When a branch breaks or is pruned from a tree, or any woody shrub, a series of mechanisms begins to protect the tree. This is so if any infection or insect infestation starts, it’s difficult for it to get through a barrier and affect the rest of the tree.
This has worked in nature for thousands of years.
But humans had the idea that tree wounds need protection like human and animal wounds do. So we created a protective coating for the wounds. And the practice of sealing a pruning cut or damage wound became common.
No pruning seal is needed
But years of research in the 60’s through 80’s showed that this is not at all necessary, and may even have deleterious effects on the tree.
The premise is that when a tree is wounded, through pruning or damage, it goes through a process called compartmentalization, where the injured wood is “walled” off from the rest of the tree. This keeps any decay organisms or boring insects blocked from the rest of the tree.
And on the surface a seal grows, called callus or woundwood.
When you use wound sealant, moisture and decay organisms can get sealed in, compartmentalization can be inhibited, and callus may be prevented from forming. The sealant does not keep the wound from bleeding its sap, or water. If the wound is dripping, the tree, shrub, or vine will seal itself. No need to interfere.
(Much of the research on compartmentalization and pruning seal use was done by Dr. Alex Shigo, of the US Forest Service, and with others, too.)
Here’s what happens when a tree is wounded
The wound doesn’t heal, but eventually it gets covered with callus on the outside. The cambium grows over the wound.
On the inside, chemical and physical changes occur that separates the wounded area from the rest of the tree, so that the tree can go on living. It’s a way to keep invading organisms in check.
Four “walls” are formed around the wound.
- The first wall plugs the xylem cells above and below the injury. Xylem cells, if you don’t remember your high school biology, form the vessels that carry water and nutrients from the soil up and through the tree. Xylem cells get plugged above and below the injury to prevent vertical progression of the injury.
- The second and third walls, on the sides, go through chemical changes that increase their resistance to discoloration and decay.
- Wall 4 is called the barrier zone, it faces the trunk; it’s the strongest and is usually able to confine the injured and decay-infected wood to the location of the injury.
The wound doesn’t heal as in animals, it just gets isolated.
A healthy tree responds fastest to injury. If it’s in decline or drought-stressed, it will take longer for the callus to grow over the wound. But the callus will grow. And it’s not needed to protect the tree.
The best pruning cuts help the most to protect the tree from infestations. If storm damage breaks branches, some clean-up of the wound may be required. Make clean cuts and cut off shaggy or loose pieces of bark.
To keep trees healthy, good pruning practices must be followed. Read what the best pruning practices are here.
There’s a couple of pruning seal exceptions
Dutch elm disease and oak wilt are vascular wilt diseases. If an elm or oak tree need to be pruned during their growing season, for safety reasons, their wounds should be painted with latex-based paint—only in theses exceptional cases. This will help prevent the spread of these vascular wilts by beetles while allowing the wounds to seal properly.
But these trees and diseases are not a concern in Nevada County.
Tree cavities should not be filled
According to Arboriculture: Integrated management of Trees, Shrubs, and Vines*, it’s best to leave cavities alone, do not fill it with anything or try to clean it. There is no scientific evidence that those strategies work. Instead focus on improving tree vigor to enable new wood and bark to grow and outcompete with decay.
No pruning seal
So when you need to prune damaged branches or you’re doing some pruning for fruit trees, safety, or improving the health of a tree, you will not need pruning seal. It won’t help and it may even impede the natural defense process in the tree or shrub.
Just prune properly, keep tools clean, keep surrounding bark smooth not ragged, and do it at the right time of the year. Unless it was an emergency fix. Read about how to prune properly here.
Pruning seal was used for a long time and many landscape professionals still use it. They have not heard otherwise yet. It’s a practice that’s been hard to stop…but now you know, please spread the word.
Main References
*Arboriculture: Integrated Management of Trees, Shrubs, and Vines, 4th ed., by Richard W. Harris, James R. Clark, and Nelda P. Matheny, Prentice Hall, 2004
Related Reading
Use These Best Practices to Keep Your Trees Healthy