Should You Use Tree Wound Sealer?

Walk through any garden centre in Calgary and you will find tubes and cans of tree wound sealer — thick, tar-like products marketed as a way to protect pruning cuts from disease and decay. For decades, homeowners have faithfully painted every cut with the stuff, believing they were helping their trees heal. The science, however, tells a very different story.

The Origin of Wound Sealers

Tree wound paint became popular in the mid-twentieth century when tree care was still guided by intuition more than research. The logic seemed reasonable: when we get a cut, we apply a bandage. Trees get cuts during pruning, so covering those cuts should protect them too. Hardware stores stocked wound dressing as an essential tool, and arborists of the era applied it after nearly every cut.

The idea persisted for so long that many homeowners today still assume it is standard practice. It is not. Modern arboriculture has moved on, backed by decades of controlled research that shows wound sealers do more harm than good in most situations.

What the Research Shows

Studies conducted by Dr. Alex Shigo at the United States Forest Service were among the first to challenge the wound-sealer tradition. Shigo examined thousands of trees and found that wound dressings did not prevent decay. In many cases, they actually trapped moisture beneath the coating, creating an ideal environment for fungal growth.

Trees have their own remarkable healing mechanism called compartmentalization. When a tree is wounded, it does not repair damaged tissue the way human skin does. Instead, it walls off the injured area with chemical and physical barriers, producing new wood that gradually grows over the wound. This process, known as CODIT (Compartmentalization of Decay in Trees), works best when the wound surface is exposed to air and can dry naturally.

Wound sealers interfere with this process. The dark coating absorbs heat from sunlight, which can damage the delicate cambium layer around the wound edge — the very tissue responsible for growing new wood over the cut. The sealed surface also stays moist underneath, which encourages the fungi and bacteria the product was supposed to block.

When Is Wound Sealer Appropriate?

There is one notable exception. In areas where oak wilt is a serious threat, arborists may recommend a light application of wound dressing on fresh oak pruning cuts to prevent the nitidulid beetles that spread the disease from being attracted to the sap. Calgary does not currently face an oak wilt risk, so this exception rarely applies to our region.

For every other species and situation in Calgary, the consensus among certified arborists is clear: leave pruning wounds alone.

What Actually Helps Trees Heal

If wound sealer is off the table, what should you do instead? The answer lies in proper pruning technique.

• Cut at the branch collar. Every branch has a slightly swollen area where it meets the trunk. Cutting just outside this collar preserves the tree's natural defence zone and promotes the fastest possible wound closure.
• Use sharp, clean tools. A clean cut heals faster than a ragged one. Dull saws tear bark and create larger wound surfaces. Clean your tools between trees to avoid spreading disease.
• Prune at the right time. Most trees in Calgary benefit from dormant-season pruning, when pathogen activity is low. Spring-blooming ornamentals should be pruned shortly after they flower.
• Avoid topping. Topping creates massive wounds that trees struggle to compartmentalize. It is one of the most damaging practices in tree care and directly shortens a tree's lifespan.
• Keep trees healthy. A well-watered, properly mulched tree has far more energy to devote to wound closure than a stressed one. Deep root fertilization can give struggling trees a boost.

Common Misconceptions

Some homeowners worry that an open wound invites insects. While certain boring insects are attracted to stressed or dying trees, a healthy tree with a properly made pruning cut is not at elevated risk. The tree's own chemical defences are its best protection.

Others worry about aesthetics — a fresh pruning cut on a prominent trunk can look alarming. Rest assured that the exposed wood will weather to a natural grey within a season, and new growth will gradually roll over the wound edge year after year.

Trust the Tree

Trees have been healing their own wounds for millions of years. They do not need our help in the form of tar or paint. They need proper pruning cuts made at the right location, at the right time, with the right tools. That is exactly what a certified arborist provides.

If you have trees that were previously topped or badly pruned, a professional assessment can help determine the best path forward — whether that means corrective pruning, crown restoration, or simply monitoring the tree's response over time.