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Mulch Out – Not Up Proper Mulching Techniques
Fortunately, we have, at last, acquired an appreciation of the aesthetics of mulch in the landscape, somewhat like the leafmold on the forest floor. Unfortunately, our tastes are influenced by what we see, and we have seen so many wrong examples of mulch around trees and shrubs that some people think they are correct. ‘When you pile the wood chips or pyramids of mulch high around the base of a tree, you are simply following a common, but sadly mistaken model,’ says Lin Diacont, President of the Virginia Green Industry Council. In trying to control several problems, you create several new ones. The heavy application of mulch probably got started with the simple directions to apply 4 inches of loose, course mulch. As the mulch began to decompose or just discolor and look less attractive, the application was repeated and repeated, until the original mulch was a foot or more deep. The next logical step has been to start off with 12 to 18 inches of mulch and then keep it that deep. From these mistakes come the multitude of problems. Incorrect mulching is a waste of time and money and is quickly becoming the number one cause of death of trees and shrubs. Over-mulching, with mulch piled high, directly against the stems or trunks, smothering the root flare zone; or with very deep mulch covering part or all of the root area cause: Root Suffocation/ Root Rot - Repeated or deep applications of mulch cause waterlogged soil by slowing water loss through evaporation. Roots must "breathe", taking in oxygen. When oxygen levels drop, root growth declines then they die, making it impossible for the plant to take up water and nutrients, leading to death. Inner Bark Death - The living tissue (phloem) just inside the outer bark must be able to freely exchange oxygen and carbon dioxide. Mulch piled high onto the trunk decreases gas exchange, killing the inner bark and then the roots which can no longer receive food from the leaves. Rodent/ Insect Chewing – Deep layers of mulch against the trunk provide a perfect habitat for these pests. If chewing is extensive (more than 50% of the circumference) or "girdles" the entire tree, little can be done to save the tree. Fungal and Bacterial Diseases – These can grow and reproduce in the thick, moist mulch next to the trunk, gaining entry into the stressed, decaying bark. Once established, cankers caused by these diseases, can encircle the tree, killing the inner bark, starving the roots and killing the plant. Excessive Heat - Thick layers of mulch begin to decay and can produce heat (similar to composting). Waterproof Layers – Thick layers of certain mulches can create impervious surfaces that do not allow water to reach to roots, especially during drought. Roots dehydrate and die, killing the tree. Because Roots Need Oxygen in order to grow they don't normally grow in the compacted, oxygen poor soils under paved streets. The Root Collar is usually at or near the ground-line and is identifiable as a marked swelling of the tree trunk. The Framework of major roots usually lies less than 8 - 12 inches below the surface and often grows outward to a diameter one to two times the height of the tree. A complex network of smaller non-woody Feeder Roots grow outward and upward from the framework roots. These smaller root branch 4 or more times to form fans or mats of thousands of tine, short, non-woody roots. These slender roots, with their tiny root hairs, provide the major portion of the absorption surface of a tree's root system. They compete directly with the roots of grass and other groundcovers Note: A few species have a Taproot that grows straight down three to seven feet or more until they encounter impenetrable soil or rock layers, or reach layers with insufficient supplies of oxygen. Between 4 and 11 Major Woody Roots originate from the root collar and grow horizontally through the soil. These major roots branch and taper over a distance of 3-15 feet from the trunk to form an extensive framework of long, rope-like roots which as 1/4 to one inch in diameter. These are important structural roots, supporting the tree against wind, etc. Death from over-mulching is gradual, with symptoms sometimes taking 3-5 years to express themselves. It starts with the decline of plant vigor and rate of growth.
Good tree maintenance is common sense – it is what trees need to flourish in nature. In the wild, the forest floor is naturally covered with a layer of decomposing leaves, twigs and other plant material. In urban settings, the most common mulch is made of wood chips of varying types and sizes. Mulch: Other Tips: What To Do? A visual inspection of the root flare zone or trunk collar (where the spreading base of the tree just goes into the soil) is the best way for you or an arborist to check the condition of the trunk for possible rot, pest chewing or diseases. If detected early on, removal of mulch to allow drying out may help curb more serious problems. Complete root flare zone excavation may be necessary and is best performed by a professional arborist. Go to our consumer website for additional gardening information at www.VirginiaGardening.com The Virginia Green Industry Council is the voice of the horticulture industry in the Commonwealth and is dedicated to enhancing the beauty of the state’s environment, the well-being of our citizens, improving our state’s economy, and improving the health and wellness for everyone in Virginia. The Council is made up of providers and consumers of horticultural products and services. The Council works to provide public and industry education, environmental guidelines and other information that will keep Virginia green and growing. For more information, visit
www.virginiagreen.org. E-mail:info@virginiagreen.org Virginia Green Industry Council |
