Can anyone tell me the best time to prune a paperbark maple? I have one about 8 feet tall, planted about 4 years ago. It's done well and the bark is quite attractive, but I'm not overly fond of the shape when it's leafed out. Looks like a giant mass of leaves in the summer. I've thought about limbing it up about 3-4 feet to give it more of a "tree" shape. Thanks.