Thickening of trunks

By Gail Theron

The reasons for wanting a thick trunk on your tree, are:

  1. To give the appearance of age - the actual age is unimportant but the appearance of age is a different matter.
  2. To give stability - your tree should have roots and a trunk which look as though they have withstood the harsh elements of wind, rain and snow for many years.
  3. To give a tree a powerful look - for me this is unimportant. I prefer trees that have subtle beauty, rather than the overstated gross trunk.

Aim for a diameter 1/6th the height of the trunk.

If you are prepared to wait for between 10 and 20 years for any marked thickening of your trunk then just proceed as you have been, repotting occasionally.

Watering and feeding:

However if you don't have that 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 which the tree has reached; If your tree is planted in the ground: for maximum effect, constant feeding and watering is necessary.

The advantage of this is rapid thickening. The disadvantages are that it is difficult to work on; you have to keep control of your whole tree in case one or two branches thicken disproportionately; the tree can only be lifted and repotted at certain times of year.

If planted in a large garden pot: The advantages are rapid thickening; better conditions to work under; fairly accurate control of branch development, also of fertiliser and moisture content. The disadvantages are only in moving heavy pots;

If planted in a large Bonsai pot: I use this method for trees where a heftier trunk is desirable, but where I still want to maintain the delicate structure and formation of branchlets; eg., Celtis.  I would use a pot with double the soil capacity than normal, feed well and then trim back periodically.

To use a branch to thicken a section of the trunk is a very good method. Try and use a branch in an inconspicuous spot so that once it has done its work, it can be removed without leaving a visible scar. Staking the branch upright makes for excellent results and rapid growth. Use this method as well as thickening in the ground or a large pot. It even works to some extent in your normal Bonsai container.

Tourniquet is a method I have read about but which I have not tried. I am a bit wary about this so don't try it on your best tree. Tie wire round base of trunk, tighten it slightly and leave until the trunk flairs. It needs constant watching, so that you don't ring bark the tree.

I feel that the biggest mistake which we all make as beginners is to try and get our trees into a bonsai pot as soon as possible. This has the good short-term effect that you then have a more or Iess presentable bonsai but not a tree of high standard.