Trim aspen trees in August or September. If pruned in the winter, they will exude a sap, which is not harmful to the tree but can be messy for you. Do not top an aspen tree and do not try to cut branches higher than you can safely reach with a pole saw. If you are unable to cut branches that are out of your reach, hire a professional to do that part of the pruning. Always wear gloves and eye goggles for safety when pruning.
Cut out all dead or diseased branches first. Use loppers or your hand saw or pole saw to remove dead branches. Use a three-cut process for removing large branches. Approximately 6 inches from the trunk of the tree, make a 1/2-inch deep cut in the underside of the branch. Then cut through the branch from the top side approximately 2 inches beyond the undercut. In this way, when the branch starts to break it will snap off cleanly at the undercut and not tear the bark and potentially damage the tree. Then, cut the 8-inch stump just beyond the outer edge of the collar (the knobby portion of the branch that is up against the trunk--do not remove or cut the collar).
Cut branches that cross or touch. Cut the smaller of two touching branches. Use the three-cut method described in Step 1 if the branches are large enough. Again, cut just past the edge of the collar, leaving the collar attached to the trunk.
Remove branches to create a pleasing shape to the tree, but do not go overboard in this regard. Aspens often require little in the way of trimming, and removing too much greenery reduces the tree's ability to make and store the sugars it needs to survive the winter. Whenever removing large branches, always use the three-cut method described in Step 1.