To Prune or Not to Prune Your Trees: Answers from Arboretum Horticulture Director, Shari Edelson

Part 1: Answers about trees from Arboretum Horticulture Director, Shari Edelson

With the anniversary of last October's damaging snowstorm fast approaching, you may be getting anxious looking at your trees still holding on to their leaves, and in fact, you may be thinking about lopping off branches now to avoid damage later. But is it really the best idea? Summit's trees are valuable and beautiful resources, so pruning wisely and at the appropriate time is important.

Reeves-Reed Arboretum's Director of Horticulture, Shari Edelson, answers questions about tree pruning:

When is the best time to prune trees?

                The best time to undertake structural pruning on most trees is during late winter when plants – and the diseases that attack them – are dormant. Late summer and fall are the worst times of year to prune, as pruning stimulates the production of tender new growth, which is then damaged by the onset of cold weather. Fall pruning also impairs a tree’s disease resistance by creating pruning wounds that do not have sufficient time to heal before the tree goes dormant for the winter. These open wounds can invite pests and diseases, causing serious stress to the tree.

When trees are pruned in February or early March, on the other hand, pruning wounds remain open for just a brief time before the tree emerges from dormancy in late March or early April, when the tree will quickly seal off these areas to prevent disease. Many trees in the rose family, such as apple and hawthorn, are particularly vulnerable to fireblight and other diseases if pruned during the growing season, and are good examples of plants that make perfect candidates for late winter pruning.

                Some trees, notably maple, birch, and yellowwood, have a tendency to weep large amounts of watery sap if pruned in late winter before leaves emerge on the branches. Though this weeping is not seriously detrimental to the health of the tree, many gardeners choose to avoid it by waiting to prune until the tree’s foliage has completely expanded, in late spring. If you decide prune your maple, birch, or yellowwood in late spring, be careful not to remove more than a quarter of the tree’s foliage – your tree needs those leaves to get through the growing season!

When is the best time to remove diseased or dead branches?

                Dead branches may be removed any time of year, as cutting into dead wood does not impact the health of the tree. Some gardeners like to remove dead wood in summer, when it’s easiest to tell the difference between dead limbs and live growth. If you choose to do your deadwooding (as it’s called) at this time of year, however, make sure not to prune into live wood, as you’ll run the risk of encountering some of the problems discussed above.

                Pruning of diseased branches, on the other hand, is best done when the tree – and the disease – are dormant, during winter. This will reduce the chances of spreading pathogens. However, if you suspect that your tree has become infected with a disease, it’s always best to seek expert guidance from a certified arborist, who will be able to provide you with an accurate diagnosis and recommend the best treatment options.

How much of a tree can you prune without endangering its health?

                In general, it’s best to limit pruning to no more than a quarter of a tree’s growth. This will enable the tree to retain its natural vigor, and will help the tree to recover healthily from pruning. I should add here that some types of pruning are never healthy for a tree – the biggest offender here is the practice of “topping,” in which the top section of trunk is removed from a healthy tree as a means of controlling height. This practice is not only detrimental to the health of the tree, but can create a hazardous situation for people as well. This is because topping often causes trees to develop a proliferation of new limbs that are attached only weakly to the tree at the point where it was topped. In high winds, heavy rain, or snowstorms, these limbs can shear off the tree, damaging homes, vehicles, or even people below.

Excellent advice. Thanks, Shari! Tune in next week for To Prune or Not to Prune, Part 2, when Shari will address pruning issues for shrubs.