A majestic shade tree, black walnut (Juglans nigra) is native to eastern North America, and especially popular in Pennsylvania. Pruning a shade tree can range in difficulty depending on the size of the tree. Young trees are easily pruned since they are shorter and more easily trimmed. Large, mature specimens require substantial planning and equipment to ensure proper cutting and management of dropping limbs. Always prune a tree to retain its natural shape, focusing on healthy growth and strong structure.
Pruning Young Trees (Under 15 Feet)
Evaluate the tree. Look for dead or diseased wood and branches, any branches that are criss-crossing or rubbing against each other, or--for narrow-crotched branch junctions--those that connect at a less-than-30-degree angle. Also note if any branches are potentially growing in a direction that may cause trouble later, such as a horizontal branch growing toward a street or house.
Remove dead branches any time of year with a hand-pruner or hand saw, making the cut 1/4 inch above the junction with another living branch or trunk. On larger branches, you may see a collar, or slightly swollen base where the branch attaches to the trunk. Avoid cutting into this collar, but make the cut 1/4 inch above the collar, preserving it, to remove the branch.
Prune away errant or awkwardly growing small branches identified in Step 1, in late winter to mid-spring. Make pruning cuts 1/4-inch above branch junctions. Light pruning can be done on branch tips any time of year, but do not remove large amounts of foliage if possible.
Ensure there is good branch structure in the tree. Remove co-dominant branches so that only one main branch, called a "leader," grows from the trunk to define its central upright tip. Also remove one of the two branches that meet in narrow-angled crotches, such as those at an angle of 30 degrees or less.
Trim away branches that are rubbing and damaging other branches. Also remove any branches that are growing inward across the central core of the tree, such as ones that grow horizontally across and through the upward branches in the canopy.
Allow the sap to ooze and run from the pruning cut wounds naturally. If pruning is not done too late in summer into autumn, there is ample time for the tree to seal and callous its wounds. Avoid painting or sealing pruning cuts.
Pruning Mature Trees
Identify the problem areas on the large walnut tree. Rotting or dead branches--and those that are growing in a manner that threatens the safety of a property or access on a street--may be earmarked for removal.
Note the size of the branches needing removal as well as the approximate diameter of the branches where you think a cut needs to be made. The larger the branch, the heavier it is. Also note the height at which these pruning cuts will occur.
Consider contacting a certified arborist to provide a free consultation on the pruning project of a substantially sized shade tree. Avoid doing the work yourself, especially if the branches are large, heavy and well off the ground. A certified arborist has been trained specifically for tree care and keeps work safety and the tree's health in mind.
Evaluate the tree as in the first section above, Steps 1 through 6. An arborist can do the large-scope removal of large branches as well as the smaller tip pruning at the tall heights, too.