Did you ever read
this topic by Scriblette? It's a great read, all of it. It might give you some insight about how to better go about structuring trees and such.
What you have her so far isn't too bad, I'd go in a direction similar to how heyy13 does it. Are you using these graphics for a game, by any chance? Or just practice?
There are a lot of different ways to construct and render the tree, so you might benefit from trying different styles. If you're not using it for a game that is, if you are, you'd want to stay consistent.
Also, if you try something new, making it slightly larger might not be a bad idea, but it's up to you.
I really don't enjoy the spheres approach, either. But if in the end-result it is well concealed then good. I just encourage you, to, once again, try different methods.