Thickening of Trunks

The reasons we want a thick trunk are:

1. To give the appearance of age. The actual age is unimportant but the illusion of age is what we should aim for.
2. To give an appearance of stability. Your tree should have exposed roots and a trunk which looks like they have withstood the harsh elements of wind, rain and snow for many years.
3. To give the tree a powerful look. This is the least important of the three as subtlety is preferred to overstatement. One should aim for a trunk diameter of roughly one sixth of the height of the tree.

If you are a patient person and are prepared to wait for between 10 and 20 years for any marked thickening of the trunk then just proceed normally with your bonsai, that is, watering, feeding and repotting periodically, however if you don't have this kind of patience you can achieve good results in a couple of years with a concerted effort.
The method you adopt depends on:

1. Variety of tree
2. Degree of thickening required
3. Stage of development that the tree has already reached.

A. IN THE GROUND

For maximum effect constant feeding and watering is necessary.

Advantages

• rapid thickening

Disadvantages:

• it is difficult to work on the tree.
• you have to keep control of the whole tree in case one or two branches or the apex thickens disproportionately,
• the tree can only be lifted and repotted at certain times of the year.

B. IN A LARGE GARDEN POT

Advantages

• Rapid thickening
• Better conditions for working on tree
• Fairly accurate control of branch development and also of fertiliser and moisture content.

Disadvantages

Difficult to move large pots

USE OF A LARGER BONSAI POT

I use this method for trees that have been in training for some years and where a heftier trunk is desirable but where I want to maintain the delicate structure and formation of branches in. For example, celtis and elms. I use a pot with double the soil capacity than normal. Feed well and trim back periodically.
To use a branch to thicken a section of trunk is a very good method. Try to choose a branch in an inconspicuous spot so that once it has done it's job it can be removed without leaving a visible scar. Staking the branch to grow upright makes for excellent and rapid growth. Use this method of thickening in the ground or a large garden pot.

TOURNIQUET METHOD

I have only read about this method and have not attempted it and advise that for the first time you try it on something young rather than on a prized bonsai. The theory is that you tie a wire around the base of the trunk, tighten it slightly and leave it until the base flares out. It needs constant watching so that it does not ringbark the tree.
I feel that the biggest mistake which we all make as beginners is to try and get our tree into a bonsai pot as soon as possible. This has the good short term effect that you have a presentable bonsai, however if you are aiming at growing bonsai of a high standard try one of the above suggested methods to markedly improve a tree.