Organic Mulch: Helpful or Harmful?
By Viveka Neveln

Organic mulch consists of any plant-based material layered on top of the soil, as opposed to inorganic materials such as plastics or gravel. Dictionaries agree that mulch is a "protective covering spread on the ground." However, some say mulch enriches the soil. Others add that it controls weeds and reduces evaporation, and still others say that mulch keeps the ground from freezing. While all of these are true, they’re not the whole story. Mulch also can cause some problems when used incorrectly. Here’s what you should know before using it in your garden.

Positives:

Controls weeds. Mulch prevents weeds from receiving sunlight that they need to grow. Larger weeds may have enough energy stored up to push through the mulch and should be removed before mulching. Layer mulch liberally to a depth of two to four inches for good weed control.

Conserves water. One gallon of water per square yard can evaporate from exposed soil on a hot, sunny summer day. A good layer of mulch helps to reduce the rate of water loss from the soil surface. This allows the plants to have a consistently adequate moisture supply from the soil when they might otherwise wilt.

Insulates soil. Because organic mulches are fibrous, they act as insulation and keep soil several degrees cooler than bare soil. For this reason, it is best to apply mulch once the soil has warmed up in the spring. Cooler soils lessen the heat stress to roots. When roots get too hot, they respire more, which uses up food reserves and reduces growth.

Provides nutrients. Organic mulch readily breaks down when worked into the soil. When it decomposes, it releases nutrients that plants can use. Partly decomposed mulches, such as composted leaves or manure , provide a better source of nutrients and are less likely to cause a nitrogen deficiency than bark, wood chips, or straw.

Improves soil structure. When combined with the soil, mulch provides organic matter which improves the soil’s physical condition. The particles of organic matter help to improve a soil’s water holding capacity, drainage, and aeration. This is especially true for heavy soils such a silty soil, clays, and clay loams.

Miscellaneous benefits. When used around trees, mulch protects the trunks from mower damage. In a vegetable garden, mulch can keep produce clean and dry, reducing the risk of spoilage before harvest. Mulch provides an attractive, neat background in the garden.

Causes nutrient deficiencies. Un-composted organic materials such as grass clippings, leaves, and straw can cause nitrogen depletion in the soil as it breaks down. However, once the mulch is decomposed, the nitrogen is released into the soil again. Use well-composted material or supplement with a commercial fertilizer.

Encourages diseases. Some disease-causing organisms, such as fungi, need a lot of moisture to infect plant tissue. When mulch is placed too close to plant stems, it may cause the development of diseases such as rots and cankers by keeping the stems constantly moist. Make sure mulch does not touch plant stems and never use mulch that contains any sort of diseased plant tissue.

Encourages pests. Organic mulch provides a damp, dark refuge for snails, slugs, and other garden pests. Mulch used to over-winter plants can provide nests for mice and other small rodents. Some insects also over-winter in plant debris. Straw and composted materials may introduce weed seeds, creating more weeds than what might have been present initially.

Causes temperature-related plant stress. The insulating property of mulch can work against it. If mulch is applied too early in spring, the soil may stay too cool. This can delay bud break on trees and shrubs or result in later planting dates for annuals. In the fall, mulch can keep soil temperatures too warm. This can force new, succulent growth on some perennials, making them more prone to frost damage.

Research conducted by agricultural experiment stations, the U.S. Department of Agriculture, and private industry all confirm that organic mulch offers a solution to a number of gardening issues such as weeds and compaction. When used properly, the benefits of mulch outweigh the drawbacks. An awareness of the possible problems is the first step for avoiding them and getting the most out of your mulch.

-Viveka Neveln

Viveka Neveln has a degree in Horticulture from Iowa State University and a degree in Plant Pathology from Cornell University. She has worked with the Iowa State extension service, the Chicago Botanic Gardens, a landscaping company, and a garden center. Her writing has been published by various magazines, newspapers,
and Web sites.