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Gardening: Tree with damaged trunk will seal off injured area to protect healthy bark

This maple tree tangled with a Mustang last week and won. The ragged bark will need to be trimmed back after which the tree will be left to seal off the damaged area on its own. No other human intervention is needed. (Pat Munts / The Spokesman-Review)

It started with squalling tires and a big bang. Someone going too fast missed the 90-degree corner below our neighbor’s house and plowed head on into his 50-year-old silver maple. The driver didn’t seem to be too badly hurt, but the newer Mustang was pretty well demolished. After all the emergency folks finished caring for the driver and towing off the car, the tree’s two trunks were left with foot-square patches of missing bark. But the tree is still standing, which is more than you can say for the Mustang.

Trees are amazing organisms. The trunks’ bare patches cover about a third of the trunks’ circumference and will no longer be able to carry water and nutrients up and down the trunk. However, the remaining healthy bark will pick up the slack and continue to feed the tree.

An injured tree won’t heal per se; a series of biological processes will isolate and seal off the damaged area to keep disease and decay from entering the healthy tree tissue. Immediately after the injury, the tree will begin to compartmentalize the damaged area by producing a type of callus that seals the edges of the wound. Over time the callus continues to grow and forms a dense rolled collar area around the wound isolating it from the healthy wood.

The tree reinforces this callus barrier by creating an inhospitable environment in the healthy wood behind it that discourages the growth of disease and decay. Using chemical and physical barriers in a process that isn’t yet well understood, the tree is able to fend off the bacteria that are always trying to find a way into the tree. Over time the callus will slowly cover the wound and keep the tree healthy.

So, what should you do if you have a damaged tree or shrub? Trim off any broken branches by cutting them back to a branch. If you have to cut it back to the trunk, look for a thin ring or collar where the branch attaches to the tree and cut just outside the ring. This is the branch collar, and it will create the callus over the wound you leave. If you have torn bark as this damaged tree has, use a sharp knife to trim off the torn bark back to solid bark and walk away.

Do not use tree wound dressing on any cut or damaged areas. While this was a universal practice in the past, recent research has shown that petroleum-based black sealer and paint do not prevent decay and can actually interfere with wound closure and do not prevent decay in the long run. They can actually encourage the development of the fungal growth that causes decay. The sealers are still commonly available at nurseries and garden stores and do have other purposes in the garden but not for repairing damaged trees.

Pat Munts has gardened in the Spokane Valley for over 40 years. She is co-author of Northwest Gardener’s Handbook with Susan Mulvihill. She can be reached at pat@inlandnwgardening.com.