How To Get A Thicker Trunk On A Bonsai?

One of the main characteristics of a bonsai tree is a thick trunk. It doesn’t need to be massive, but any sort of thickness will always make a bonsai tree look better. It will help with the scale and make it look much more mature and powerful.

However, getting a thick trunk on a bonsai is often a challenge, especially when you are starting out with humble nursery material or seedlings.

This article will look at the different ways you can increase the thickness of your bonsai trees trunk.

What Makes A Bonsai Trunk Grow Thicker?

The trunk of a bonsai tree will thicken to compensate for an increase in root and leaf growth.  

Basically the trunk is the link between these two systems. Water travels up from the roots to reach the leaves and once sugar is made by the leaves it is pushed back down through the trunk into the roots.

The water and sugars travel up and down the tree trunk in a thin layer just under the bark called the cambium. (Imagine it like a lot of little water pipes going up and down inside the trunk).

The more leaves and branches a tree has, the more water it is going to need to sustain itself.

As this demand for water increases, the cambium has to expand to cope, making the trunk thicker. (The little water pipes will need to expand to send more water up and down the tree).

Every year a new layer of cambium is formed. This is the “growth rings” you see when you cut into a tree trunk.

Not only does this growth help with the water demand, but it also acts structural support.

The more leaves and branches there are on a bonsai, the heavier the tree becomes. The trunk has to thicken to support this weight and make sure the tree doesn’t fall over.

How To Get A Thicker Trunk On Your Bonsai       

So, to get a thicker trunk on your bonsai you just need to let your tree grow. The more leaves and branches it grows, the thicker the trunk will become.

However, you can’t just let a bonsai tree grow indefinitely, or it will just become a full size tree.

We need to use a few techniques that allow us to grow the tree and to allow it to thicken, but also keep it from getting too out of control at the same time, so it is still usable as bonsai.

Trunk Chopping A Bonsai Tree For A Thicker Trunk

For deciduous bonsai trees this is the most common approach. You allow the tree to grow freely for a few years until it reaches a good thickness and then totally chop it back hard.

You are then basically left with a tree stump. As deciduous tree will re-grow on old wood, you will get new shoots all over the stump which can then be grown and trained into a bonsai.   

This is one of the quickest ways to get a thick trunk on a bonsai, but it comes at the cost of leaving a huge cut scar where you chopped it down to a stump.

These scars are usually too big to ever fully heal and often have to be incorporated into the design through carving to make it look more natural.

(You can read more about Trunk Chopping A Bonsai)

Cutting A Bonsai Back To Create A Thicker Trunk

A slightly different approach is to allow the bonsai tree to grow unchecked all season, and then cut it back hard at the end of each year.

After a number of years the tree will still thicken a lot (it will take longer than just leaving it totally untouched for a number of years).

This will also still leave scars, but as you are doing it once a year, they will not be as large. This technique exchanges one large scar for a number of smaller ones.

The smaller scars are more likely to fully heal over, or can be hidden more easily. Some people prefer to take a bit longer and use this technique so they can avoid one huge scar on their trunk.

Using Sacrifice Branches To Thicken The Trunk Of Your Bonsai

This is where you treat the tree like a bonsai apart from one branch which is allowed to grow fully unchecked.

The growth of this one branch will result in the trunk thickening to support its weight.

Once the trunk has thickened, the sacrifice branch is removed. This is where it gets its name, as you ‘sacrifice’ this branch by removing it, in order to benefit the rest of the tree that is left.

This technique is excellent for bonsai trees that will not back bud, like pines. This way the tree can be allowed to grow to the size you need, while other parts are kept small and remain suitable for bonsai.  

Best Place To Grow Your Bonsai Tree For A Thicker Trunk

Now we know our bonsai trees need to grow a lot to thicken them up, we can start to look at ways this can be affected. You want all your bonsai trees to be healthy by giving them enough sunlight and water.

However, where you grow them can massively affect the speed at which they will grow.

Growing Bonsai In The Ground For A Thicker Trunk

Growing your bonsai tree in the ground, or in some sort of grow bed will get you the strongest and quickest growth.

A bonsai tree grown in this way will be have no restrictions and will be able to increase its size quite quickly. The only problem is that the soil can often not be great, meaning the roots can become large and thick, which is not ideal for bonsai.

This can be corrected in time, but it is something you need to think about when deciding to try this technique. The longer it is in the ground, the more likely its roots will get out of control.

(You can read more about – Growing Bonsai In The Ground)

Using A Big Pot To Thicken The Trunk Of Your Bonsai

This is very similar to growing in the bonsai in the ground, but the size of the pot can add some restrictions, which can slow the trees growth down a bit.

This method is a bit slower, but you do get some extra control. Using a large pot means you can add better quality soil, which will help keep control of the roots and will stop them getting too thick.

It is also common to grow a bonsai tree in a ground for a few years and then put in a large pot to transition it from the ground to a pot. The space of a large pot helps with this and also helps with producing the finer roots that you need for a bonsai.

Can You Grow A Thick Trunk On A Bonsai In A Small Pot?

You can try and grow a thick trunk on a bonsai in a small pot, but it is going to take you many, many years.

A small pot just does not have the space to allow a bonsai tree to grow out. I have grown many trees in small pots and they do grow well, but they are still tiny compared to trees grown in larger pots.

If you want to grow small bonsai trees, you can try a small pot. However, I would suggest you grow your trees in a large pot for a number of years and then put them in to a small pot.

This way your trunk will just be more impressive and you will not be stuck with a stick in a pot for decades.

Good Soil Will Help You Get A Thicker Trunk On A Bonsai

No matter where you grow your bonsai trees, you should try and use the best soil possible as this will drastically improve your roots and your trees ability to grow .

Bonsai soil has plenty of small gaps in between the particles, which allows for plenty of air and water to get to the roots. 

It is this constant supply of air and water that will get your bonsai tree to grow better, so you will end up getting a thicker trunk faster.

(You can read more about – A General Guide To Bonsai Soil

Other Techniques To Get A Thicker Trunk On A Bonsai

Letting your bonsai tree grow and then cutting it back is the most common method to get a thicker trunk, but there are also some other techniques that can also thicken the trunk up.

Bonsai Fusion

Fusion is where a number of young trees are planted really close to each other and usually tied together in some way.

As they grow and expand, they grow into each other. As they are constricted in some way, usually by wire, they can only grow into each other. After a period of time the trees start growing together and start to fuse into one tree.

Eventually these many trees will become one. As you can imagine, being able to turn 10 small trees into 1 larger tree is going to create a good trunk. It is almost like you are melting them all together into one tree.

However, as these trees grow into each other, it can be obvious when you look that  them that it is a number of trees together.

It’s almost like you can see the seam where they where stitched together. These fusion marks will soften over time and eventually you will not be able to tell. At this point the trees all look like one tree.  

This method can produce some good results, but it can be hard to do and a lot of people do not like it, they do not see it as a pure bonsai for some reason. like its cheating?  I’m not sure, I think it’s pretty cool.

(You can read more about – What Is Fusion In Bonsai?)

Tourniquet/Wire

This is where wire is wrapped around the trunk of a bonsai and it is allowed to cut in.

As the tree expands and grows it slowly eats the wire up. As the tree is forced to swell around the wire, it actually looks thicker in these sections.

This will only work for bonsai trees with thick bark, which will eventually, hide the wire scars. It is often done on pines.

You also need to make sure that the wire does not dig in and cut the flow of cambium, otherwise this section of the tree will die. You also need to apply the wire in the correct positions or you will have swelling in some parts, but not in others, so you must be careful using this technique as you can easily mess it up.

Branch Splitting

This is a pretty extreme technique where you basically split the trunk of the bonsai down the middle and then allow it to heal over. As it heals and the tree fills in the split area, making it become thicker.

To be honest, I can’t even recall if I have actually ever seen this done and I wouldn’t recommend it.

It seems very risky and will probably result in some crazy looking scars down the trunk. The fact that I can’t remember seeing it done makes me think it is not very effective unless you really know what you are doing.

Conclusion

Growing a thick trunk on a bonsai is easy, but it takes time. You can try and speed it up by letting your tree grow like crazy for a few years while in the ground, but it still takes time.

There are no real short cuts in bonsai, so you will have to wait. However, if you take the time, you will be rewarded with a great trunk.