Not so long ago, arborists and other tree professionals would coat fresh pruning cuts with a latex- or petroleum-based paint. The goal was to prevent the advancement of decay, which could ultimately kill the recently pruned tree. However, one rarely sees arborists painting freshly pruned branches anymore.
To understand why the practice has fallen out of favor with most tree-care professionals, it is necessary to understand why trees decay, how trees respond to such threats and why painting fresh wounds is often counterproductive.
Microscopic fungal spores (as well as insects, viruses and bacteria, which harm the tree in other ways) are everywhere, and they bombard trees on a daily basis. While many of these organisms are harmless, or even beneficial to a tree’s health, others are destructive pathogens, intent on stealing resources from the tree, and advancing their own lifecycle. Often, this comes at the tree’s detriment.
Most fungal spores bounce off a tree, or otherwise fail to gain a foothold – they need a vulnerability to exploit. But, those that find a weakness – such as a freshly cut branch – often successfully colonize the area and begin feeding and multiplying. As these pathogens begin eating through the tree’s tissues, the tree enters a downward spiral. Decay spreads throughout the tree, potentially causing large and dangerous cavities to form.
Tree Defenses and Compartmentalization
Contrary to the claims of some wound paints, which purport to “accelerate the healing process,??? trees do not heal. They do not – as humans and other animals do – produce new tissues to replace the damaged ones. Instead, trees attempt to wall off the wounded region and grow around it.
Arborists call this the process of compartmentalization (it is sometimes referred to as CODIT, which stands for the Compartmentalization Of Decay In Trees). Most trees exhibit this response to decay, but some species compartmentalize better than others do. Trees compartmentalize their wounds by creating four different barriers, often called walls. Each wall prevents the spread of decay in a different direction.
Wall one attempts to stop the decay from traveling vertically from the wound. However, the first wall is a relatively weak barrier in many species, thereby allowing the development of long swaths of decayed material. Wall two is comprised of a tree’s growth rings. Growth rings present a much more robust boundary to decay than the first wall does. A tree’s rays make up the third wall. Rich in starches, the rays provide very strong protection against the spread of decay. A tree’s cambium layer forms the fourth wall. This wall helps to prevent the decay from progressing into new wood.
In addition to the process of compartmentalization, trees create something called woundwood at the site of injuries. Once it forms completely, woundwood helps to protect the bark-free wood.
The Problem with Paint
Instead of bettering a tree’s chances of recovery, paint actually increases the likelihood that the tree will develop problems, when applied to fresh wounds. The paint tends to prevent the formation of woundwood, thereby increasing the length of time that the tree is susceptible to pathogens. Additionally, the paint traps any bacteria or fungi present, where they can feast on the freshly cut wood. Paint also traps moisture inside the wood, which further retards the rate of woundwood formation and helps nourish any fungi present. Some paints even serve as a food source for pathogens!
These facts have been verified by myriad studies, but one of the first works that examined the issue empirically was authored by Alex L. Shigo and Walter C. Shortle, and published in a 1983 issue of the Journal of Arboriculture. In fact, the authors revisited the issue in 2012, and came to the same conclusion: None of the materials tested prevented decay.
Exceptions and Caveats
Wound paint does make sense in a few circumstances.
If your freshly pruned trees face a specific threat, then targeted chemicals may improve its chances for long-term survival. However, there is a big difference between covering all tree stubs with paint and carefully evaluating a tree’s health, vigor and susceptibility to specific pathogens, and treating as necessary. For example, some insects that feed on the tissues of freshly cut branches can cause oak wilt in places where the offending fungus is present. While not yet a problem in California, it may be advisable to paint fresh pruning cuts on red oaks in afflicted areas to prevent transmission of the disease.
The Best Solution
The best way to prevent decay in trees is to skip the paint, provide proper supportive care and make all pruning cuts in the correct locations. This helps to bolster the tree’s defenses and accelerate the process of compartmentalization. If your trees are in need of pruning, contact Arborist Now -- the Bay Area's leading arboriculturists -- and schedule a tree pruning with one of our ISA-certified arborists.