I’ll bet the English were the first to take tropical and subtropical plants and trees and try and keep them alive in a colder climate by bringing them into winter shelter. My only reason for thinking that is because of the reputation that the English still enjoy as your quintessential over-the-top garden lovers. I could be wrong, but I guess it doesn’t matter. Whoever thought of it first really did the rest of us plant lovers a big favor and I for one will be forever in their debt. But just because we need to shelter our plants from the winter, it doesn’t mean that they have to hide from the summer too. All my plants spend the summer outside and that’s especially true of my indoor bonsai trees.
The one thing though that you absolutely have to pay attention to is the night temperatures. Don’t bring your trees out too early and don’t leave them out too late. I have a few horror stories from making both mistakes. Just because the nights are frost free, don’t take it for granted that your tropical bonsai tree won’t be seriously set back by a night in the 40’s.