I’ve had a bougainvillea for several years that I’ve trained into a tree that is about 6 1/2 to 7 foot tall. Can that be cut down to make a bonsai? I’ve been reading on the techniques you say, but they don’t seem to touch on this one, I would appreciate any information you could give me about this. I have attached two pictures so you can see what I’m talking about. Thank you.
Lea.

No picture atached.

Yeah, I think you could. There seems to be a decent trunkline here (But keep in mind, bonsai is 3D, pictures are 2D):

Keep in mind, bougainville is VERY slow to close big cuts.

Hey Lea.
Yep, Bougainvilleas are very popular here in Brazil, and people do cut them back, like LB said, they're slow healing cuts and will need special attention on this terms, but yes, can be done, and they root big cuttings, so probably you can propagate it from the prunning leftovers.