BRT grow like weeds. I have to keep mine pruned regularly or they get out of hand, even in winter.
However with all that growth, the trunk is relatively slow to thicken, particularly if you live in the northern latitudes where the outdoor growing season is short and the temperatures cooler compared to places like Florida.
If your starting out in a more conventional type pot (ie flower pot, fairly deep, narrow) thats fine, but at some point you are going to want to put it in a wide, shallower pot to encourage the roots to grow laterally instead of down. To thicken the trunk, you will need to let them grow some IMO. They will thicken if you keep them cut back but they will thicken slower. BRT are heavy feeders to support that growth so keeping them regularly fed with fertilizer will help too. If you are in a northern climate where you need to keep them inside in the winter, you absolutly must give them good supplimental light if you want them to grow well.
I have 4 brazilain rain trees, two are pretty much already with a decent sized trunk.
The best example I can show of growing a trunk on a tropical is my willow leaf ficus.
This picture was in 2016 before I started to select and prune for final branches, its even thicker now.
The tree was the size of a pencil when I got it.
I repotted it into bigger bonsai training pots as it grew and now its base is about 2.5 to 3 inches.
Sorry I dont have an updated picture at the moment. Thought I had taken one this year but I guess I forgot to.
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