Answer: River birches should be pruned in the spring, after they have fully leafed out. If you prune in the winter while they are still dormant they will ooze sap in early spring, which will attract insects. Wait until the leaves are out, then prune out anything that is mis-placed or damaged, including crossing/rubbing branches or side shoots that might be starting to compete with the leaders. The growth habit of clumping river birches is to send out branches near the bottom of the trunk and shoots from the roots (that's how they clump and colonize an area). I'd just allow them to grow at will, removing only damaged or crossing, rubbing, broken branches. Best wishes with your new trees!
Q&A Library Searching Tips