Woundwood: How Trees Heal and Why It Matters

When a tree is wounded — whether from a pruning cut, storm damage, insect activity, or mechanical injury — it cannot heal the way animals do. Trees don't regenerate lost tissue. Instead, they wall off the damaged area and grow over it, producing what arborists call woundwood, sometimes incorrectly referred to as a callus. This woundwood is new vascular tissue that forms at the cambium around the margins of a wound, gradually rolling inward over time. A healthy tree with good vigor will produce woundwood relatively quickly, eventually occluding the wound entirely. The speed and quality of that closure tells you a lot about the overall health of the tree.

Wounds come from many sources, and understanding the cause matters as much as treating the result. Improper pruning — particularly flush cuts that remove the branch collar, or stub cuts that leave dead wood protruding — are among the most common causes of poor wound response. Storm damage, construction injury to the root zone, lawn equipment strikes, girdling roots, and pest activity such as borers or canker fungi all compromise the cambium and disrupt the tree's ability to respond effectively. The location of the wound matters too. Wounds low on the trunk, close to the root flare, are generally more serious than those higher in the canopy simply because of the concentration of vascular tissue in that zone.

The biological process underlying wound response is called CODIT — Compartmentalization of Decay in Trees — a model developed by Dr. Alex Shigo that fundamentally changed how arborists understand tree health. Rather than healing, trees compartmentalize: they chemically and physically wall off the wound, limiting the spread of decay into healthy tissue. Four walls of compartmentalization are established, with the boundary formed between the wood present at the time of wounding and any new wood grown after. This is why respecting the branch collar during pruning cuts is so important — the collar contains specialized parenchyma tissue that drives the compartmentalization response. Remove it and you've compromised the tree's primary defense mechanism at the wound site.

The best thing you can do to support a tree's wound response is to optimize its overall health. A tree under stress from drought, compacted soils, nutrient deficiency, or root damage will compartmentalize slowly and incompletely, leaving it vulnerable to decay fungi and secondary pathogens. Soil care — aeration, organic matter, appropriate moisture — gives the tree the resources it needs to respond vigorously. Where wounds are the result of poor past pruning, corrective work to remove stubs and dead wood can help reactivate a stalled wound response. Wound dressings and sealants are not recommended and have been shown to provide no benefit and in some cases trap moisture. The tree's own chemistry is the tool — your job is to give it the conditions to use it.