Note – this is a light edit and re-post of an article written for the BSSF.org site in 2014.
Many trees, when first considered for bonsai, lack a sufficient trunk to become a good mature tree if left confined to a small container. Young trees and older-but-underdeveloped material will dramatically increase in quality with some judicious augmentation of trunk height and/or girth.
A 5-10 year plan for improvement.
Most bonsai instruction and reference material is aimed at development of branching with the assumption that the trunk is of sufficient size and quality; unfortunately, the vast majority of material available at bonsai vendors and sales lacks the assumed qualities. Luckily, with many deciduous species it is possible to start development of the refined branch structure while simultaneously increasing the size and character of the trunk. We can do this by using sacrifice branches and large containers and/or a ground growing.
To accomplish simultaneous branch and trunk development start by repotting the tree into a large box or pot; this will allow it to establish a larger root system. The type of pot should be tailored to the growth habits of the tree. Japanese Maples, Hornbeam, Elm and many others react well to wooden boxes, Tridents will grow in air-pruning containers. Willow need copious amounts of water so use ceramic containers. Oaks need larger deeper containers or ground growing to put on significant girth.
Train all new growth with wire, do not pinch any tips or restrict growth in any way during the first year. Keep an eye of the top third of the existing tree and think about one or two branches that can be used as a sacrifice branch. The position of the sacrifice is ideally about two thirds of the way to the top of the mature tree. In figure 2 above the right portion of the new leader will be used for the apex while the left portion will be developed into the mature branching.
Typically, trees in development need heavier fertilization than trees that are being refined. When growing in good bonsai soil use a combination of solid organic fertilizer and liquid (either chemical or organic.) For some deciduous trees large amounts of fertilizer can make growth overly long or difficult to control. For our purposes, fertilize more slowly at first, particularly while establishing the nodes that make up the new trunk section and then gradually increase fertilizer once the new trunk section and sacrifice branch are established.
Nebari and Ground Growing
After a year in a large container, transplant the tree at the appropriate angle into the ground. Edit the roots when placing the plant in the ground and place the tree on top of a board or other flat object that is 12-15″ across. This will allow the tree to be more easily dug later and will prevent roots from growing straight down.
One of the most difficult things to control while growing bonsai in the ground is the nebari and the size of the roots. With the use of the board the roots will have a tendency to run past the board and then down into the soil; the unhindered growth will usually cause them to bulk up more than desirable for your finished roots. Similar to the sacrifice branch, selective reduction of the vigorous roots will encourage the finer root structure that will eventually be the mature nebari. Use a raised bed with removable sides to allow access to the roots while the tree is growing in the ground. Remove the sides and trim the roots around the outside to encourage finer rooting; trim only a few of the large roots each year to keep the top growth and the entire plant vigorous.
While trees are in the ground they should be treated just like a tree in a pot – keep the bonsai soil in the vicinity of the trunk and water as you would other trees. You must keep the feeder roots that are in this area alive so that there is less shock to the tree when it is transferred back into the container. The mass of bonsai soil should be 3-4 inches in depth above the board. Add some mulch or sphagnum moss to prevent the soil from drying out as quickly. If you do not have a place to put the tree in the ground continue using a large box or pot but repot every 3-5 years.
Continue training the branches. Encourage the strong shoot near the top of your tree as a sacrifice branch. Choose it such that when it is removed the cut is vertical rather than horizontal and preferably so it is on the back of the tree. Allow the sacrifice branch to grow without cutting or pruning of any kind for at least three years.
A second strong shoot from the previous top should be trained into a new trunk section and top; adding a trunk section can improve many trees by increasing the movement and taper. Keep control over the mature branches-wire all new growth and cut back after shoots elongate and harden off (you can pinch selectively, but it may not be productive as a technique.)
Prevent the sacrifice branch from shading out the lower branches by pruning side branching selectively. Consider bending the sacrifice branch backward so it is not shading what will be the finished top of the tree as it continues to grow. Many trees have a tendency to re-center their growth over the root base by growing back toward the invisible vertical line that projects from the center of the root base upward. Stake or wire the sacrifice if needed to avoid shading the lower branches.
In figure 3 we see the tree after 3-5 years depending on quality of growth; the sacrifice branch should be well-established and fattening the trunk. Continue developing the finished branches by controlling their growth carefully. With a strong sacrifice and the tree in the ground it may be difficult to control them completely. If the smaller branches get weak consider cutting back some, but not all, of the sacrifice branch to re-establish balance; this will slow development of the trunk.
In Figure 4 we see the tree after 7-10 years. You should have a significantly improved trunk within the first 5 years and with some luck you may have a good branch structure as well. The “X” in the drawing indicates cut location for the sacrifice. It’s tempting to cut off the sacrifice branch once some marginal improvement has been made but a few extra years can allow for mature tree characteristics to develop more quickly – mature bark and “muscle” in the trunk will all be more prevalent in trees that are allowed to grow out longer.
Re-examine your tree each year to consider whether the design would be improved by altering your plan; consider the taper and movement of the trunk and alter the plan as needed for improvement.
Styles, Shapes and Sizes
This article is not meant to advocate a particular style, size or type of tree. The drawings in figures 1-4 above are one way to develop a young tree that up to that point had little in the way of taper or movement. The author imagined that the example tree started as a 12″ tall with a 1″ trunk and ended at about 22″ tall with a 4″ trunk. The same starting point could be developed differently into a larger tree by the same method but with different details. Finally, starting from the same point with a 1″ trunk the tree could be reduced to a 3-4″ in height and then branching developed. However, even for shohin trees a sacrifice branch can be a good idea; I recommend following all the same steps, simply reduce the tree first and make the curves in the branching tighter. The sacrifice would need less time to grow. Judge removal of the sacrifice based on the taper of the trunk.
I have applied this technique to different styles and species, the results are typically a dramatic improvement in trunk quality. Give it a try!