When to prune trees in central Ohio

The general Ohio window

Trees close wounds by forming callus tissue, and the rate at which they do it varies by species and by conditions. OSU Extension's factsheet on pruning and the care of tree wounds reasons from that: although the optimal time for pruning varies by region and species, the time to prune many trees is when rates of growth, and therefore callus formation, are the greatest. In Ohio, it says, this means pruning should take place in the spring to early summer, March through June.

There are two standing exceptions in the same guidance. Hazardous tree limbs should be removed regardless of the time of year, to minimize the risks of injury and damage. And some species have their own constraints, including problems with winter pruning where freeze injury is a risk, and different timing where fruit production is the goal.

If significant pruning is on the table, the factsheet's own advice is to check with a local professional garden center, an Extension service, or a certified arborist rather than working from a general calendar.

Oaks, and why the dates are different

Oak wilt is a vascular disease caused by the fungus Bretziella fagacearum, and OSU Extension reports that it has been found, with variable frequency, in the majority of Ohio's 88 counties. Oaks in the red and black group are extremely susceptible and can die within a few weeks of infection; white group oaks are more tolerant and may decline over one or more years.

The route that pruning opens is overland spread. Sap-feeding beetles are drawn to fungal mats on infected trees, pick up spores, and then fly to fresh wounds on healthy trees, attracted by the smell of sap. Wounds are believed to be attractive to those beetles for only about three days, which is precisely why the timing of a cut matters so much.

The published prevention is a date range. Overland spread can be hindered or interrupted by ensuring that trees are never wounded between April 15 and July 1, when most of these beetles are flying. A more stringent approach avoids wounding oaks throughout the growing season, April 15 to October 1, since additional summer flights are possible. If pruning an oak during the growing season is unavoidable, the factsheet says it is imperative to dress the wounds, which can be done with latex paint: this slows wound closure but deters beetles from landing.

Most infections do not travel by air at all. OSU estimates that around 90 percent of new infections pass between neighboring trees through root grafts, which is why control of an established infection involves severing root contacts by trenching rather than anything done in the canopy.

Wound paint, and topping

For trees generally, wound painting is no longer recommended. The reasoning is straightforward: callus formation needs oxygen at the wound surface, and painting a wound with material that impedes oxygen access delays or prevents closure. The one exception OSU names is areas where oak wilt occurs, where wound paints may be useful in preventing insect spread of the pathogen.

Season does not rescue a bad cut, and neither does it excuse topping. OSU Extension states that an ISA-credentialed arborist would never recommend topping a tree, that topping encourages multiple weakly attached lateral branches prone to breakage, and that the large horizontal wounds it leaves do not close, exposing the wood to interior decay that descends through the trunk. Removing the tops of trees can be required for utility line clearance; that is a different situation from a landscape tree.

What the calendar does not change

Timing is a horticultural question. Permission is a separate one, and it does not move with the season: if the tree stands in a Columbus street, park or other public space, pruning it needs a free city permit applied for by the company's certified arborist, in June exactly as in January. The guide on permits for tree work in Columbus covers what that involves.

Sources for this guide: OSU Extension factsheet PLPATH-TREE-06, Pruning and Care of Tree Wounds, at https://ohioline.osu.edu/factsheet/HYG-3311-09; OSU Extension factsheet PLPATH-TREE-02, Oak Wilt, at https://ohioline.osu.edu/factsheet/plpath-tree-02; and OSU Extension factsheet HYG-1032, How and Why to Hire an Arborist, at https://ohioline.osu.edu/factsheet/hyg-1032.