After selling a couple of my deciduous trees at a recent auction I decided that I had entirely too few trees which are naked for the short San Francisco winter. The Chinese elm that I had been training for 5 years from nursery stock fetched a high enough price that I was able to take a trip to Lone Pine and buy a dozen more trees, with money left over for fertilizer, akadama and other supplies. I bought two Texas Cedar Elms, a European Beech, a European Hornbeam, three Winter Hazels in gallon cans and four small scrub oaks in 4-inch pots. (okay, so they’re not deciduous, but at least they’re not pine trees, I already have too many of those.)

But what I realized after I brought all my new plants home was that all of them, although in some cases already resembling a bonsai, had trunks that were really just too small. I decided to plant the two Elms in my front yard to grow wild for a couple years. The Beech has a nice inch-thick trunk, but the root base was horrid, so I immediately started an airlayer, right below the best tapered section above the roots. The tree will have to grow for a couple years or longer before it can start fine branch training. The Hornbeam has a nice trunk, but the roots are also a mess, it was potted up from a small container with little attention paid to circling roots. I will air layer it as well as soon as I get a chance, right above the nebari that are there now, then I’ll let it grow out for a couple years to fatten the trunk from an inch to two or more inches.

The Winter Hazels can be great trees, but it will take a while. The most fantastic of these that I have ever seen graced the cover of Golden Statements a couple months ago, I imagine it was imported from Japan, but all the same, it’s a nice tree, made nicer by the profuse early-spring blooms and the beautifully plump buds that adorn the tree all winter.

All this got me thinking about trunk fattening. There have only been a very few times when I went to a nursery and returned with a tree that had a trunk with good taper, and a nice big caliper. Maybe I’m cheap, or maybe the material is hard to come by. Whatever the reason, we can all benefit from some facts about tree physiology and how they affect the growth of the trunk. Keeping these facts will allow us to maximize the growth of the trunk so that the little starter trees can get bigger.

In Bonsai Today #102, Will Heath reviewed a little-known book called “The Growing Tree” by Brayton F. Wilson. This is not a book about bonsai, but a book for the forestry student about the way in which trees grow. It is quite detailed, and you might check it out if you have the time. I ordered a copy of it and read through some of the more interesting chapters after it arrived.

If you’ve read a stack of bonsai magazines you will already know that when people are growing the trunk of a tree they tend to either let a large branch escape out of the top of the tree or they allow many sacrifice branches to grow from near the base to fatten the trunk. In both cases the techniques outlined increase the girth of the trunk rapidly because there is a lot of branch growth going on. The distribution of growth however, is the difference between the two techniques. The top escape branch will uniformly coat the existing trunk with new wood, increasing the size of the trunk, but not increasing the taper from bottom to top. The second technique will increase the girth and the taper of the tree, but will leave the tree with more scars on it after the sacrifice branches are removed. A combination of the two techniques can be used good effect as well.

What the stack of bonsai magazines probably didn’t tell you is the more technical details of wood growth. Cambial activity is responsible for the increase in size of the trunk; the cambium deposits new wood; makes new bark; and replicates to surround the enlarging girth of the tree. In his book, Brayton Wilson states “Although initiation of cambial activity occurs at about the same time in trees of different growth rates, cessation usually occurs earlier in slow-growing, suppressed trees than in fast-growing, dominant trees. Variation in ring width between trees is, therefore, due both to differences in rate of cell production during cambial activity and to the length of time that the cambium is active during a season.” What does that mean to the bonsai grower? Well this, along with a few other facts I found in the book, indicates that when trying to fatten trunks, allowing wild growth for a few years is the single most important thing for fattening the trunk. But beyond that, keeping the tree healthy and growing late into the season is also very important. People typically stop fertilizing bonsai when the first of November rolls around, but with a tree being grown for stock it may be a good idea to continue fertilization until winter is definitely here. In my yard, the Chinese elms typically drop leaves in the second half of December. If you can get an extra month and a half of growth each year, your trees will be that much fatter.

Making sure that the tree you are fattening is fast growing involves a number of different things. If you plan to grow the tree in a growing box or other large container you will want to fertilize regularly and make sure that water and oxygen are always available in the root zone. Use a slow release organic fertilizer, which typically last about 6 weeks and provide the tree with a small amount of food each time it is watered, but also supplement with chemical fertilizer once per week which provides the tree with a large shot of nutrients. If you plan to put the tree in the ground, it will take longer to get established, but once it has a good number of roots it will grow even faster than a tree in a container. During the period of establishing roots provide enough water and fertilizer to get the tree growing well. Also, make sure that the soil immediately surrounding the trunk is well draining like the soil in a pot; this combined with regular watering of this part of the soil will ensure that there are small roots near the trunk when it comes time to dig the tree out of the ground.

The amount of fertilizer applied will vary from species to species and depend on the size of the container a tree is planted in.  However, it is worth noting that, when using organic slow-release fertilizer the danger of burning the trees is reduced dramatically, allowing you to use far more fertilizer than you might otherwise try.  Also, a fast-growing tree will absorb far more nutrients than a slow-growing one.  So, don’t be shy; once you see the tree taking off for the sky apply more and more fertilizer, removing the old cakes or pellets and adding new ones each month.

If you are lucky enough to have come across a copy of Bonsai Today #20 you may have read the article on growing pines from seed in plastic colanders (it has been reprinted elsewhere).  The trees are planted in coarse bonsai mix in a colander and watered and fertilized heavily.  The roots, instead of circling the bottom of the pot, bud back endlessly causing the base of the tree to fatten up faster.   You can get plastic colanders at most 99 cent stores, or you can order pond baskets, which are basically the same thing but are typically black instead of metallic or fluorescent pink.  I am currently trying this technique with a number of different trees and my hope is that it will apply to many different tree types.

John Boyce told me once that in bonsai “You either want them short and fat, or tall and skinny.”  So, if you’ve come to the conclusion that you have enough tall skinny trees, then it’s time to try to fatten some of them up.  Afterall, everyone needs variety in their trees, both in shape and in species.

Tags: bonsai stock, Fertilizer, Pines, yamadori