Mulching is Important for Tree Health
by Michael Kuhns, Extension Forestry Specialist
Most Utahns have planted or cared for a tree at some time in their lives. Few, however, realize the importance of mulching a tree's root system.
In order to understand the importance of mulch to a tree, think back to the last time you walked through a forest. You may have noticed that the forest floor was covered, not by grass, but by organic matter. Twigs, leaves, dead flowers, rotten wood, and other debris cover the forest floor. This material can be thought of as a mulch. It shades and cools the soil, adds organic matter and nutrients to the soil, reduces compaction, and helps keep grass and other plants from growing under and competing with the trees. Shade from surrounding trees also keeps soil and roots cool and moist in the forest.
Trees that are native to heavily forested areas, therefore, are well adapted to having a lot of organic matter covering their root systems. Trees roots are very shallow, within 6 to 12 inches of the soil surface, and this organic matter or mulch helps them survive. Roots do best under moist, cool conditions and need plenty of oxygen in the soil. These conditions are ensured by a good mulch layer.
Conditions under typical landscape trees bear very little similarity to the forest described above. The trees we plant in our yards are usually native and best adapted to a forest setting. We usually plant them, however, in the middle of grass lawns.
Sunlight hits the soil above the tree's root system much of the time, increasing soil temperature and decreasing moisture. Grass competes with tree roots for nutrients and water, but doesn't do very well because of shade from the tree.
Chemicals are used to kill weeds in the lawn and keep grass growing quickly. Compaction may be high due to foot or bicycle traffic, reducing soil oxygen. Mowers and string trimmers bump into and kill the tree's sensitive cambial layer just inside the bark, causing wounds and rot. Tree roots may have to be removed occasionally as they emerge above the grass and become nuisances.
If we think of trees as wanting forest-like conditions for their roots, it is obvious that much of what we do for our lawns is not good for trees. The solution, for healthy trees and healthy lawns, is to mulch under your trees and shrubs. This duplicates the conditions of a forest floor, improving root system health and tree health. Mulch beds around trees or groups of woody plants also help segregate tree and grass areas. Heavy fertilization, herbicides, mowing, and other treatments and practices needed for healthy grass are then kept away from tree roots and trunks. Grass can still be grown where it does best, in high light areas away from trees.
A good mulch bed should extend out at least three feet from a tree's trunk in all directions, though wider is better. Mulch can be anything organic. The best mulches are fairly coarse and woody, like wood chips or ground bark. Leaves, pine needles, compost, straw, and grass clippings also work fine. Mulch bed depth should be maintained at 4 to 6 inches, though fine mulches may tend to mat and should not be piled as deep. Edging around the mulch bed is not needed, though it can be used if you don't dig more than 2 to 3 inches deep to install it. It is better never to dig under a tree at all to avoid damaging shallow roots.
Plastic sheet "mulches" should never be used under trees or shrubs. Though they keep weeds out for a few years, they kill roots by not allowing oxygen and water into the soil. A good, deep layer of organic mulch will keep weeds out just as well and helps the tree instead of harming it. Lava rock, marble chips, gravel, or other hard, non-organic mulches also should not be used. They lead to soil compaction and do nothing to cool the soil or add organic matter. Stick with organic mulches only.
Mulching can be done at planting time, but it can also be done around established trees. Just spread a deep layer of mulch right over the existing grass. Before long the shade of the mulch will smother the grass. Any grass that comes through can be pulled or sprayed with a weed killer.
Let's learn from nature and give trees what they need. Mulch is the key to maintaining a healthy landscape, both trees and grass.