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With below normal precipitation - as much as 10 inches - in Rhode Island during the fall and winter, a water shortage is likely for the warmer months of 2002. In Newport County precipitation is reportedly 49% below normal.1 To conserve water, gardeners and tree care givers like me encourage mulching. In this article, we will discuss the "why," "what kind," "where," "when," and "how" of mulching.
Why should I mulch?
The benefits of mulching could fill a book, but we will be brief. It :
reduces moisture loss, as much as 10-25% (the sun warms bare soil causing water evaporation);
provides insulation keeping roots warm in the winter and cool in the summer;
increases growth rate of newly-planted trees and increases survival rate as it eliminates competition for nutrients in surrounding area;2
serves as a physical barrier to lawn mowers and weed trimmers that can cause bark and root injuries;
reduces soil compaction caused by foot traffic and rain;
reduces maintenance area and reduces weeds and grass; and
improves soil structure, nutrient level and prevents soil erosion.
What kind of mulch should I use?
A composted, medium-textured organic mulch. Organic mulch decomposes and releases small amounts of nutrients and organic matter into the soil. Composted mulch compounds have sufficiently broken down and will not rob plants of soil nutrients like fresh mulch will. Properly composted mulch should have a pH between 6.0 and 7.2. See chart for types of mulch and feedback on each.
Fresh, or unweathered, organic mulch will rob the soil of nitrogen during its early stages of decomposition.
Peat moss and double-chipped mulch is too fine and can hold too much water which can create other problems. Sawdust is also too fine, but if composted and mixed with other materials it, too, is fine for mulch.
Be cautious of large stockpiles of organic mulch, especially leaves and woodchips that often go through anaerobic decom
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position and become very acidic - pH of 3.0. This low oxygen/high moisture decomposition produces byproducts - methane and alcohol - toxic to plants. The mulch will emit a vinegar, sulfur and ammonia odor. The detrimental effects include leaf chlorosis, leaf scorch, defoliation and/or plant death. Damage generally occurs with 24 hours of application.
Where, or what area, should I mulch?
Ideally, mulch should be applied to a tree's or shrub's drip line - edge of the branch spread. Make sure area is free of grass and weeds before applying. The mulch will kill grass it buries.
Apply mulch in saucer-like form, shallow in center and rising at the edge. Do not pile the mulch (like an anthill) and keep mulch at least 3-4 inches from bark to prevent touching bark that could lead to bark rot.
When should I mulch?
Mulching can be done annually in late Spring once soil has warmed. Replenish mulch to 2-4" in depth when needed. Mulch can be raked to rotate decompos
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ing material.
How and how much should I mulch?
Check the soil acidity, soil texture and drainage. Some trees may not need mulch if area is naturally moist. Applying mulch to continuously wet soil can potentially create problems, such as progressive invasion of root rot pathogens. Indications will be mushrooms growing around trees. In some trees, mulch can make soil too wet and adventious roots may inadvertedly escape excessive moisture by growing into mulch. Excessive moisture can lead to cankers and fungus growth.
In heavy soils, too much mulch can cause root death. Roots need to absorb oxygen. Soil must be porous enough to contain 10% oxygen in the top 6-8 inches of soil. Sandy soils may benefit from 4-6 inches of mulch.
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1.State of Rhode Island Water Resources Board, February 14, 2002.
2. Journal of the Society of Municipal Arborists, July/August 1996. An 8' diameter of mulch around a newly planted tree will result in a four-fold increase in smaller feeder roots growth compared to trees competing with encroaching grass for nutrients.
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