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.
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.
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.