I'm helping my tree to help itself
“On my way out of that garden, the gnarled trunk of a centuries-old oak tree caught my attention and literally opened my eyes to a hitherto unknown domain: the world of bark. It turned out to be a revelation that changed the course of my life.”
— Cedric Pollet, “Bark: An Intimate Look at the World’s Trees,” 2010
French photographer and landscape designer Cedric Pollet traveled across five continents to study and photograph hundreds of trees in neighborhoods, parks and woodlands. The result was the publication of a hardcover book filled with photographs of all kinds of trees with all kinds of bark: smooth and bumpy, single-colored and multi-colored, striped, spotted, even bark that looks like puzzle pieces.
Like Pollet, I am fascinated by tree bark — by the way it looks and feels, and by the way it helps nourish and protect trees. In many ways, tree bark is like human skin but, unlike skin, bark does not regenerate. Dead cells from wounded bark remain for the life of the tree, and this is why I’m so concerned about a large wound on one of the scaffold limbs of a weeping birch (Betula pendula ‘Youngii’) in my backyard.
The birch has been a focal point in the garden even before we bought our house 11 years ago. I love its twisting white branches and hanging green leaves that rustle in the breeze. I would hate to lose my weeping birch tree.
The tendency to protect the tree by dressing its wound is understandable. After all, I did exactly that for my children’s cuts and scrapes, just as my mom did for me. However, most wound dressings are petroleum-based products, which are not healthy for trees.
Even dressings made from natural materials do not prevent organisms from penetrating the bark and beginning the decomposition process within the inner tissue. In fact, wound dressings seal in moisture, which speeds up decay, and the materials they’re made from can even feed pathogens.
In addition, wound dressings, particularly those with black coating, eventually crack from sun exposure, allowing pathogens and insect pests to invade. For all of these reasons, the use of wound dressings has been discouraged by most horticultural experts for decades, even though an ample number of products are still available in stores.
So what to do about the terrible wound on the limb of my weeping birch? I talked with local arborist Mike Bartlett, and he assured me that even though trees can’t heal their wounds, they have evolved effective mechanisms for damage control.
When a tree branch or the trunk is damaged, the tree responds by generating a special kind of wood that forms a callous roll, beginning at the outer edge of the wound and eventually filling in the center. The tree also begins producing chemicals that help ward off invasions to the wound by insects and disease.
In this way, the tree isolates, or compartmentalizes, the wound so decay and discoloration doesn’t spread. Applying sealants to the damaged area prevents the tree from initiating self-defense mechanisms that have evolved over millions of years.
Instead, Mike recommends a three-step process to help save my weeping birch.
First, gently remove loose bark around the wound, then use a sterilized, sharp razor to slice off dead bark to the edge of the callous roll, making sure not to damage the new growth.
Second, look for tiny holes made by wood-boring insects. Apply insecticide to the limb if insects are present.
Third, help the tree help itself by making sure the rate of new growth exceeds the rate of decay. I can do this for my weeping birch through proper irrigation (birches like lots of water), consistently replenishing the soil with nutrients, and spreading a layer of mulch around the root zone of the tree.
Providing adequate growing conditions will free up energy for the tree to generate callous wood more quickly, although large wounds may never be fully filled. If a wound causes structural weakness, then the limb should be supported to prevent further damage. Finally, the limb or tree should be removed if it presents a safety hazard.
Also important is to try to find out how/why the tree was damaged. One of the first questions Mike asked was whether the damaged branch was facing southwest (it is). Come to find out, birches have thin bark and are particularly susceptible to sunscald when the afternoon sun is hottest.
Sunscald kills off healthy tissue cells and causes lesions in the bark; these openings allow insects and infections to exacerbate the damage. I can’t protect my tree from the scorching sun with a hat, the way I protect myself, but perhaps I can grow a tall trellis of green beans in front that will cast an afternoon shadow.
The wound on my weeping birch is not pretty, but my main concern is the overall health of the tree. After all, I’ve developed some rather unsightly wounds during my 57 years, and I like to think of them as emblems of an active life. Besides, if I want to look at beautiful bark, I can read Cedric Pollet’s book.
Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at Rnowak39@gmail.com. For more about gardening, visit her blog at http://blogs.esouthernoregon.com/theliterarygardener/ and check out her podcasts and videos at https://mailtribune.com/podcasts/the-literary-gardener, and her website at www.literarygardener.com.