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Developing Large Trunks for Bonsai

by Brent Walston


Perhaps one of the least understood concepts in bonsai is that plants are grown and trained for years to develop large and interesting trunks. Small bonsai do not become large bonsai. Plants are grown out in large training pots or in the ground to attain the trunk size and character desired before they ever come near a bonsai pot. I have a large Pyracantha bonsai that was trained for twenty years before it finally got its bonsai pot. Once plants are potted in small containers they nearly cease to grow. This is the time to develop other aspects of bonsai such as leaf reduction and ramification (development of fine branches). Beginners often rush to have that first tree in pot, and thus deprive themselves of the opportunity of having a really fine bonsai.

The following discussion pertains mainly to deciduous trees. Pines are a special case and have been discussed the article Training Black Pine for Bonsai.

Growing for the Long Term

I usually spend between 5 and 10 years training my trees before they ever reach a bonsai pot. The best ones take 15 to 20 years. I often tell my students, I don't grow trees, I grow trunks. Next to the nebari development, trunk development takes the most time to achieve.

There are no strict guidelines for how long it takes. It depends on how you want your tree to look. When presented with a plant for bonsai treatment, the first question I ask is: How big do you want it to be? Almost everything else follows (assuming a style has been selected). Development plans must fit the eventual size and shape of the tree. This is very hard for beginners to grasp because they have not seen many trees and have a great deal of trouble visualizing the end product. Nonetheless it is essential.

For really large trunks, 3+ inches, planting in the ground is probably the fastest way to go, assuming good soil, water, and you don't live in the artic tundra. This works best for deciduous trees that back break buds easily, as do elms and maples. Often I don't even think about growing branches until I have my 3 inch trunk. If you have been growing branches all along, they probably will have gotten too fat for your finished bonsai.

Deciding on Branch Placement

I do think about branch placement however, because I really like gentle bends in my trunks, even the large ones. So I might grow a tree in the ground or in a pot for 3 to 5 years, get a 2 inch trunk, then cut it down to the level of the planned first branch. Where is this? One third the decided height of your finished tree. This is why it is so important to have first visualized the tree.

After the trunk cut, the tree will explode with new growth, and hopefully a new leader will develop at the top of the cut (the first branch position). Let this leader and all the wild branches under it grow for several years. All these lower branches are really sacrifices that will be cut off later, so don't worry about them getting too fat. The new leader will form the trunk section between the first and second branch. Continue the same process until you get the trunk that you want.

If you need a formula for desired branch placement try this, it is an adaptation of proportions selected by the 'Golden Section':

Somewhere near the end of the process you can start growing the branches that you want to keep at the bends in the trunk that you created. The timing depends on the species of tree and its growth characterics. I have grown 5 inch trunk crabapples from cuttings in 6 years with this method, they have tremendous taper and crooked trunks, but I am just now beginning branch development.

Growth Rates and Taper

Cutting back the trunk, as described above, will actually slow the increase in diameter, but it will increase the amount of taper. This is the price that is paid for taper. The larger the trunk you desire, the longer you allow the new leader to grow. In approximate terms, let it grow until it reaches half to two thirds the caliper of the desired trunk or trunk section. For example, if you desire a three inch trunk, it makes little sense to make the first trunk cut until the stem has reached an inch and a half. After it reaches this size, cut it down with a perpendicular cut just above where you want it to break buds for the new leader (see How to Make the Cut below). Make the cut at one third the desired height of the finished tree. This will be the position of the first branch. Allow a new whip to develop from nearest the desired position. This will form a nice soft curve in the trunk of the finished tree. Restrain all the other shoots by pruning them back slightly, but let them grow. In other words, let the new leader be dominant.

The soft curve in the trunk results from the new leader growing at an angle to the first trunk section. This is the place to grow the first branch. Repeated cuts of additional leaders will continue to increase taper because each new leader will have to start from a bud while all the lower sections continue to grow. Each cut at the top of a new section of trunk provides the position for the next branch.

Click here to view a trunk chopped Zelkova in training.

Anchoring the tree in the ground will give you more taper, buttressing, and a better nebari, due to the stress fractures that will form from the wind waving the whip. Unstaked trees in studies at UC Davis have grown larger and stronger trunks than staked ones.

Many deciduous trees will form a jungle of low branches in addition to the desired whip which will form the next trunk section. Leave these low branches on the tree. Branches increase the diameter of the trunk up to their point of attachment. These will greatly increase the taper. They should be removed when you begin to work your final branches, or when you have achieved enough taper, or when they result in a 'knob' that gives you reverse taper (Chinese elms, Ulmus parvifolia will often do this).

Controlling the Direction of Growth

Repeat the whole process of locating the cuts at the position of the other desired branches as many times as you like, but usually three is sufficient. Each new trunk section should be shorter than the previous one to get diminishing intervals between the ascending branches. You direct the growth by selecting a leader, side branch, or bud where you want the new growth to go. I like my trees to ascend in nice soft spirals.

When to Make the Trunk Cuts

You should probably never perform this operation as the leaves are coming out, wait until the new leaves have hardened off, usually in a month or two. Before the leaves emerge, the roots are at maximum storage capacity. If you prune then, all that food is going to look for buds to expand, and the growth will be explosive, coarse, and with long internodes. This is exactly what you want if you are only looking to develop the next section of trunk, the portion between branch 1 and branch 2. This will give you the most rapid development. Identify the new leader quickly and protect it. If you are lucky it will be right at the top of the cut that you made.

If you perform this operation after the leaves have hardened (or sooner), you do it when the roots are depleted. They spent a great deal of food (energy) to produce all those new leaves and shoots. This is not conducive to developing a new leader unless you want a weak one with close internodes, such as if you want to develop a new apex at the top of tree. It is also preferable for trying to get buds to break for new branches on fast growing trees, because the new growth will be more refined with closer internodes.

How Far Can You Cut Back the Trunk?

It is not true that you should never cut back below the lowest branch. With trees that back-break buds easily such as elms , maples, zelkova and others you can cut back nearly to the ground or a really low stub and start your new leader to develop tremendous taper. I routinely cut Ulmus parvifolia down to 2 inches because I really like a swelling and curve right at the base. I let a new leader grow and then cut it back to the position of the first branch. You need to know your trees here, better to ask first if you aren't sure. Some trees resent this treatment. Never do this to conifers.

I often will cut back to a branch, especially if it has a narrow angle to the trunk, that is, it points upward. If it is near the correct position it is much better to go with a sure thing than to take a chance that a bud will break in just the right position.

How to Make the Cuts

I've been doing trunk cuts for some years now and can report what I have learned. At first I did 45 degree cuts as recommended by most books. In fact I spent a lot of time carving the crater shapes at the same time. I have come to the conclusion that this is mostly a waste of time. I now just give them a perpendicular whack, and save the angle cuts and carving for later, after the dieback is complete.

You must understand what is happening when you cut off a trunk. You are creating a wound that the plant will wall off and heal by itself. If you cut back to (or near) a side branch, the plant will usually wall off an area that reaches around the collar of the top of the side branch and then extends downward at an angle behind and below the side branch. The area that lives is the area that has connective pathways to the branch. The area above this dies (unless it can break some buds in this area). This may be a 45 degree angle, it may be more, it may be less. It makes little sense to try to guess what this angle will be. It makes much more sense to wait a year and see how far it dies back, then cut off the dead wood and carve out the wound if necessary for clean closure. It is going to die back to this point anyhow, so why carve it out, or create such a large wound so close to the tissue that is going to survive?

If there is no side branch and you are cutting back to just a stump, the same argument still holds. Cut a little bit higher than the position you want bud break and hope you get it where you want it, or inspect the trunk closely for the small bumps that may be dormant buds. By making an angled cut just above where you want bud break, you are creating a larger wound and increasing the chances that it will dieback more than you want. Once you do get bud break and you choose a new leader, you can proceed as above. One interesting and powerful trick is that dieback will usually proceed until it hits a preformed bud, or the collar of an existing branch, or the connective tissue of an existing branch. If you cut back to a side branch and there is another branch lower and on the opposite side, dieback will almost never go lower than the collar of this lower branch. This can help you limit the dieback by choosing the position and branches properly, OR you can pretrain your tree by pruning it back the year before to create more lower branches before doing the final chop. This also strengthens the lower 'tree' because there will be many more preformed buds on the 'stump' after the final chop.

Growing Sacrifice Leaders and Branches

A related process is to grow sacrifice leaders and branches to increase trunk or branch caliper, or correct a reverse taper in a developed tree. This process involves growing a wild whip somewhere out of the trunk, or less frequently, out of a branch to increase the caliper up to its point of attachment. The difference in this case, is that the sacrifice is simply a tool, an artifice, that will be removed completely when it has done its job of increasing the caliper. Sacrifice branches can be used for deciduous or evergreen trees, but they are especially important for developing conifers.

It is important when growing out sacrifices not to shade out the areas below it , or overly weaken the areas beyond it. I usually let the sacrifice grow as a long unpruned whip with all the leaves and small branches cut off of it for several feet to keep from shading the 'tree' below. Sacrifices can be as long as ten feet or more, depending on the degree of enlargement desired.

Use sacrifice branches and leaders to correct a problem when your tree already has good form and finished branches. Remember that branches increase trunk caliper up to their point of attachment. To increase the diameter along the entire trunk allow a sacrifice branch to grow near the apex of the tree, but not at the very tip of the apex or it will destroy its delicate structure. If this occurs you will have to grow a new apex to achieve the final diminishing taper. I often cut out the sacrifice before it has finished its job and start a new one a little lower to preserve taper.

You will have to remove your tree from its pot and put it in the ground or in a larger training pot to achieve the vigor necessary for the sacrifice branch to do its job. To achieve caliper and taper, select positions lower on the trunk for the sacrifice branches. Do not let sacrifice branches grow from existing branches (water sprouts) or you will overly fatten the branch and put it out of proportion to the trunk. They can, however, be used to correct the diameter and increase vigor of weak branches.

When sacrifices are used to strengthen branches as well as the trunk, one must be much more careful. Development can come very quickly, and overdevelopment can occur in a single season. Overly large branches are a common fault and are difficult to correct. If overly large branches occur, all you can do is place a sacrifice above the fat branch to increase the trunk size to restore the balance.

Growing Large Straight Trunks for Formal Upright Style

The best way to get formal uprights with good taper is to plant them in the ground or use the escape technique with them planted in a five gallon can. In the escape technique, you allow the roots to escape out the drain holes of the nursery can and into the earth. Continue watering through the can. When it comes time to harvest the tree, simply cut the roots at the can (which still contains an intact root ball). The top must receive its trunk cut and allowed to recover before you can do this.

Let them grow wild to about fifteen or twenty feet where they can wave in the wind. This will develop enormous buttressed trunks in about five years since they are anchored in the ground. Then break the tops and jin the upper portion as I have described for Cedars. This will give you an abundance of bud breaks for new branches for such species as Sequoia sempervirens, Taxodium disticum, Cedrus sp, and Metasequoia glyptostroboides.

The jin and steep diagonal cut are the two most common methods to create taper. Another that I have been playing with is to make the back cut at a steep angle visible from the front, in other words down the side, so that when the top is broken and pulled down a section of the side of the trunk comes with it. This will give you more taper and it will look natural since the jin is carved or better yet broken and pulled down with pliers. A section of live bark must be retained toward the back so that one side of the tree will not be devoid of branches. It is not a perfect solution but it does help.

The perfect solution is to allow whips to grow and continually cut them back at intervals as frequently as one year,adding trunk sections in diminishing amounts. Each time a leader is cut a new one of smaller diameter replaces it. Since the cuts are made frequently the trunk curves are less noticeable and will completely disappear in a matter of years. Once the taper is achieved you can let it grow wild and break the top or whatever. You don't see this much because it is an extremely slow process. Rather than building bulk each year from an ever increasing amount of foliage you periodically remove over half of its growth capacity and force it to start over.

And finally

I suppose the bottom line here is that one should not get too involved in choosing just the right seedling or young plant for most bonsai. For even small bonsai, trunk development as I have described it here makes critical selection meaningless, except for selecting plants with good nebari. Never pass up a potential bonsai candidate with good nebari.

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