The trunk of a Bonsai deserves specific attention, as it is one of the most eye-catching features of a tree. The following features will be discussed: The Nebari, tapering, thickness and overall shape of the trunk.
The Nebari (root-flare) is important in providing a Bonsai with a well-balanced appearance. More information can be found at the root-flare page.
A trunk should be thicker at its base than at the top of the tree (see photo 1, below); this is called tapering. Although it is hard to improve irregularities in the thickness of a trunk (select trees well before buying them) it is possible to achieve minor changes:
By allocating the tree’s growth to the branches just above the thin part of the trunk this part will benefit from an increase in the flow of nutrients and as a result grow thicker. Allocating growth can be done by pruning the entire tree except for the branches above the thin part on the trunk. This method is slow and it will take at least two years before you will start noticing any changes. As said before; selecting trees on its tapering trunk is important.
As it is impossible to make a trunk thinner, the only way to make the trunk tapering is to thicken the other parts of the trunk, using the method described above. However, it will take even longer before you can notice significant change. You might conceal the thick point by creating a Jin or Shari instead.
Bonsai plants often look older than they are when they have a thick and gnarly trunk. The only way for a trunk to grow thicker is to let the tree grow freely in a large container, without pruning it for several years. Once you are satisfied with the thickness of the trunk you can train it again and place it in a smaller pot.
The overall shape of a trunk is one of the most eye-catching aspects of a tree (see photo 2, above). As it is nearly impossible to bend a trunk once it reaches a certain thickness (until that point you can use a trunk-bender) it is important to take the shape of the trunk into account when buying it.