I'm still a novice at gardening, so I need advice. Can I prune my Boomerang Lilac during the summer? I did cut back the old blooms this spring. Now, I want to prune it to keep it from getting too big for the space in my garden. There are new shoots that are about 5 feet high, about 2 feet taller than the rest of the bush. Can I prune them back 2 feet?