Hello - I have several mexican bird of paradise shrubs with beautiful yellow flowers that I planted a year ago. I can't find any information on when I should start pruning them to tree form. They are all between 3 and 5 feet high now and each have several trunks. Should there be all these separate trunks for a tree? I don't know how to make them into nice bushy trees and looking on google has not helped me. Right now they are not very bushy. For example, the tallest plant has a single 5 foot long stem. I am using them as a screen. Thank you.