Beginners Corner - Roots and Trunks

by Dorothy Franz


There are two aspects regarding the roots that we need to look at when we start the development of a bonsai. The first is of horticultural importance - the life of a tree; and the second is the visual importance - the design of the bonsai.

Horticultural importance:

The soil a tree finds itself in is the source of food and nutrients for that tree and in bonsai we provide these nutrients. Our main purpose is to produce growth through encouraging and developing a network of fine roots. This is done during repotting by giving the tree a haircut or a root pruning. Pruning encourages secondary and tertiary roots the same as happens above the ground when one prunes branches. It gets rid of old and dysfunctional roots to be replaced by active feeders through the development of fine root hairs. The fibrous roots are the ones that feed the tree. The thick roots are the storage, which in bonsai are unnecessary as we are providing all the nutrients by fertilising, watering etc. By cutting away the old and encouraging the new, the tree has a perpetual young root system.

Regular root pruning relieves congestion in the pot, thus improving porosity and air circulation. The soil mixture also provides porosity by the stones we mix into our soil.

The amount of ramification one is able to achieve below the soil is directly proportional to the amount of ramification above the soil so it stands to reason that if you achieve a very well developed fibrous root system below the soil you will have a healthy and potentially wonderful tree.

The other function of roots is to anchor the tree, which it cannot do if one cuts off the main thick storage root and one, has no fibrous system. It is sometimes necessary to tie the tree in to the pot when rep otting for the first time because of the absence of sufficient fine roots.

Visual Importance:

In bonsai, the roots nearest the trunk are of great visual importance. It is usually the main reason why one selects a particular view as the front. The spread of these roots is also what visually ages a tree.

When selecting material your first consideration is what lies beneath the soil. This will give you an idea of the size of the base. The base should create a feeling of stability and power, as it is the part that visually anchors the tree to the ground. One should be able to see the beginning of the roots at the base of a tree and they should fan out. This creates a very powerful impression. A buttress at soil level is also the beginning of the taper of the trunk.

How to achieve a good root spread:

In young trees you can tease out the roots and spread them. Applying fertiliser round the edge of the pot encourages roots towards the fertiliser. Placing the tree in a large shallow container and/or placing a flat plate of some sort under the base will force roots to spread out horizontally.

In older trees you can wire the offensive root in a better position. Roots make bark when they are exposed to air and light and become thicker. To create a more pleasing look split a thick root in two and place an obstruction like a stone, piece of wood or cork in between to keep the sections apart.

Encourage thickening in a particular root by allowing a branch to grow just above the root a little above soil level.

This will cause thickening immediately below it.

Making longitudinal cuts right through to the hard wood will cause the cambium to expand forming scar tissue and more rapid thickening will take place.

There are several ways by which one can encourage roots where there are none.

  1. Method 1 -Grafting. The easiest way is to make an approach graft. The best time to do this is in spring. One cuts a "v" in the trunk where you want a new root. Obtain a rooted seedling of the same species. Apply root hormone to the cut-out and place the seedling in the "v". It should fit in snugly. Bind it well or screw them together with brass screws. Seal all around the wound and allow it to grow for a season. This method is particularly successful with deciduous trees. It will take considerably longer with conifers.
  2. Method 2 - Drill holes. Drill two or three holes in the trunk where you need roots. Apply root hormone to the end of a matchstick and plug the holes. Cover with soil and wait until roots appear. Holes must be plugged and covered otherwise they will just callous over.
  3. Method 3 - Split the bark. By splitting the bark and lifting it gently instead of drilling holes one can achieve the same result. Again apply root hormone, place a pebble to keep the flaps lifted and cover with soil.

If the tree doesn't have enough of a splayed base you can improve it by splitting it in two. Cut from underneath the root ball. Take out a section and ease the two sections apart. Insert a plug to ensure that they stay apart.

To sum up, the aim is to have strong surface roots of differing thicknesses, radiating out at irregular intervals from all around the base of the trunk at ground level. Below the surface the roots should subdivide into secondary and tertiary roots forming a network of fibrous roots to sustain the tree.


Horticultural Aspect

The trunk of the tree has two main functions from the horticultural point of VIew.

  1. To support the tree, it's branches and flowers.
  2. To transport and distribute water and dissolved nutrients, which have been absorbed by the roots and to take the food produced by leaves by photosynthesis to the rest of the tree.

The inner layer of cells in a tree, called the xylem, carries the water and nutrients. These cells are continually dying and being renewed outwards, which means that the dead cells form the hard wood in the centre supporting the tree. This continuous renewal of cells accounts for some of girth of the trunk. The second layer of cells just underneath the bark called the phloem, moves the food manufactured in the leaves.

Bonsai Aspect

In bonsai the trunk line determines the style of the tree and gives the tree it's character which is then enhanced and broadened by the branches.

As you all know, bonsai styles are named according to the trunk line. I am not going to go into styles as this information is easily available in every bonsai book. What is important is that one determines the trunk line at the outset and then develops it. Trunks should be visible from the front for at least two-thirds of their length. If one cannot see the trunk clearly then one effectively has a bush and not a tree.

Trunks should have no visible pruning scars in the front unless they can be hollowed or carved so as to make them a feature.

A bonsai should have a firm broad base and the trunk should taper to the top. The apex should bend slightly forward. Be careful not to bring it too far forward otherwise it will look as if the tree is falling out of the pot. Bring it forward and then up.

Eventually the height of your tree should be roughly one-sixth the diameter of the trunk.

In practice, one initially decides the height either because suddenly at some point in the trunk there is no taper so you cut the tree at that point and begin with a new leader by taking a suitable branch nearby and wiring it up. Or, one comes to a point in the trunk which is boring, dead straight and out of character with the lower section. One then needs to cut off the upper part at that point and start with a new leader.

Increasing Girth

There are several effective ways of increasing girth:

  1. The most effective way of achieving girth is by planting in a large container or in the ground, feeding copiously and trimming periodically. The biggest mistake made by novice growers is to place a tree in a bonsai pot too soon. This has a short-term effect. One has a presentable bonsai but it doesn't grow rapidly into a wonderful tree.
  2. Make vertical cuts in the cambium, which stimulate growth to repair the scar.
  3. Let the apex grow wild and cut it back every year until you have achieved the required diameter. This will increase girth for the entire length, as food is moving all the way up and all the way down.
  4. If one needs to thicken the lower part only, allow sacrifice branches to grow wild just above the area that needs to be thickened. This is very effective in deciduous trees. Make sure the sacrifice branch doesn't get too thick otherwise you will have an ugly scar. Also try to keep these branches in inconspicuous places, like at the back or side. They need careful monitoring. Sacrifice branches can be used anywhere in a tree. Wire this branch upward for maximum result. The growth is rapid so be very observant and cut it off in good time before it is too thick. One may get thickness where it is not required or these branches may cause weakness in other branches so be very vigilant. Do not leave them to grow for more than one year. Remove them and start with a new sacrifice branch until the result you require is achieved. In conjunction with allowing sacrifice branches to thicken certain sections, one can keep over vigorous sections back by trimming well and partially defoliating. In this way you can regulate the areas of growth.
  5. One can try flexing the trunk regularly to increase girth. This works like stretching. Do not try it on conifers or any tree that produces resin. Also only flex in one direction not in all directions.

Another way of achieving an interesting result is by cutting the trunk at the apex and growing a new leader. Keep doing this continually until you have built up your tree.

Shaping of trunks by wiring can be quite difficult if they are fairly thick and rigid. The best method is to wrap the trunk with wet raffia before wiring, which will give it some protection. Clamps can also be used for heavy bending.

There are certain trunks that are considered unattractive which one should avoid namely:

  • Cylindrical trunks with no taper
  • Potbellied thickening above the soil or pigeon breast.
  • Repeated zigzags or curves
  • Trunks leaning away or backwards
  • Ugly visible pruning scars.

The aim is to develop a broad base and a trunk, which tapers evenly towards the apex and a trunk with interesting movement setting the mood of the tree. The branches are then developed to expand the design.


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Sunlight dripping from tree

Slides off the periwinkle

And splashes on ground

Random Bonsai Tip

You can bend thick, hardened branches by undercutting. A wedge is cut underneath where the bend is needed and then the branch is eased down anw wired into place. Thick, coarse branches could also be removed completely and replaced with new branches by thread grafting or approach grafting