How to Build and Strong and Thick Bonsai Trunk

  • By: Josh Koop
  • Date: July 27, 2023
  • Time to read: 6 min.
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Narrow or weak trunks are common problems gardeners run into when it comes to Bonsai trees. Fortunately, there are many different ways you can promote tree trunk growth and thicken your Bonsai . Remember, weak trunks can impact the longevity of your Bonsai and cost you the tree entirely.

The best ways to thicken a Bonsai tree trunk is to choose one, or a combination of these methods:

  • Growing a sacrifice branch
  • Splitting the trunk
  • Transplanting the tree
  • Cutting back the trunk
  • Inducing swelling
  • Intentional bark scarring

While Bonsais have a beautiful and ornamental quality, it is important to remember these are still living miniature trees. This means a weak trunk or an improperly thickened trunk might kill your Bonsai .

Here we’ll explore the correct ways to promote trunk growth and what that means for a healthy Bonsai tree.

Promoting Healthy Trunk Growth In Bonsai Trees

Bonsai trees can be created from almost any perennial woody-stemmed tree or shrub species, though some species are more popular than others.

It is important to consider what type of tree your Bonsai is before deciding the best course for trunk thickening.

The main point of the art of Bonsai is to use specific cultivation techniques to shape miniature trees in such a way they mimic the shape and scale of a full-grown tree.

As this often means restricting the roots, it can be difficult to promote even growth which can lead to thin trunks, discolored leaves, and other problems.

By promoting healthy trunk growth, you can avoid many other worries associated with Bonsai cultivation.

Growing Sacrifice Branches

The best way to thicken the trunk of a Bonsai tree is through the growth of a branch that will eventually be removed entirely. This is a great method because it works on nearly any species and won’t have a drastic impact on your tree.

Any low branch can be used as a sacrifice branch but it does help to keep in mind, as this branch will eventually be removed there will be a scar and a wound in the bark for several years.

With that in mind, it is usually a better idea to choose a branch on the back or side of your tree.

You will want to wire this branch sideways and away from the tree so that as it grows, the attachment to your trunk will be small and in turn a smaller scar when it is time to sacrifice the branch.

Then it’s as simple as letting the branch grow until the trunk has reached your desired thickness. It is important to note, this method only works for the trunk below the sacrifice branch, anything above the sacrifice branch will not thicken.

If there are many areas along the trunk that you want to thicken it is recommended you do multiple sacrifice branches starting from the highest point you want to thicken and then moving down.

Splitting The Trunk

While the sacrifice method takes several years to show the results Bonsai cultivators might be looking for, this method will show results quickly.

This technique is as easy as it sounds but while you will see a difference almost immediately it can take years for the trunk to fully heal. With this method, you also need to make sure that your tree species can manage such a dramatic injury.

If going this route, for best results it should be done at the beginning of a growing season and you will need a large pot to let the bonsai heal in.

For this process, you simply remove the Bonsai from its pot, clear the dirt away, and then using a saw you split the trunk directly up the middle from the roots until you reach about halfway.

When you replant the tree you will want to use wooden wedges or wires to keep the trunk halves separate.

Allowing your Bonsai to recover from this procedure in a large pot will encourage faster healing, it is also suggested that you allow your bonsai a wild growth season with no training after splitting the trunk.

After the wild season, you can trim your Bonsai’s roots and place it back into a small shallow pot.

Transplanting The Tree

Since Bonsai trees are simply full-size tree species that are cultivated to stay small via restriction in a small pot, reasonably if you want them bigger the answer is simply a bigger pot.

If you remove your Bonsai from its small pot and transplant it into a larger one for a short time you will experience faster growth and a thicker trunk.

Some gardeners will plant their Bonsai trees outdoors for a season if the species of tree and weather permits, but if that isn’t an option a pot five to six times larger than their regular pot should encourage growth.

If choosing the outdoor method it’s important you consider weather, temperature, soil,  pests, and diseases.

Planting a Bonsai outdoors without considering those key points could kill your tree so think carefully before choosing this option.

Cutting Back The Trunk

This method is one of the best for creating thick, tapered Bonsai tree trunks. Conversely to other methods we’ve mentioned, this one is most often used on trees grown outdoors directly in the ground.

This technique involves letting the tree grow until the base of the trunk has reached your desired thickness. At this point you’ll make the first cut, even if the tree has grown over 5 ft. tall, you’ll cut the trunk down to just a few inches.

After the first cut, you continue to let the tree grow and will likely make at least three more large cuts for a realistic look.

Using sacrifice branches may also be important to add thickness to the trunk of the tree, but letting them get too big will result in big scars on the trunk so choose carefully what branches you allow to grow.

Inducing Swelling

An older but effective method used in Bonsai maintenance is called the swelling or tourniquet method. This is when you take a wire tourniquet and place it around the base of a tree to induce swelling and promote thick trunk growth.

This is a relatively quick method and usually will produce results after one season. The drawback of this is it will leave a noticeable scar on the trunk and is only useful for creating a tapered trunk.

The method does not work to induce thickness anywhere on the trunk aside from the base where the wire is tied.

Intentional Bark Scarring

Certain species of trees can have their trunks thickened through the intentional scarring method. This usually works best with pine or juniper trees.

With this technique, you use a knife to create vertical carve lines on the trunk of the tree through the bark to the sap layer. The tree will produce “scar tissue” to heal from the wound which thickens the trunk.

It is best to not exceed three of four cuts a year to make sure the tree is able to properly heal.

Will These Methods Hurt My Tree?

The species of tree used for the art of Bonsai are often hearty and while some of the methods above might seem counterproductive for overall tree health, done correctly they won’t hurt your tree.

It is much better to split a Bonsai trunk and promote growth and the ability to transport nutrients better than to slowly let a Bonsai starve or struggle with a weak or thin trunk.

 If done correctly and in moderation, these methods will cause a bit of stress on your tree for a short period of time in order to give them a strong healthy foundation to grow for decades.

Final Thoughts on How Do I Thicken My Bonsai Trunk

In the information above there were multiple methods provided to give you an opportunity to build up your trunk for your bonsai tree.

I think there is a balance to be struck in the look and aesthetics and not all trunks should be widened so make sure to think about where you are going so that you make the best decision!