As Susan and Jeffrey were showing us around their yard, Jeffrey stopped near a young native dogwood tree. It was just a little taller then we were, with a two to three inch diameter trunk. This tree obviously wasn't doing very well. One of its limbs was all dried up and many other leaves didn't look healthy.
We'd just finished a breakfast on their patio which included fresh raspberries from bushes nearby. Our hosts were proudly showing us around the beautiful and edible landscape they had created in the narrow, 20-foot-wide strip which separates their house from their neighbors' fences. We'd seen a fruiting pear tree, the strawberry patch and raised beds with vegetables and herbs. In that context, this tree really stood out.
Jeffrey pointed to some patches of green on the dogwood's bark and asked what he should do about them, thinking they might be the cause of the problem. This wasn't the first time that a homeowner had asked me how to get rid of the lichens that naturally live on tree bark, but they certainly weren't the source of the trouble. That was on the trunk near the ground.
The man who maintained their lawn was rather overzealous with his weed whacker. Its high-speed rotating plastic string had cut into the dogwood's bark right at ground level, exposing the vital cambium layer. People who remember their botany will recall that the cambium, the layer just beneath the bark, is the critical passageway for water and nutrients going up to the leaves, and for sugars traveling down to storage in the roots. Removing the cambium all the way around the trunk, which is called girdling, kills most trees.
We looked around the yard and found other trees and shrubs which had been damaged the same way. With all the insults from pollution, climate change, imported insects and diseases, we shouldn't further test a tree's durability by abusing its cambium.
My recommendation to Jeffrey, besides educating his contractor, was to spread some mulch, or covering for the soil, around his trees. This solution provides multiple benefits. Firstly, the mulch signals weed whackers, lawn mowers, and other dangerous and noisy machines to stay away. Mulch can also smother the grass underneath it. Tree roots feed mostly in the top six-to-twelve inches of soil. Grass and other plants growing between the trunk and the drip line will compete with the tree for nutrients. A mulch of organic materials (the only kind I would recommend) encourages worms and other useful soil organisms. And, it will eventually break down to nourish the tree.
A tree's own leaves are the preferred mulch. That's nature's way. Other leaves, shredded bark, wood chips and hay also work well. Try to use a local waste material. Don't use sawdust, because it can temporarily deplete the soil of nitrogen as it decomposes.
Be careful not to spread the mulch too thickly or pile it up next to the trunk. A three-inch thick mulch is about maximum. Mulch piled against the bark keeps it wet and can cause fungus problems and frost damage, especially on young trees. Mulch piled right next to the tree also invites mice to move in and feast on the cambium. It's worth doing a little hand weeding around the trunk to avoid damaging this important part of the tree.
Remember that flowering dogwoods are understory plants. They appreciate a bit of shade and woodsy soil. Their ever-changing beauty and benefits to wildlife make them a good choice for landscape use. Trees are one of the most important elements for creating comfortable outdoor spaces.
Since trees clean the air and lichens grow much better when the air is unpolluted, more trees also mean more lichens. There may be nearly no lichens visible in a city, while in the state forest or on the Maine coast, lichens seem to cover everything many layers deep, in a wonderful array of colors. Lichens can live on rocks, buildings. bark and in other inhospitable places. They provide nitrogen for trees and benefits to other organisms as well.
Caring properly for our trees will result in a more livable environment for all of us. We should respect them.
This is Bill Duesing, Living on the Earth
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