Tag Archives: Branches

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 woodref (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 autophagyref – 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.


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 studiesref:

  • 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.

bonsai wire

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 xylemref (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 hoursref 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
Angiosperm tension wood above the branch

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 MOEref, 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.

link to table here

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.

Ramification of Branches and Foliage

After establishing trunk and branch structure, ramification (a fancy word for ‘branching’) of branches and foliage (as well as roots) is a key goal of bonsai. This makes a tree look older and more sophisticated, and gives the bonsai enthusiast options for continued development of the tree.

Ramification is created by branching the stems. Stem branching usually* requires buds, as a new bud creates a new stem. The pattern of stem branching for a particular species will depend on its ‘phyllotaxy’ (leaf morphology) and pattern of buds.

Usually in bonsai we don’t want more than two stems from the same location, the general guidance is to fork into two at any given junction. This is because strong growth of multiple branches at a junction leads to a bulging area on the trunk which bonsai judges don’t like. In the real world, many trees have reverse taper and bulging branch junctions though, so it’s your call. To avoid this situation, remove buds which are in places you don’t want by rubbing or cutting them off.

To improve ramification, you need to encourage as much budding as quickly as possible, then select the buds you want to develop. Pruning the growing tip is the main way to encourage budding, because pruning removes the apical bud (the dominant bud at the end of the stem), diverts resources into buds lower down the stem and sensitises those buds to respond to auxins and develop into shoots. In deciduous trees this should result in at least two buds generating from the stem instead of the one which was there. Another great way to create ramification on deciduous trees is through bud pinching – see Harry Harrington’s detailed explanation of how to do this. Bud pinching removes the entire primary meristem except for two outer leaves, this encourages the buds at those leaf axils to grow, along with two new buds at their bases.

Different species have differing abilities to respond to pruning, so try to get a sense by observing your tree of how well it will cope. Deciduous trees are designed for regeneration so in general they take pruning reasonably well, although if you take it too far they might send out suckers instead of new buds from the branches. With evergreen conifers you want to ensure there is some foliage and at least some buds remaining after you prune, otherwise it may not regenerate (unless it’s a thuja, or a yew, these guys are refoliating machines). I have cedrus seedlings in my collection and by cutting back the apical leader from not long after they germinated, and every year since, they have become extremely bushy and well-ramified (although, at the cost of developing a think trunk).

Gratuitous image of one of my cedar bonsai

Anything which stops or prevents tip extension will drive bud activation and ramification further back on the tree. In the case of conifers, the presence of flowers on the growth tips (as you see in juniper) has this effect as well, and can cause back budding. Lammas growth (a second flush in summer) can give you another round of ramification as long as you’ve pruned beforehand (otherwise it will just add to the existing stems).

Research has found that bud outgrowth is “controlled by plant hormones, including auxin, strigolactones, and cytokinins (CKs); nutrients (sugars, nitrogen, phosphates) and external cues”.ref In particular the sugar sucrose has been identified as a key driver for promoting bud outgrowth and accumulating cytokinins – this is generated by photosynthesis.

In one study on apple trees, foliar application of a synthetically produced cytokinin 6-benzylaminopurine (BA) was found to generate three times the lateral bud growth on currently growing shoots compared to controls (but not on old growth)ref and at the same time reduced the length of the main stem. BA was used to encourage better growth of bean sprouts in China before being bannedref and has been shown to increase the number of leaves (ramification!) on melaleuca alternifolia treesref (melaleuca is the source of tea tree oil), and on some conifersref. Could BA (also known as 6 BAP) be useful in bonsai? You can (like most things) buy this product in foil bags on ebay, but there is a product in the orchid world called Keiki paste which also contains 6 BAP – so maybe some judicious use of ‘crazy keiki cloning paste‘ might also help ramification and shoot development in your trees?ref You can also purchase BAP (as its also known) from vendors involved in hydroponics and suchlike as it’s used in in-vitro plant micropropagation.

If you baulk at paying £18 for 7ml of keiki paste, there is one other source of cytokinins which is a lot cheaper, more sustainable and clearer in its provenance – compost. This study found that compost created particularly from waste collected throughout springref contained 6 BAP. Frustratingly there weren’t any free to read articles analysing compost leachate for cytokinin content, but if it’s in solid compost it’s a fair assumption there are cytokinins in leachate as well. Which makes me feel a lot better about the £300 I recently spent on a Hotbin composter! Which incidentally, produces gallons of leachate, which can be diluted and added as a liquid fertiliser.

* I’ve recently read a study which states that “apical meristems can be surgically divided into at least six parts and these then become autonomous apical meristems.”ref What this suggests is that you could slice growing tips into 6 (or better, two since we don’t want more than two stems from a node) and they would become two stems instead of one! One to try next spring.

** By the way – it’s not auxins which cause apical dominance! Check out page 215 of this book, it’s nutritional status and phyllotaxy which determine the apical stem’s sensitivity to auxin which is present.