How To Make A Bonsai Trunk Thicker?

Over the last few years of dabbling with the art of Bonsai, my eyes have always been drawn to any species of bonsai tree with a nice thick trunk, yet my trees have thin trunks. This prompted me to figure out, how can you make a bonsai trees trunk thicker?

Bonsai tree trunks will only thicken up when the tree is growing a lot. A thick bonsai tree trunk can be achieve with a sacrificial branch, trunk merger or the bend to grow technique. Let’s find out more… 

The most common techniques to thicken a bonsai tree trunk are:


-Growing the bonsai tree in the ground

-Trunk Merging

-Growing A Sacrificial Branch

-Cutting The Trunk

-Bend To Grow


1. Growing The Bonsai Tree In The Ground


In simple terms, the reason any bonsai tree is a miniature, rather than a full size tree, is because the root growth is being restricted by being in a small pot. Based on this logic, if you were able to remove the bonsai tree from the pot and plant it directly in soil, the root mass will develop and the trunk will eventually thicken up, a lot. Note, I say ‘eventually’. This really won’t be a quick process by any means but if you aren’t in any sort of hurry, it will definitely thicken up your bonsais trunk. 


A downside of growing your bonsai tree in the ground is there are a lot of possible problems or complications with this such as..

-The nutrients in the soil could be low quality 

-Pests and disease


-Lack of drainage could cause the roots to rot



Make sure you ensure that your specific species of bonsai could actually survive living outside in the soil all year round, before you make the decision to remove it from the bonsai pot. If the bonsai tree species can usually survive in nature, in your climate, you will be fine to grow it outdoors. Should you be say growing a sub-tropical bonsai whilst living in a cold climate, this process is just going to kill your tree and shouldn’t be attempted. If you want to know more about this, have a read of my article: How To Make A Bonsai Tree Grow Faster.

2. The Trunk Merging 

Trunk Fusion – Photo By Swindon Bonsai Society

Merging the trunk is actually the fastest way to achieve a thick bonsai trunk, however, it will not work on an existing bonsai tree. You’ll essentially need to grow a number of sapling trees around a conical object, with them tied as tight to each other as possible using a light rope. During the growing season, the saplings will of course grow dramatically, but with a limited area to go, the trunks will essentially merge and become, aesthetically, one tree. Ideally you want to slightly interweave the saplings from the beginning, to avoid any one sapling dying later on and ruining the trunk. 


This method isn’t too simple for amateur bonsai enthusiasts, however, it definitely has the perks of being fast and not leaving any scarring on the bonsai trunk. On the flip side, it won’t actually have anything inside the trunk, due to the way the saplings are grown. In the future this can lead to other complications that come with growing a hollow trunk bonsai tree. 


3. Sacrificial Branch

Having a sacrificial branch is definitely my favourite way of thickening up a bonsai tree trunk. However, much like the other methods I’m sharing with you, it has its pros and cons. The sacrificial branch techniques revolves around growing out a side branch near the base of the trunk, which will thicken up and feed the trunk a large amount of nutrients. These nutrients will then encourage the trunk itself to thicken up a lot, without having to do anything such as growing the bonsai in the ground. 

Of course though, you won’t want the big sacrificial branch as part of your bonsai tree once the trunk has thickened up enough, so you’ll have to remove it. It’s a very simple process to remove the branch with a pair of clean concave cutters, however, it’s no doubt going to leave a pretty big scar on your bonsai. With that in mind, it’s best to plan forward and ensure that your sacrificial branch is at the back of your tree, to minimize the effect on the aesthetics in the future. 


4. Cutting The Trunk 


For this technique it’s best to grow the tree in the ground. This is definitely one of the best ways to create a thick trunk and a nice taper to your tree. The first large cut you’ll need to make is at the point along the trunk where it has reached the thickness you ideally want from this tree. It’s important to make sure that you don’t cut this too low or too high on the tree, you’re really looking at the cut being at about one third of the desired height of the tree, no lower. 


The next step is to wait until the tree starts growing branches. You’ll need to pick one branch to wire up as vertically as possible, as this branch will be creating the next stage of the trunk. Make sure to keep growing the other branches sideways too for the immediate future, in order to thicken up that lower trunk. 


5. Bend To Grow


This is definitely a good way to grow a bonsai with a nice thick trunk, with more or less no scarring on the bonsai. Bend and Grow technique will only work with a species of bonsai that grows a lot of side branches, as that is what you’ll be using. Firstly, you need to let the tree/sapling start growing out until near the end of the growing season, at which point you’ll want to bend the main trunk downwards, whilst bending the most vertical branch upwards in its place. 


Over the months, this will give the bonsai a lovely taper as the side branch will thicken up slower than the main trunk. After a while when the bonsai is looking as desired, you could either leave the main trunk in place for a while as a sacrificial branch (as talked about above), or trim it off. The issue with trimming it off is of course the large scarring that will be obvious on the bonsai. My advice would be to use some concave cutters and try to prune the tree to ensure that the scar is at the back of the tree. 

How To Thicken Bonsai Trunk Video

Final Thoughts


These are the most popular ways to thicken up a bonsai tree trunk. Personally I have gone down the route of a sacrificial branch and growing in the ground, with a new tree, rather than any of my current trees. It’s worth noting that if you’re going to be growing a bonsai tree from scratch to try and achieve a trunk trunk, you’ll most likely want to use a fast growing species like a ficus. By providing the optimum growing conditions, fertilizer and frequently repotting you’ll be able to grow a bonsai tree faster, to achieve a thicker trunk naturally. 

If you have any experience in thickening the trunk of a bonsai tree, I’d love to hear about how you did it in the comments!

Relevant Articles – How To Make A Bonsai Tree Grow Faster / How To Prune A Bonsai Tree


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