are rarely compatible, and interfamilial (between different families) grafts are essentially always incompatible.^ref

To find out the genus of a particular plant, you can search on http://www.theplantlist.org/ – for example Pinus is a genus and Pinus sylvestris is a species. So in theory you should be able to graft any pinus onto another one. The Pinus genus is a member of the Pinaceae family and there are examples of intrafamilial grafting working in this family – for example grafting Cedrus atlantica scions onto Pinus strobus stock.^ref

In order to create connections between the two vascular systems, each stem needs to be cut to expose the vascular tissue, then the vascular cambiums of both plants are aligned as closely as possible and held tightly together with tape or a rubber band (or similar). Since the vascular cambium can be extremely narrow (depending on the species, but 3-10 cells if you look at the images in The Plant Stem by Schweingruber & Börner – see References for details) – it can be extremely challenging to get the positioning correct. After this the graft is ‘sealed’ to the extent possible – beeswax has traditionally been used.

Below is a diagram showing the sequence of events in a successful graft. It’s not the case that the vascular systems just line up and start working, like you’ve connected pipes together. When plant cells are wounded they die (see repairing damage), so these can’t just reconnect to another plant. Initially there is “a necrotic layer of one or two damaged cells” between the wound sites. When the two wound sites are placed together, the plant activates a process known as autophagy^ref – incidentally this is a similar process which is invoked when humans fast – it clears away and recycles dead or damaged cell material. Although their vascular systems are not connected, there is some communication between cells at the graft boundary, otherwise they would not detect each other and activate autophagy (which isn’t activated if another plant is not present at the wound site).

At the same time auxins and sugars start to accumulate at the wound site (since there is nowhere for them to go) and callus tissue starts to form – this is what ultimately joins the two plants together. Callus tissue is a mass of unorganized cells that forms in response to wounding – this can then regenerate the entire plant based on the plant growth regulators which are present.^ref The callus tissue differentiates into vascular tissues which act as a bridge between the two plants.

https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.14383

A key point with grafts is that even after the graft is completed, you will always have two genetically distinct individuals with a joining layer between them (which apparently includes transfer of DNA between the individuals, but only for a short distance)^ref. The upward supply of water and mineral nutrients as well as the downward flow of photosynthates are modified and so is the root–top interchange of hormonal signals.^ref This can result in graft failure many years afterwards, due to more long-term genetic incompatibilities. The best way to reduce this risk is use the closest genetic match as possible – the same plant (best), variety (good), species (good but not if very different varieties) or genus (OK but risky).

To optimise the chance of success of a graft there are a few factors which contribute to better outcomes (aside from compatibility), according to studies^ref:

Grafting technique – some types of graft work best with specific plants – for example in conifers terminal fissuring and lateral plating are used.
Use vigorous stock and scions
Use younger stock and scions, unless you are bud grafting, which seems to be also successful from older plants
In some species winter grafting is more successful
Temperature can affect success – depending on the species – you don’t want it too hot or cold
The graft union needs to be held together, and protected as best as possible from drying out or from pathogens. One study found that paraffin wax was effective.^ref This might be one situation when the slightly dodgy tree ‘wound sealants’ would actually be useful.

So bonsai nerds, what does it all mean? If you are considering using grafting techniques, my first piece of advice is to find someone or a book which has proper detail in it about the process. As noted I have the Grafter’s Handbook by Garner. Brent Walston also has instructions for grafting pines on his website.

Do some research about the species you are looking to graft, to find out the most successful techniques for that species.

Consider using bud or chip grafting – it’s supposed to be one of the easier forms, doesn’t disfigure your tree and allows you to use the same plant as the donor, reducing incompatibility issues.

Repositioning Branches

A major part of bonsai practice involves moving branches into more desirable positions to meet a particular vision for the look of the tree. This is done using a wide range of different tools and techniques, which are not really the focus for this website. What I want to look at it is how some of these techniques affect the tree from a physical and biological perspective.

The first and most commonly used technique is to use flexible wire which is wound around a branch and then bent into position. To learn how to do this you can look on just about every bonsai website out there. But how does this work, how far can you go, how long does it take to work, and what do you have to watch out for?

Basically the way this works is that it forces a branch into a different position, it’s as simple as that. As long as the branch doesn’t break, you leave it there until the branch ‘hardens’ in the new position, and then you take the wire off. So two questions arise – how does it harden into this new position, and how long does it take before the wire starts to cut into the bark?

The first question is interesting. Basically what you are doing by moving a branch out of position is that you are creating a new shape with the branch, and when the new layer of xylem, phloem and bark grow and the xylem lignifies, the branch will be set in that new shape.

The majority of the structural strength of a branch or trunk comes from the xylem^ref (sapwood & heartwood). Xylem cells are dead cells impregnated with lignin, a polymer produced by plants which strengthens these dead cells. Lignification happens after cell death is achieved – one study found that clearing out a new xylem cell took 96 hours^ref and that lignification continued for several hours after that.^ref

In order to set a branch in a new position the xylem needs to grow enough new layers of cells to hold the branch. The amount of new xylem needed to achieve this probably depends on the bending force exerted within the branch – the higher this is, the more xylem will be needed to hold it. So repositioning a branch which is easy to move might need less than one season of xylem growth, a branch requiring more force might need more than one.

The force that can be applied using wire wound around a branch is limited by the structure and strength of the wire and this is dissipated by the winding which distributes the force along the wire. Hence a heavier branch or a harder bend will require heavier wire or might not be possible at all using this method. An alternative is to attach a branch to something else (like the pot, or a piece of deadwood on the same tree). Wire, cable or any strong material can be used. In my experience, you can achieve a lot more force with less wire in this way, because you are using the resistance of the tree itself as a counterweight. Branch bending tools available for bonsai operate on a similar principle.

Trees create something called reaction wood to counteract strong forces – for example a branch which grows out horizontally from a trunk exerts a lot of force on the trunk.^ref To handle this force a gymnosperm creates compression wood – specially structured xylem cells at the bottom of the branch’s join to the tree. This has the effect of providing extra support to hold the branch up. Angiosperms create tension wood, which is above the join. See below for examples from the Bushman’s Friend blog. Once you remove the wire or whatever is holding your branch in place, depending on its position and weight you might expect reaction wood to form.

**Gymnosperm compression wood under the branch**
http://bushmansfriend.blogspot.com/2010/12/reaction-wood-compression-wood-and.html

**Angiosperm tension wood above the branch**
http://bushmansfriend.blogspot.com/2010/12/reaction-wood-compression-wood-and.html

Obviously when bending into a new shape, you don’t want to break the branch. In general, younger branches are easier to bend than older ones because they have less lignified xylem holding them in place. The ability for wood to bend is measured by the ‘modulus of elasticity’ – a low MOE means it doesn’t resist bending. MOE is correlated to density – the higher the density the higher the MOE^ref, so in general denser woods will be harder to bend – usually it is angiosperms which have denser wood than gymnosperms. Conifers are easier to bend on average although there are some exceptions. You can see some species below.

Now we come to the question of how to avoid wire (if used) cutting into a branch it is repositioning. There are two meristems producing secondary growth on branches – the vascular cambium which adds biomass to xylem and phloem, and the cork cambium which adds biomass to the outer bark. Note however that they are both doing this in layers underneath the outside bark layer, so the bark itself isn’t growing up over your wire, it is being pushed from underneath by new cells. The problem with this from a bonsai perspective is that any damage to the outer bark layer may be permanent (unless you have a species which sheds its outer bark).

The rate at which secondary growth happens depends on the species of tree (it’s genetic) but usually there is a growing season (based on the tree’s phenology). During this season is when the branch and bark will expand and this is when you risk getting wire marks. Two ways to avoid marks on the bark due to wire are (1) leave some slack in it so the branch can expand underneath it and (2) keep an eye on it and remove or adjust the wire when you see it getting tight. Obviously if you have a species which puts on a lot of xylem & bark every growing season it is harder to keep wire on for longer periods – this is one of the reasons why I avoid bonsai wire wherever possible. Using the attachment method described above allows you to keep a branch held in position without wire marks becoming an issue, while wire can be used for smaller branches which set quickly and can have the wire removed within one growing season.

The Endosphere

Although it might sound like we’re veering into science fiction territory, the endosphere is actually part of a plant’s microbiome, like the rhizosphere and the phyllosphere. It is the community of microbes which live inside the plant itself – that is, between and in its cells. It’s only in the last few decades that research on the endosphere has accelerated – this has found that in fact a wide variety of microbes including bacteria and fungi live inside plants for at least a part of their lifecycle.^ref They are known as endophytes – and some of these are symbiotic whilst others can be pathogens.

Endophytes are found throughout the plant, in leaves, roots and stems, in spaces between cells as well as within cells themselves; the greatest number are found in roots, then leaves, then fruit/flowers. The types of microbes in residence depends on the microenvironment in each part of the plant, the specific physical and chemical characteristics in each environment attract different microbes.^ref

To enter the plant in the first place, microbes come from outside, through the root tips and hairs, through stomata and trichome pores in leaves, fruit & flowers, through holes in the stem made by insects, or by producing enzymes which break down plant cell walls to create an opening. Often these microbes are present in the rhizosphere or phyllosphere, and they migrate into the plant for all or part of their lifecycle.^ref Usually they live between cells, but some examples of bacteria and fungi entering plants cells have also been found. Endophytes can be transmitted vertically (from mother plant to seed), and horizontally (from the outside environment).^ref

Of all the spheres, the endosphere is the hardest to study, so there isn’t a huge amount of research which demonstrates what endophytes actually do when they are inside plants and how the host plant might benefit. Some findings are that endophytes are able to detect Reactive Oxygen Species (“ROS”) and may be able to help plants fight high ROS levels (eg. acting as an anti-oxidant).^ref Others have found endophytic fungi which produce the plant growth regulators gibberellic acid and indole acetic acid (auxin), and that this contributes to greater root & shoot mass.^{ref1 ref2} One study found an endophyte which conferred resistance to Dutch Elm Disease in vitro^ref. Finally a large number of endophytes associated with trees have been found to produce Taxol^ref, the best-selling cancer drug ever manufactured^ref and this promises to be a way for greater volumes of the drug to be created.^ref So like bacteria & fungi across the microbiome, these microbes appear to be pop-up pharmacies within the tree.

The endosphere probably doesn’t need to be your prime concern from a bonsai perspective. Like the other components of the tree’s microbiome, you want to foster a healthy one, which benefits the tree, and not an unhealthy one. Doing this mainly involves not killing them off!

The Phyllosphere

The phyllosphere is the community of microbes which live in and on a plant’s leaves. I had no idea that this even existed before writing the section for this website about the microbiome. Of course, if you think about it for a microsecond, it must! Our world has more microbes than anything else by several orders of magnitude, so, there must be microbes in a tree’s leaves. But the phyllosphere has been less publicised due to the intense interest in the rhizosphere (root microbiome) and in its beneficial microbes which can help plants grow by manipulating the soil and root environment.

The phyllosphere is different to the rhizosphere in that its main microbial members are bacteria and not fungi, although fungi are present, along with some archaea. It has been estimated that there are 1 million -10million bacterial cells per cm² of leaf surface.^ref And worldwide, the phyllosphere is an important microbiome, with a possible 10²⁶ cells! But it’s a relatively hostile environment, with fluctuating temperature & humidity and limited nutrients on the leaf surface. The shape and structure of the leaf at a microscopic level provides a range of microhabitats for bacteria, including the bases of trichomes, stomata, hydathodes (leaf pores), grooves along the veins, epidermal cell junctions, and cuticle depressions.^ref A study into tree phyllospheres found 129 bacterial species were significantly associated with the gymnosperms including Armatimonadetes, Actinobacteria, Bacteroidetes, Acidobacteria, TM7, TM6, Deltaproteobacteria, OD1, Fusobacteria, and FBP and 79 with the angiosperms including Chlamydiae, Proteobacteria, Gammaproteobacteria, Alphaproteobacteria, and Firmicutes.^ref

Bacteria on a leaf surface, from: https://www.ethlife.ethz.ch/archive_articles/090915_blattleben_kw/index_EN.html

What determines the microbial mass and mix on leaves is a combination of different factors, including the nitrogen content of leaves, the specific leaf area (related to carbon availability), wood density and seed mass^ref and the largest part of the variation seen between phyllospheres comes down to the host species. Conifers have a different phyllobiome than other species, for example they have less ice nuclei active bacteria (bacteria which can cause ice crystals to form) and they have Frankiaceae which is involved in nitrogen fixing in the soil.^ref Location also plays a role, with urban trees displaying a different phyllosphere makeup – correlated to ultrafine particulate matter and black carbon on the leaves.^ref

Bacteria usually require an available carbon source. You might be surprised to know that similar to roots, leaves also produce exudates (substances they exude into the environment). These include a wide range of carbon compounds, such as carbohydrates, amino acids, organic acids, and sugar alcohols, primarily products of photosynthesis, as well as proteins, oils, secondary metabolites and mucilage.^ref These carbon sources are not the only ones – the Methylobacterium species can use methanol exuded from the leaf from the breakdown of pectin as its only carbon source.^ref One of the bacterial families found on birch – Rhodospirillaceae – is able to photosynthesise, removing the dependence on leaf carbon sources. Another study discovered that certain phyllosphere bacteria can use diesel for their carbon source!^ref

Similarly, bacteria in the rhizosphere produce a range of substances just like they do in the rhizosphere – biosurfactants which reduce surface tension, degrade hydrocarbons and improve moisture levels and dissolved nutrients on the leaf surface, plant growth regulators which open up the leaf cells and cause them to leak nutrients, enzymes which help break down nutrients and protect the bacteria from solar radiation, and phytotoxins (if the bacteria is a pathogen).^ref

The benefits of phyllosphere microbes to their host are similar to those in the rhizosphere – for example Acetic Acid Bacteria have been found to perform nitrogen fixation within the needles of Pinus flexilis^ref, others confer resistance to Bursaphelenchus xylophilus-induced pine wilt disease^ref, some phyllosphere fungi produce zeatin, a cytokinin (plant growth regulator)^ref and others auxins, some also produce anti-freeze proteins which lower the freezing temperatureon the leaf.^ref Bacteria are implicated in the bioremediation of harmful chemicals or pollutants^ref, improved tolerance to stress, production of proteins which trigger the plant to mount defences against pathogens as well as those which attract populations of beneficial fungi.^ref

So, just like the rhizosphere, the phyllosphere is a very active place with many microorganisms playing different roles and constantly interacting in a dynamic ecosystem. What this means for bonsai is that there likely are organisms in the foliage which benefit your plant. Similar to the advice in general around the microbiome, applying fungicides, anti-bacterials and chemical pesticides can kill phyllosphere organisms so avoiding this is a good idea.

Choosing a pot

Of course your choice of pot has a lot to do with the aesthetic vision you have for your tree, and I’m certainly not going to get into a debate about ‘feminine’ and ‘masculine’ trees and pots (hint – I’m not a fan of gendered bonsai!) Or glazed/unglazed (etc).

The pot for your bonsai is more than just its physical receptacle, it is also a life support system, holding water, soil & microbes and providing physical support. There are some physical attributes of pots which promote or inhibit a tree’s health, including materials, volume, width/depth ratio, drainage and even colour.

To start with materials. I had quite an unsatisfactory experiment with concrete in the form of hypertufa, for a while I was trying to save money by making bonsai pots using this material. Hypertufa is a combination of cement, sand, an organic material like moss or coir, and perlite/vermiculite. It worked well for making pots, but I found that they dried out really quickly; further reading told me that cement and particularly hypertufa is very porous. It also can leach calcium & silicon, which may or may not harm/benefit your tree depending on how much comes out.

A study on tomatoes showed better results for plastic pots in winter, and clay pots in summer, which related to the temperature of the pot.^ref Clay/ceramic pots did not heat up to the same degree as plastic. As an illustration of this is below – a thuja occidentalis has suffered heat stress damage on one side of the pot.

https://www.researchgate.net/profile/Avner-Silber/publication/332235948_Chemical_Characteristics_of_Soilless_Media/links/625e92ad709c5c2adb86e7cb/Chemical-Characteristics-of-Soilless-Media.pdf#page=324

This comes down to the principle of temperature buffering – or, the ability to withstand temperature variations without transmitting these to the roots. Buffering is improved when the pot is larger, and when the surface area at the top of the pot is reduced. On the other hand, some species respond well to having warm (not hot) roots.

Be aware that a dark or black pot will get hot out in the summer sun. In one study, a black pot caused the growing medium to be up to 10 degrees C higher than the air temperature.^ref In a sunny or hot locale, this could prove deadly to roots if maintained for too long. Where the pot is positioned and the foliage of the tree in it will affect how much sun hits the pot. One study found that plants grown in white pots had 2.5 times the root density of those grown in black or green pots.^ref

The geometry of the pot affects evaporation rates, since more surface area provides more space for evaporation to take place. You can see the differences in the table below. Yellow highlights show two pots of similar volume. The 7cm radius pot with 5cm of depth has a similar volume to the 9cm radius with 3cm depth. But the second pot’s surface area is 1.7 times larger than the first. This will significantly increase evaporation. It’s probably no great surprise to anyone who has bonsai that a shallow wide pot requires more careful attention to watering.

Another aspect to consider in choosing a pot is the geometry relative to the tree being blown over. Whilst a heavy pot can compensate for geometry somewhat, a tree wired into a pot is effectively a giant lever with the fulcrum at the edge of the pot. Wind coming sideways onto the tree will push the lever and if the pot is too narrow relative to the height of the tree, the tree will fall.

You can calculate the force needed to turn a tree over based on the fulcrum position of the edge of the pot. The larger the difference between the pot radius and the centre point of wind force on the tree, the less force is needed to push it over. I’ve done some calculations on a 2.5kg tree with a 30cm-ish diameter foliage canopy, and you can see below that once it gets to 50cm tall with a 20cm wide pot, the wind needed to push it over becomes much lower.

If the centre point of wind force on the trunk moves upwards, the surface area of the foliage increases, or the pot width decreases, you can soon end up with instability. My suggestion is to test this when you’ve repotted, push the tree at the point where you think the wind will be centred, and see how much force is needed to push it over.

Biochar

(Thanks to Dr. Karen O’Hanlon of Probio Carbon for answering some of my questions about biochar).

Biochar is a product which has been advertised as a beneficial component of bonsai soil over recent years. So what exactly is it?

Biochar is basically charcoal which has been “produced from organic waste using pyrolysis technology under temperatures ranging from 400C to 700C where oxygen is either absent or depleted”.^ref Pyrolosis means decomposing carbon-based materials through the application of heat.^ref So a feedstock (source material) is acquired and heated in the absence of oxygen for a given period of time to create what you would probably recognise as charcoal. The structure of biochar is shown in the image – as you can see, it has many, many holes in it.

Scanning Electron Microscope image of biochar
https://www.rhs.org.uk/soil-composts-mulches/biochar

So why would you add biochar to your bonsai soil? There are a few good reasons. It has been proven to improve water availability^ref, act as a fertiliser reducing the need for chemical fertilisers^ref and increase microbial biomass^ref (ie. it attracts beneficial microbes).

An experiment conducted in Colchester, UK by the Bartlett Tree Research & Diagnostic Laboratory amazingly found that ash trees treated with biochar did *not* get infected by ash dieback disease over a period of 4 years even when the disease was present in adjacent trees on the same site. They believed the reason for this was that the biochar enhanced the trees’ immune system and improved root growth.^ref

The microbe aspect of biochar is really interesting – in one study it was found that microbes living in it were able to ‘mine’ the biochar pores for phosphorus. So it appears to have synergy between its composition (with nutrients for plants) and its attractiveness to microbes which can help get those nutrients into plants.

One of the key physical properties of biochar is that it has a massive surface area, relative to its size – in one study on malt spent rootlets (a residue from brewing) it was 340 m² per gram!!^ref That’s larger than the size of a tennis court for every gram of biochar.^ref This increased surface area along with the physical structure of biochar having lots of tiny pores, results in greater water retention in the soil.^ref

Biochar can be made from basically any organic material, from forestry to food production to agricultural by-products and this source material is the main determinant of its chemical properties.^ref So when choosing a biochar for your bonsai soil, you want to know what it has been made from, and what this means in terms of its properties. Some of the properties which vary significantly include pH, surface area and cation exchange capability/electrical conductivity. For bonsai I would say you want high surface area & pore volume (to assist with water availability) and high microbial mass. The fertiliser aspects are probably a nice-to-have. Looking at the table below this means probably biochar made from a wood-based source material is best.

There is quite a bit of research out there on different biochar properties, which I will summarise here for you to read through. Unfortunately I haven’t found any research which looks at volume of microbes for each feedstock, but I would expect this to be positively associated with surface area.

Biochar Feedstock	Properties
Wood	Highest surface area (leading to better water retention) and highest pore volume (a factor of 10 higher than manure) Lowest cation-exchange capability Largest amount of C Contain less plant-available nutrients More electrical conductivity Lowest ash content (associated with lower pH) Micro-nutrient content mixed (see table here) Total bioavailable nutrients mixed (see table here)
Crops & grasses	Highest average particle size Highest K content Lowest calcium carbonate equivalents Micro-nutrient content mixed (see table here) Total bioavailable nutrients mixed (see table here)
Manure	Lowest surface area and lowest pore volume Highest cation-exchange capability Highest calcium carbonate equivalents Lowest average particle size Highest ash content (associated with higher pH) Greatest N, S, P, Ca, and Mg concentrations Highest micro-nutrient content (Fe, Cu, Zn, B, Mn, Mo, Co, Cl) Total bioavailable nutrients mixed (see table here)

Source: https://link.springer.com/article/10.1007/s42773-020-00067-x/tables/1

The temperature at which the biochar is created makes a difference too. Increasing pyrolisis temperature leads to “increased biochar C, P, K, Ca, ash content, pH, specific surface area (SSA), and decreased N, H, and O content”^ref

Like many things in life though, you can have too much of a good thing. In some studies, too much or the wrong biochar in soil has led to phytotoxicity^ref You might also be wondering why it doesn’t just remove all the nutrients in the soil like activated carbon, which is used in aquariums and drink bottles to remove metals, chlorine and contaminants. When asked this question Dr. Karen O’Hanlon at Probio Carbon said it was because biochar is not ‘activated’ to the same degree as activated carbon. Reading more about this, the absorbent properties of biochar are “1/6th to 1/12th that of high quality activated carbons”.^ref Activation forces more pores and surface area into the charcoal, this is done by varying the temperature and pyrolysis process. So whilst there probably is some nutrient absorption, it’s not going to be at the same level as activated carbon and can be compensated for by the nutrients within the biochar themselves and the increased microbial activity.

Rauhs Model

Rauh’s model represents the Cupressaceae family (cypress, juniper & redwood), some Araucariaceae, the Pinaceae family including most Pinus species, the Podocarpaceae family as well as angiosperms such as oak, maple and ash. It is a very common model for trees we encounter in bonsai.

The architecture according to Rauh’s model includes a monopodial trunk (one which continues to extend, and does not terminate) which grows rhythmically (on a seasonal cycle) and so develops tiers of branches, the branches themselves morphogenetically identical with the trunk (ie. they develop in the same way). Because the branches are identical, the trunk can be less dominant in this form and another stem can take over if the trunk is removed or damaged. Flowers and reproductive organs are always lateral and without effect on the growth of the shoot system.^ref Often these are on short shoots.

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers20-09/09318.pdf

An essential feature of Rauh’s model is that branches develop mainly by ‘prolepsis’, from dormant lateral buds close to the resting terminal bud.^ref Prolepsis in this context means ‘the discontinuous development of a lateral from a terminal meristem to establish a branch, with some intervening period of rest of the lateral meristem’. So basically there is a gap or period of dormancy before the bud extends to form a branch. Whilst this might seem obvious to European readers, actually this mode of development is not what happens in other parts of the world, particularly the tropics, where continuous growth occurs, and this difference creates differences in the tree architectures visible in those different places.

It was noted in one study that Apple trees follow Rauh’s model during their juvenile phase but a different one during their reproductive phase (ie. their flowers terminate shoots and affect the branching after this point).^ref

Massarts Model

This architectural model is associated with many conifer families including Abies, Picea, Sequoia, Metasequoia, Cedrus, Taxodium, Taxus, Cephalotaxus, Ginkgo & Ilex aquifolium. The pattern for this architecture is a vertical, dominant trunk with rhythmic growth and which consequently produces regular tiers of branches at levels established by the growth of the trunk meristem. Branches are plagiotropic (horizontal) either by leaf arrangement or symmetry. The position of flowers/cones is not significant in the definition of the model (which means they don’t terminate the branches and have any effect on the structure).

One study in Indonesia looked at rainfall ‘stemflow’ and ‘throughfall’ (basically how much water runs off the tree into the ground causing erosion) and found that the Massart’s model tree (Pterocarpus indicus) had the highest leaf surface area and caused the least erosion from water runoff.^ref However the study used angiosperms with broadleaves and not gymnosperms with needles.

Tree Architectural Models

Halle & Olderman in the 1970s created a model of 23 types of architectural models to which all tree species are believed to conform. They started with the idea that the shoot apical meristem/s (“SAM” – the primary growing tip) is/are the ‘treemakers’.^ref The behaviour of the SAM over time determines the form of the tree. They identified four types of meristems (active growing shoots) which led to different forms – those with a single meristem (like a palm), those with modular construction which follow a precisely repeating pattern, trees with trunk/branch differentiation and those in which the meristem changes direction to produce both trunk and branch.^ref These depended on factors like whether the SAM is reproductive or vegetative, whether it grows vertically or horizontally, whether growth of the SAM is continuous or rhythmic (ie. has a period of dormancy or a growth pause) and the chronology of growth of each meristem.^ref

Below is an updated version which includes some models added later (and one which was removed as nobody found a real tree which conformed to the theoretical model).

https://gali-izard.arch.ethz.ch/francis-halle

Each architectural model has a unique combination of growth, branching, axis differentiation and position of sexual structures.^ref Not all of these are relevant to bonsai – as you can see Holttum & Corner don’t branch and Tomlinson & Bell branch underground. Many of these models are relevant for palms, cycads and tropical trees which aren’t commonly used for bonsai.

Two of the architectural models represent many of the trees used in bonsai at least in the Northern Hemisphere^ref – Massart’s and Rauh’s model – their main difference is in the branching angle with Massart’s at an angle to the stem and Rauh’s growing upwards. In both models the trunk is monopodial – it keeps extending upwards and is dominant.

Massart’s model represents Abies, Picea, Sequoia, Metasequoia, Cedrus, Taxodium, Taxus, Cephalotaxus, Ginkgo & Ilex aquifolium. More forms and discussion of Massart’s model are represented in this post.

Rauh’s model covers the Cupressaceae family (cypress, juniper & redwood), some Araucariaceae, the Pinaceae family including most Pinus species, the Podocarpaceae family, as well as angiosperms such as oak, maple and ash. This is shown in more detail in this post.

If you’re looking to understand the architectural model for your particular tree, you might consult this book – it mainly focuses on tropical trees but gives some pointers on working it out.

Some other models include Attim’s model for Eucalyptus, this is similar to Rauh’s model but follows a continuous growth pattern – in these trees as one leaf expands outside the bud it is replaced by a new bud initiated at the shoot apex.

Troll’s model is applicable for hemlock, acacia, beech, where “axes are all plagiotropic (ie. horizontal), the architecture being build by their continual superposition; main-line axes contribute part trunk, part branch, the proximal part becoming erected, most often secondarily after leaf fall” – it is believed that reaction wood is involved in determining this architecture (the type of wood created to stabilise a branch against gravity – compression wood developed under the branch in the case of gymnosperms, and tension wood developed above the branch in the case of angiosperms). Hemlock is a gymnosperm with this model.

Finally, trees can move from one model to another when they move from their juvenile vegetative phase to their reproductive phase. For example Apple has been found to conform to Rauh’s model when juvenile but Scarrone’s when reproductive:^ref