If I want to grow a bougainvillea in a pot or hanging basket, how big dies it need to be? Relatedly is there a metric for the size if the pot to the bougainvillea?
Lastly, I've always perceived bougainvilleas as shrubs, but I saw a video with one shaped like a tree. How would I make it this shape?
There are many different varieties with different growth habits, but they don't really mind being grown at any size from a bonsai to a 25-foot-tall tree.
If you want a hanging basket, you probably want a variety that grows as a vine rather than a bush or tree, so it will naturally hang down.
The minimum size of container is about 30cm diameter. You should be able to keep the plant pruned back to about 1 meter tall grown like that. They don't mind being pot bound so long as you give them enough feed. Prune after flowering or in early spring.
