Thickening Bonsai Trunk: A Comprehensive Guide

Care & Tools

To thicken the bonsai trunk, the leaf area and the size of the root system should be increased at the same time through repotting in a bigger pot, growing in a sieve, growing in a field, or sacrificing a branch.

Role of leaf area and root system in bonsai trunk growth

Role of leaf area and root system

There are only 2 aspects of a tree that affect its growth:

  1. the number of leaves, and
  2. the size of the root system.

Leaves are the primary organ for photosynthesis, the process by which a tree converts light energy into glucose to grow and survive.  Trees do not need to eat food as we do; they can create energy by themselves, and the number of leaves on a tree plays a crucial role in how much energy they can produce to grow.

As to the roots, they absorb water and nutrients from the soil, providing the tree with the necessary resources to support growth.  Trees need water (and CO2) to create energy through photosynthesis.  Nutrients absorbed from roots are dissolved in water and move from roots to stems and leaves for growth and reproduction.

So, it seems you either have to increase the leaf area or the size of the root system to make your bonsai trunk bigger.  But it is not the case here.  You have to promote the growth of both leaves and roots for a thicker bonsai trunk.

Interplay between leaves and root systems

The number of leaves on a tree and the size of its root system are interconnected and affect each other’s growth.  A tree with many leaves needs a larger root system to absorb enough water and nutrients to support its increased photosynthetic activity.  Similarly, a tree with a larger root system can support more leaves, which in turn can lead to faster growth rates.

When a tree’s root system is restricted, it cannot absorb enough water and nutrients to support its leaves, which leads to a smaller tree size.  This is what is happening to bonsai trees, which is obviously what we want.  But this means the growth of your bonsai tree’s trunk is very slow when you restrict your tree’s root system.

So, if you want your bonsai tree trunk to grow bigger, you have to increase the leaf area and the size of the root system at the same time.

How to thicken bonsai trunk

There are a few techniques you can use to thicken the trunk of your bonsai tree as the following.

  1. Repot in a bigger pot
  2. Grow in a sieve
  3. Grow in a field
  4. Sacrifice branch

Repot in a bigger pot

One of the easiest ways to thicken a bonsai trunk is to repot it in a bigger, deeper clay pot to expand its root system and subsequently leaf area.  In general, plant leaf area, shoot volume and the size of the root system enlarge as container size increases.

One of the reasons for this is that a bigger pot can contain more soil, which provides a tree with more water and nutrients to grow.  The other reason is, with a larger space in a pot, roots can spread out even more to develop a bigger root system.

The size of the pot is obviously important and so is its material.

Pot material should have good aeration, drainage, and adequate water retention, in addition to its heat absorption and dissipation ability which has a good effect on root growth.  An ordinary clay/terracotta pot has all these characteristics and should be used for the purpose of growing thick trunk.

Grow in a sieve

Growing a bonsai tree in a strainer or sieve is a technique that is used by professional bonsai growers of pine trees in Japan to thicken the trunk of a bonsai tree.  The idea behind growing a bonsai tree in a sieve or strainer is to promote the branching of the roots by exposing them to air.

When roots encounter air, they stop growing because they need to be in touch with the soil to absorb water, oxygen and nutrients.  The energy of that root is then redirected to other roots that are still in the soil, causing them to grow more vigorously and branch out more.  This leads to a finer root system that allows the tree to take more nutrients to grow.

Finer and younger roots are more sufficient in their task.  Water and nutrients are absorbed in the tips of the roots and they are found in greater numbers on younger roots than on older roots.  Also, younger roots tend to be more active in growth and metabolism, leading to more efficient absorption of resources.

So in all, a finer, more ramified root system makes a bonsai trunk grow bigger which can be attained by growing a tree in a sieve.

There is one serious shortcoming of growing a bonsai in a strainer or sieve though; it can cause problems with watering.  Because all of the soil surfaces are exposed to air, the bonsai tree is extremely susceptible to water shortage, especially when it is hot.  If you need to water your normal bonsai tree twice a day in summer, you may need to at least double the amount of watering.

And it is also difficult to keep the soil at the right moisture level for the tree’s health.  It may be either too dry or too wet.

Grow in a field

Field growing is a technique used by professional bonsai growers to grow bonsai trees in the ground in a nursery or natural setting.  These trees are later dug up and properly shaped as bonsai.  As the trees are planted in the ground, they can develop a stronger, larger root system, which helps them grow bigger and faster.

Field growing is done by professionals called “seedling makers” in Japan who specialize in growing bonsai trees in the field.  Instead of planting bonsai seedlings in a container after germinating seeds, which is usually the case, they plant the seedlings in the field (thus the name “field growing”).

But there is a huge problem in growing bonsai trees in the field: the trees risk becoming normal trees in nature if left to grow naturally.  So, the seedling makers place tin plates or roof tiles under the seedlings so that the roots do not grow down deep.

Seedling makers then cultivate the trees, make them big and thick, prune them at the appropriate time, make the root system strong, and sometimes transplant them in another field.  By repeating this process for a decade or more, you can obtain good nursery bonsai trees that are ready to be trained as bonsai trees.

This Youtube video shows field-growing bonsai trees by a bonsai professional.  The video is in Japanese but he says his trees are at least 40 years old and the older ones are in the field for 90 years.

So, field growing is not as easy as you might think and can take quite a long time.  I should add that the work of maintaining the tree can be difficult because it requires a good sense of perspective and has to be done while lying down on the ground.

Sacrifice branch

The “sacrifice branch” technique is a traditional bonsai technique used to thicken the trunk of a bonsai tree.  A sacrifice branch is a branch that is intentionally elongated even though it is unnecessary for the styling of the tree shape.

To thicken the trunk, a sacrifice branch is left to grow without pruning, which enables the tree and the trunk to grow big.  And when the trunk reaches the desired thickness, you remove it.  Hence the name “sacrifice” because you sacrifice a branch to make the trunk bigger.

Here is what a sacrifice branch looks like on a fully-grown Japanese black pine bonsai.

Sacrifice branch

Sacrifice branch (inside the red circle)

Here is what it looks like on a Japanese pine seedling after a few years.

The idea behind this technique is that trees can grow more vigorously when they have more (branches and) leaves because they are able to produce more energy through photosynthesis.

However, if a sacrifice branch is left growing for a long time, the wound after its removal would be large and noticeable.  So, if one sacrifice branch is not enough to achieve the desired thickness of the trunk, you can thicken the trunk gradually by switching from the first sacrifice branch to the second sacrifice branch and so on until you get the desired thickness of the trunk.

Environmental factors affecting bonsai trunk growth

A bonsai tree’s trunk growth is affected by a variety of environmental factors, including light, water, soil, and nutrients.

Sunlight

Light is essential for photosynthesis, and a lack of light can limit the energy a tree can produce.  Most bonsai trees are sun-loving plants; i.e. plants that grow in open places and cannot withstand a long period of time in the shade.  So, most bonsai trees should be kept outside with plenty of sunlight and airflow to grow big.

Water

Water is also essential for tree growth and giving an adequate amount of water is necessary for photosynthesis and the absorption of nutrients.  Remember that too much water is also bad for bonsai trees, causing root rot.  The key is to give water when the soil is half dry.

For more about how to water a bonsai tree, the following post might be helpful. 

“How often to water bonsai tree” (link here)
Soil

The soil is another important environmental factor.  Good bonsai soil must meet the following requirements.

  • Good water retention,
  • Good drainage,
  • Good aeration,
  • Adequate soil pH (slightly acidic), and
  • Low in nutrients.

Normally, bonsai soil should contain little or no organic matter to control tree growth so that we can shape it in a desirable style.  But if you want to thicken the trunk, it might be necessary to mix organic matter more than usual to support the growth.

For more about what is good bonsai soil, please read the following post.

“How to make bonsai soil at home” (link here)
Nutrients

Nutrients play a key role in the growth of leaves and root systems.  Bonsai trees need a variety of nutrients to grow, including nitrogen, phosphorus, and potassium.  Once or twice a month from spring to fall, except for hot summer is the average frequency for normal bonsai trees.  So you might want to add more when thickening the trunk.