Most people have heard the old adage, “you should only prune your oak and elm trees in winter”. Have you ever wondered why, especially only for oak and elm trees? What is different about those two species? Well, it is not just a marketing ploy by arborists to drum up business in traditionally slow periods. The main reason for only pruning from mid October through mid April is to deter spreading fungal diseases. Oak wilt is the concern for oak trees and dutch elm disease is the concern in american elms. There are also a number of other general reasons that make winter pruning a good idea for oaks, elms and most other species.
Oak wilt is caused by the fungus Ceratocystis faacearum. This disease affects just about all species of oak trees. The disease starts on individual branches that wilt and die as the disease progresses. Infected trees will eventually die as the disease spreads through the branches, trunk, and roots. Trees in the red oak group get infected and die quickly, in some cases less than a month. Trees in the white oak group have lesser symptoms and typically take multiple seasons to die.
Oak wilt survives in still living oak trees as well as oak trees that have recently been killed by the disease. In the spring, the fungus produces fungus pads on dead or dying oak trees. These mats crack the bark of the tree and attract picnic beetles or oak bark beetles. Beetles pick up spores of the disease on their bodies and carry these spores to healthy trees. The disease can also be spread to adjacent trees through root grafts.
Picnic and oak bark beetles are attracted to healthy oak trees that have been recently damaged by wind, storms, or by pruning during the part of the season where beetles are active. The beetles can travel up to a quarter of a mile a year spreading the disease. The period where they are active starts typically in April and goes through mid summer. It is very important to not prune oak trees during this period due to the attraction and rapid response of the beetles carrying the spores of oak wilt.
Dutch Elm Disease
Dutch elm disease is caused by two fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi. This disease also typically starts on a single branch, which is indicated by wilting branches. These branches leaves continue to turn yellow, then brown, and then drop off completely. Dutch elm disease also spreads from the top of the tree down through the roots. The spread of the disease varies with some trees dying within one season after infection and some lasting for several seasons.
The fungus that causes Dutch elm disease moves from infected trees to healthy ones either by elm bark beetles or through root grafts formed with adjacent trees. Elm bark beetles are most active in spring and fall and are attracted to wounds on elm trees. These wounds can be from breakage from wind and storms, or from pruning cuts.
Winter pruning of oaks and elms greatly decreases the likelihood of spreading lethal fungal diseases, due to the fact that the insects are not active. Other reasons to wait until winter to prune trees and shrubs include the following. It is easier to see the structure without the leaves – and since the buds for spring have already formed, dead branches can easily be seen. Less impact to surrounding landscape around trees such as turf and perennials, as they are dormant as well.
Board Certified Master Arborist