Falling leaves and early snows are a reminder of what is just around the corner — winter! Bare winter trees are ideal pruning candidates for more reasons than you’d think, especially for those susceptible to infections. Two such varieties that should only be pruned in the dormant months are oak and elm, which are susceptible to fungal infections like oak wilt and Dutch elm.
Oaks are at risk of disease oak wilt (Ceratocystis fagacearum), a fungal disease that can be lethal, especially to red oaks. The growing fungus disrupts the vascular tissue of the tree, hindering water transportation, eventually causing wilt and death. Oak wilt spreads via sap-feeding beetles. These beetles will move from tree to tree feeding on sap and unfortunately sometimes fungal material. If a tree is freshly pruned, beetles will be attracted to those fresh wounds, potentially leaving behind oak wilt.
Dutch elm disease (Ophiostoma ulmi and Ophistoman novo-ulmi) works much in the same way; by attacking an elm’s vascular system. It too is spread by sap-feeding beetles, who often lay eggs in dead or dying infected elms. When adult beetles emerge in the spring, they will migrate to healthy elms nearby, spreading infection.
Since dormant trees do not produce sap, and sap-feeding beetles are not active in the colder months, they are much less likely to invade new pruning cuts to spread disease. Once the leaves have fallen, infection is often easier for an arborist to spot as well.
Avoiding spread of disease is not the only benefit to pruning trees from November to February:
The best way to prevent disease is to prune your oak or elm from the months of November to February. In fact, some areas have strict restrictions on when these species can be pruned.
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