One thing I've found out about bonsai, it is always a work in process. By planting a tree back in the ground will increase trunk size. A bonsai pot is too restricting and slows growth. As far as trunk tapper, the entire trunk should have a nice flow to it. This not always easy to achieve. Two good video series on youtube are orlandobonsai and bonsaistudent. Both of these gentlemen I've learned a great deal from. Bonsaistudent has a set of videos in which he does a time clip of replanting a tree. It is amazing in a year, how it thickens the trunk and branches. I hope this helps.

I completely disagree with you Jason. Constant pruning on a tree and keeping it confined in a tiny pot would serve no purpose for thickening the trunk. Planting it in the ground and letting it grow wild is the right way. You want it to go crazy with growth. The more leaves, the faster it'll grow and thicken.

I agree with Ryan. Think of a trunk as a river. The more tributaries (branches/leaves) it has, the faster it widens. More foliage = more food produced = more files of cells laid down in the cambium = more thickening. The root restriction, high temperatures, and other negative cultural conditions associated with containers inhibit caliper increases. Plant your tree in the ground if possible for rapid development of the trunk & primary branches. Move the tree to a pot for finer ramification. If you can't put the tree in the ground for it's early development, use a very fast, well-aerated soil and a very large container. Let the roots run into the ground through the container bottom whenever possible.

Al

Agreed, the only way to thicken the trunk is to drastically increase the amount of weight it has to support, which means letting the plant grow as large and lush as possible. You can't do that in a bonsai container, it needs to be out in actual dirt where it can really branch out (literally, no pun intended).

What works for me is, I bury a dinner plate upside down and plant my tree over it for a year. Spreads roots and helps taper.

For Serissa I was told not to cut the leader branch which will make the trunk thicken.

Nancy

If you want to induce the taper you need for your bonsai to look proportional and believable, you'll need to plan some strategic trunk chops.

How much weight a trunk supports strengthens the trunk because the added flexing stimulates lignin production, but it doesn't thicken the trunk. Thickening is id a direct relation with cell files laid down in the cambium. The more food production there is, the more cells laid down in the cambium. So, more foliage/light = more food production = more cells laid down in the cambium = a fatter trunk and branches.

Unless you're purposely after a wispy tree or literati style, it's difficult to develop a good tree without trunk chops. It's POSSIBLE with skillful restriction of top growth over a long period of time, but generally planned chops are the preferred method of inducing taper.

You can see by the cut paste where this maple has been chopped once. It will be chopped again and the main trunk removed at the crotch between the main trunk & the little branch wired up as the new trunk. Then, I'll select another small branch low on the wired branch as the new leader, and let the rest of the wired branch grow wild - to thicken everything below it. This is how we build taper into our trees.
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Al

The first thing to remember is: Trunks will only fatten up when the plant gets a lot of growth. Bonsai trees in small pots where we effectively limit the growth will not show a lot of growth in trunk thickness. Growing trunks mean growing your tree. This is one of the main reasons why –as a rule of thumb- serious bonsai growers will start off by worrying about the Nebari and trunk, before growing a canopy. Effectively, you start building a tree by creating a good root structure, then a good trunk and then the branch structure.

That's not true. An increase in trunk/branch caliper is an integral part of any growth. There is a direct relationship between growth rate and thickening of the longitudinal axes because cell division drives both additive and multiplicative growth. Slow growth simply means the trunks and branches thicken more slowly than during periods of robust growth, but thicken they do - which is why bonsai artists must be very vigilant because wire quickly bites into the periderm as branches and trunks on even highly refined bonsai with congested roots in very small pots thicken.

I agree with what you said about nebari. For trees other than conifers, it's the primary consideration when selecting a tree for purchase. Building the tree is easy, building a solid nebari - not as easy. Note the roots on the tree above. I was dissatisfied with the roots it was on, so rather than go through the trouble of fixing what was there, I layered the top of the tree & ended up with an impressive root system (for a little squirt of a tree).

Al

This can be a long process requiring patience, at times it can impair the look of your tree temporarily until the desired effect is achieved. For instance on this Ficus Retusa, the aerial roots give a very unique style which I love. However by trimming the aerial roots the trunk will thicken giving it the desired taper effect. This will temporarily take away from the style of the tree however even more roots will grow back long term and you will achieve a strong thick trunk. I go more in depth on trunk thickening on this section of my page.

Trunk Thickening