Wound dressings, pruning paint and cavity filling: what extension services say

This page reports what the named codes, regulators and makers say. It is not training and does not teach the work. The publications below are extension fact sheets for tree owners and professionals, not standards or law. For the pruning terms in the standards see the pruning standards guide. For the courses that teach pruning see the chainsaw tickets guide and the certifications guide.

How the sources say a tree answers a wound

Mississippi State University Extension (publication P3533) says a tree responds to a wound by compartmentalizing, forming sealing, strengthened wood and callus tissue that grows from the wound edge toward the centre. It says the new wood stays for the life of the tree, limits the spread of decay, and that after these processes decay will not spread unless something breaks the barrier. It also says decay cannot be reversed. The University of Kentucky fact sheet PPFS-OR-W-01 (September 2015) says wounded trees do not technically heal: they close over damaged tissue with woundwood and callus, and large wounds need several growing seasons to close.

What each source says about dressings

Source	What it says about wound paint, sealer or dressing
Mississippi State University Extension, P3533	Covering wounds with paint is described as detrimental. Many products are petroleum-based (tar, asphalt, paint) and impede callus tissue and compartmentalization, seal in moisture that encourages decay, and can feed fungi. In most cases wounds can seal themselves and are left to natural processes. A dressing applied to one cut oak limb flaked off after 2 or 3 years and had only acted as a barrier between callus and the cut surface.
University of Kentucky, PPFS-OR-W-01, September 2015	Wound sealants and paints are said not to prevent decay and are not recommended.
Iowa State University Extension, SUL-7, January 1999	Putting a wound dressing on large branch stubs is described as ineffective against decay-causing organisms.
Penn State Extension, Pruning Landscape Trees, November 5, 2021	Among the signs of proper pruning it lists cut surfaces not treated with wound dressing or tree paint, which it says can injure trees.

The Mississippi State sheet lists the 1983 Journal of Arboriculture paper by Shigo and Shortle, "Wound dressings: results of studies over 13 years", among its suggested reading. The sheet does not state that paper's findings in its own text.

Cavity filling

Mississippi State says cavity filling was once an accepted practice, done for appearance with materials such as cement or brick after the cavity had been cleaned back to sound wood, and that the cleaning usually broke the tree's own barriers so decay kept spreading. In its account, small cavities are best left for the tree to close, and large ones may never close yet cause no trouble when the tree walls off the wound. It reports research suggesting a filling can give new callus something to grow against, and weighs that against the cost, the decay a filling does not stop and the harm it often does to the boundary between decayed and sound wood. Its conclusion is that the cleaning, more than the filling, tends to cause further harm, and that existing callus is left undamaged when a cavity is filled.

Where the sources place the cut

Mississippi State says pruning cuts should be smooth and clean, leave no stubs, and keep the branch collar, because cutting above the collar leaves a smaller area for callus to cover. Kentucky names the three-cut technique of an undercut, a cut through the branch and a final cut. The Kentucky sheet also lists mulching to moderate root zone temperature, irrigation in dry seasons and fertilizing by soil test results as ways to maintain vigor.

The oak wilt exception

In regions with oak wilt, state agencies do advise a sealer on oak cuts made in the risk season. That is a disease-control rule, separate from the general view above. The dates and wording by state and province are on the oak wilt guide, and the disease itself is covered on the oak wilt page.