Techniques of Making a Trunk Thick

A great trunk in bonsai is a trunk that has a weathered old look, is firmly settled in the soil with nice root flare (nebari) and has nice thinning of the trunk towards the apex (taper). The various styles in bonsai are determined by the trunk line which gives the tree its character and each style is named according to the trunk line. When a bonsai is viewed, the trunk should be visible from the front for at least  2/3 of their height. If the trunk is not seen clearly then it will look like a bush and not a tree. The bonsai tree should have a trunk with a broad based tapering to the top.

The thumb rule is that the diameter of the trunk at the lowest point determines the maximum height of the tree. If the diameter is for example 3 inches, the tree can be six times the diameter that is 18 inches high. The canopy of the tree starts at about 1/3rd of the height of the tree and in this case, roughly 6 inches above the soil. Side branches should not be thicker than 1/3 of the thickness of the trunk at the attachment. The apex of the tree should be bent slightly forward and then it should be bent up. There should be no visible scars in the front of the tree and if there are any, they should be hollowed or carved so as to make them look like a feature. 

Trees are of various kinds but most trees work in the same way. The different parts of the tree work in unison to help the tree get food, water and nutrients it needs to survive. The trunk of a tree is important for two reasons- 
First it acts as a support rod giving support to the crown or foliage and also gives the tree its shape and strength. Secondly, it acts as the central plumbing system forming a network of tubes that carries water and nutrients up from the soil to the leaves through the roots and food in the form of  sugar from the leaves down to the branches, trunks and roots. The trunk consist of five layers of tissue:-

1. Outer bark-  that is hard and protects the tree from diseases, pests, strong winds and extreme temperatures. 
2. Phloem- is the 2nd layer from outside and acts as a food conduct by carrying sap from the leaves to the rest of the tree.
3. Cambium- is the 3rd layer which is very thin which is responsible for producing new cells and for making the trunk, branches and roots grow thicker and larger in circumference.
4. Xylem- is the youngest layer of wood and has a network of thick walled cells that bring water and nutrients up from the roots to other parts of the tree. As the tree develops and grows, the xylem cells in the centre of the trunk.
5. Sapwood- When a tree develops and expands in circumference, it builds a new ring of xylem around the original xylem tissues known as Sapwood or secondary xylem.
6. Heartwood- As the tree grows, the xylem cells in the centre of a tree become inactive and die, losing their function of conduction and forming a hard skeleton that serves only as a support to the tree. This heartwood is usually darker in colour than the sapwood.

Making the Trunk Thick

Trunks become thick only when the tree gets a lot of growth. The process of creating a bonsai tree is to first create a good root structure followed by a good trunk and finally the branch structure. That is why bonsai enthusiasts tend to worry more about the roots (nebari) and the trunk before growing the canopy. There are a few techniques common amongst the bonsai artists to make the trunk thick:

Growing in the ground or in large grow bags

The oldest and the most common technique to thicken the trunk is to plant the tree into the ground. Trees thicken faster in ground because they are able to send their roots for enough to seek nourishment and the longer and thicker the roots become, the thicker the trunk base gets. Through this is one of the best methods of making a trunk thick, it has its limitations and drawbacks. Firstly, one should have enough suitable space for the tree to grow into a large tree and the space available should meet the sunlight and water requirements.
Secondly, when planted in the ground, the bonsai tree may suffer if the soil is not fertilised properly or if the soil does not drain well. Finally, if there are no fine roots near the base and the tree is sensitive to the disturbance or does not grow new roots from the cut off stubs of the thick roots, it becomes a problem when it is time to lift the tree from the ground.
The alternative is to plant the bonsai tree in a large grow bag, oversized pot or a large container. In this method, close attention has to be given to watering fertilizing and every year the soil should be changed in order to maintain good growth. This method has the advantage of controlling the root system, which makes it easy for replanting into a bonsai pot when the trunk becomes thick.  

Cut and Grow Technique

This technique will result in thickening the trunk and also in creating a taper. First a tree is planted on the ground or in a large container/grow bag and left to grow. The first cut is made at the point where the trunk has reached the desired thickness. This cut point should be at 1/3rd  of the height of the desired tree's final height. Once the tree start growing branches from the cut point, one branch or leader is selected and wired as vertical as possible. It is this branch that will form the next section of the trunk. Any other branches are presented. This leader is allowed to remain dominant and grow till the desired thickness is achieved. 

Sacrificial Branch Techniques

A sacrificial branch is any side branch growing on the lower portion of a bonsai tree that is allowed to grow long and thick, so that it directs the nutrients to the trunk causing it to thicken. This branch is usually left on the trunk temporarily till it achieves its purpose of trunk thickening. In the meanwhile, the rest of the tree should be kept under control. When the trunk reaches the desired thickness, the sacrificial branch is cut off the trunk using concave cutters. When selecting the sacrificial branch it should be remembered that there will be a scar when this branch will be removed, that will remain for years. Hence, it is advisable to always select a branch at the back of the bonsai. 

Bend and Grow Technique

This is used on the species that have a tendency of growing lots of side branches. The basic idea is that as the young plant grows and the main stem starts to become rigid and woody, the trunk is bent down and a side branch is put on its place b wiring it straight up. By this repeated bending the main trunk and putting a side branch in its place, a good taper is related with very small and a few cut marks. The disadvantage is that the trunk grows much slower and the taper is not as extreme as in the cut and grow method. 

Trunk Splitting Technique

This should be used only on trees which can tolerate and heal serious injuries- not recommended for the deciduous species or species that are prone to rot. During the growing season, when a tree is beginning to grow new foliage, it is pulled out of the soil, roots are washed, the trunk is split or sawn in half from the most base up to the main body. The tree is frequently misted. Once the split is made, a piece of wood is inserted from the bottom, slowly forcing the split open. The tree is then reported in a larger pot with the wedge still in place and is allowed to heal.