How To Make A Bonsai Tree Trunk Thicker

When growing a bonsai tree, one of your goals is to grow a thick tree trunk. A bonsai’s trunk serves two purposes. First, the trunk supports the entire tree against the elements. Second, the trunk transports water and nutrients to the foliage and moves carbohydrates from the leaves to the roots. A thick trunk will ensure the longevity of the bonsai plant.

In this article, we will provide an overview of how to make a bonsai tree trunk thicker.

To thicken a bonsai tree trunk, the plant needs to experience a lot of growth. That means the tree must be removed from its shallow pot to encourage rapid growth. In this article, we will explore the common techniques used to thicken a bonsai tree trunk.

The techniques are:

  • Growing in the Ground
  • Sacrificial Branches
  • Trunk Merging
  • Cut Down the Trunk
  • Split the Trunk
  • Bend the Trunk

Trunk Thickening Method: Growing in the Ground

Bonsai trees are not a special kind of tree. They are full-size trees that have been restricted in a small pot to keep its miniature size. If you remove a bonsai tree from its pot and plant it directly in the ground, it will ultimately grow to its normal size. It will take several years for the tree to grow to its full-size, so many bonsai gardeners will plant their trees in the ground for a short time to encourage trunk growth. Once the tree trunk is at the desired thickness, gardeners will remove the bonsai from the ground and put it back in a shallow pot to stop the rapid growth. 

It can take years to thicken a bonsai truck using this method. Gardeners who use this method prefer to use fast-growing tree species such as Chinese Elm or Ficus.

You also need to evaluate and solve potential problems when you plant your tree in the ground.

Weather & Temperature

Before growing your bonsai tree in the ground, make sure your bonsai tree can survive living outside all-year-round. Tropical trees that require high humidity and warm weather will not survive harsh winters. 

Soil Nutrients and Drainage

If you plant your bonsai tree in soil that is not properly fertilized, it may not grow properly. If the soil does not offer proper drainage, you risk root rot which may ultimately kill your bonsai.

Pests & Diseases

Planting your bonsai tree outside exposes it to pests and diseases. Routinely check on your plant so you can treat it quickly if needed.

An alternative to growing a bonsai in the ground is planting the tree in a large pot. Growing a bonsai tree in a large pot provides the gardener more control during the rapid growth period.

Trunk Thickening Method: Sacrificial Branches

Another popular method to thicken a bonsai tree trunk is using sacrificial branches. The sacrificial branch technique focuses on growing a side branch near the base of the trunk that will feed the trunk a lot of nutrients. The nutrients will encourage the tree trunk to thicken up. The side branch will grow out while the rest of the tree is pruned and kept restricted. Once the bonsai trunk has reached the desired thickness, the sacrificial branch is removed so you can continue to focus on pruning and shaping the tree. The sacrificial branch technique does not require growing the bonsai in the ground and is a great option for gardeners with limited space.

It will take several years to thicken a bonsai trunk using this method. Any side branch can be used as a sacrifice branch. The branch you select will be very thick and will leave a significant scar. Gardeners work for several years to close the wound on the bark. When using this method, plan to grow the sacrificial branch in the back of the tree to minimize the scar’s effect on the aesthetics.

To grow a sacrificial branch, you will need to wire the branch so it stays straight. Choose a branch that is halfway up the trunk. As the branch grows, it will add girth to the trunk below the attachment point. Anything above the sacrificial branch will not get thicker. This is a good way to develop a tapered trunk. Many gardeners grow multiple branches to obtain a smoother tapering in the trunk. For a better taper, choose higher branches first and then moving to lower branches over time.

Trunk Thickening Method: Trunk Merging

Merging the bonsai trunk is a fast way to develop a thick trunk. This method, sometimes called trunk fusion, will not work on an existing bonsai tree. Instead, you’ll need to tie together several sapling trees so they can eventually merge and create the illusion of becoming one tree. Trunk merging works best with fast-growing tree species like Chinese Elm or Ficus.

Trunk merging is not a traditional way to thicken a bonsai tree trunk, but many gardeners use this method because it is a fast way. Merging can occur after a few months of the saplings being tied together. The trunk merging method gives gardeners more control of the tree design. They can plan the size and begin tapering early.

There are some disadvantages to using this method. If any of the saplings die during the fusion process, it can leave gaps in the trunk and lengthen the fusion time. You’ll need to set aside a few seedlings to replace any dead trees. If you choose not to use a rooted cutting from a parent tree, you risk merging saplings with different characteristics. This will ultimately affect the aesthetics of your bonsai tree as each side of the tree can have different foliage. One sapling can dominate the resources and cause other saplings to die off. It’s important to ensure there is enough light, water, and nutrients to encourage growth in all the trees.

Trunk Thickening Method: Cut Back the Trunk

Cutting the bonsai trunk back is one of the best methods to create a thick, tapered trunk. This technique works best when your tree is grown in the ground. As your tree grows, you will cut down the trunk to encourage it to get thicker and tapered.

Once you see a part of the trunk where it has reached the desired thickness, you will make the first large cut. This will be about one-third of the desired height of your bonsai tree. Your tree may be very tall, sometimes up to 10 feet tall, before making the first cut. You will cut down the trunk to several inches. After that, you will let the tree grow out again.  

After the first cut, keep your tree in the ground and let branches grow. You will use the sacrificial branch technique to encourage more thickening at the lower portion of the trunk. You will continue to make more large cuts as the trunk thickens. For a realistic look, you will need to make at least 3 major cuts. In the end, you will be left with a strong, tapered trunk.

Cutting back the trunk leaves, the bonsai will have big scars that will take several years to heal. It can be difficult to establish a formal upright or broom-style design using this design.

Trunk Thickening Method: Split The Trunk

Splitting a bonsai trunk is the quickest way to achieve trunk thickness. With this technique, you will physically split your bonsai trunk down the middle and use wires to keep the two halves of the trunk separate as it heals. Splitting a bonsai trunk produces almost instant results. It should only be used on trees that can tolerate this treatment. Recommended tree types are Juniper, Chinese Elms, Ficus, and Boxwood. This method should be implemented at the beginning of the growing season.

You will remove the bonsai from its shallow pot and hold it upside down. Using a saw or rotary tool, begin to split the trunk down the middle until you reach the halfway point. When you re-pot the bonsai tree, use wires or wood wedges to keep the two parts of the trunk separate. It’s best to plant the tree in a large pot to speed up the healing process. This will give your bonsai tree the illusion of a thicker trunk immediately. It will take years for the trunk to fully heal.

Trunk Thickening Method: Bend The Trunk

Bending the bonsai tree trunk is a great method to thicken the trunk without having large scars. This technique focuses on tree movement and flow. It’s designed for tree species that grow a lot of side branches. You will need to start with a tree sapling. It is difficult to bend the trunk if it is too thick so it’s best to start with a young tree.

At the end of the growing season, use wire to bend the main trunk downwards and bend the side branches upwards at the same time. Over time, the side branches will thicken up slower than the main trunk and create a nice tapered look.

Final Thoughts

Developing a thick trunk is essential for the aesthetic of a bonsai tree. A good bonsai trunk will be thick, tapered, and look aged. There are several techniques that a bonsai gardener can use to achieve a thick trunk. Many techniques will take years and some will result in heavy scarring. There are many considerations in determining which technique is appropriate for your bonsai tree. With proper dedication, patience, and practice, any bonsai gardener can learn how to make a bonsai tree trunk thicker.  

How To Make A Bonsai Tree Trunk Thicke

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