I don't have time (perhaps in the future), and maybe someone has already tried this, but I was thinking that Weigelia might make a flowering bonsai. The reason why is that I took cuttings in October (zone 5) and have been wintering them over in my house. One stem is in bloom.
The only drawback might be, from other threads I've read, is that it has fairly large leaves. Very easy to root cuttings though to start with.
The new little My Monet Weigilia would be perfect for Bonsai
