Is it true that it’s important to apply pruning paint, paste or some other sealant (I’ll call all of them “wound dressings” from now on) to prevent rot when you cut off a tree branch?
My father certainly used to think so. He’d have us carry a pot of paint (yes, any kind of paint: some of our trees had bright blue or orange spots when he finished!) and a brush and hand both to him whenever he cut off a branch when he pruned his apple trees. “Any hole larger than a quarter needs to be dressed,” he insisted, convinced he was saving his trees from that supreme tree killer, rot.
And that was considered standard practice until well into the 1970s when an upstart biologist named Dr. Alex Shigo, later to be considered the world’s greatest arborist, first began to take a serious took a serious look at how trees grew … and shot that theory to smithereens.
Shigo discovered that trees actually do a pretty good job of protecting themselves. Almost as soon as a branch is removed, the tree starts to wall off the wound, plugging up the cells to keep sap in and air (and microbes) out. Over time, bark will grow over the wound, sealing it off entirely.
He found that tree dressing (then as now, a petroleum-based product was usually used) damaged the cambium around the cut, slowly the healing process and preventing a new layer of bark from forming over the wound. And instead of keeping the wound drier, it actually kept it moister, sealing in fungus spores and giving them the ideal conditions under which to germinate. Plus tree dressings eventually cracked, giving disease organisms that missed their first chance to infect the tree a second opportunity.
As a result, trees treated with tree sealants were actually more subject to fungi and thus rot than ones that weren’t treated at all.
In most cases, the best thing to do after you prune a tree is … nothing at all, at least, other than evening out the wound to leave as small an exposed surface as possible if it was caused by breakage or tearing. Just let Mother Nature do her job.
Don’t believe me? Watch a certified arborist at work. They never use wound dressings any more.
Of course, the above assumes you’re pruning living trees when they are dormant, in late fall or early spring. If they are in a period of active growth, notably in from mid-spring through early fall, it may be wise to apply, not wound dressing, but an insecticide/fungicide to trees susceptible to insects or disease, such as elms. And ideally, you’d avoid pruning them at all during the summer.
And, by the way, you can remove dead wood at any season. I almost feel like adding “of course,” as that seems obvious to me, but a lot of people don’t realize that.