While mulches are not new to landscapes, interest in them is growing as gardeners adopt water conserving habits and other environmentally sound practices. Mulch is an organic or inorganic material applied to the soil surface during the growing season or applied over the plant during the dormant season. An important characteristic of mulch is its ability to insulate a plant and its roots from the effects of extreme temperature fluctuations.

Ideal mulches must allow water and air to enter and exit the soil. They should be attractive, odor-free and stay in place. Ideal mulches should not compact or become a fire hazard. Although no single mulch material will meet all these requirements, select one that best suits the needs of your site.

Benefits of a Mulch:

• Conserves soil moisture
• Moderates soil temperature by insulating the soil surface
• Reduces soil compaction caused by equipment and people
• Reduces soil erosion from wind or water
• Slowly increases soil fertility through decomposition (organic mulches only)
• Reduces incidence of disease by protecting above-ground plant parts from splashes that carry soil-borne inoculum
• Reduces fruit rot by eliminating contact between fruit and soil
• Reduces winter injury by minimizing temperature variation, reducing water loss in plants and decreasing heaving of plant crowns and roots
• Aids weed control

Characteristics of Organic Mulches:

Most gardeners use organic mulches, which are derived from plant material and imitate naturally occurring forest or prairie litter. Gardeners can develop their own organic mulches by recycling yard waste such as chopped or shredded leaves, branches, wood chips or grass clippings. Organic mulches can also be purchased from garden centers.

An important value of organic mulches is that they continuously add organic matter to the soil surface. Earthworms and other organisms will incorporate this material into the upper soil area. When the planting bed is renovated, the gardener can work this organic mulch into the soil deeply enough to actually incorporate it into the root zone.

Characteristics of Inorganic Mulches:

Inorganic mulches include lava rocks, pebbles, plastic or landscape fibers which neither break down and improve soil structure nor add nutrients to the soil. If a site requires renovation, inorganic mulches can be difficult to remove, whereas organic mulches can be easily incorporated into the soil.

Dark-colored inorganic mulches, such as plastics, warm the soil more than organic mulches. Black plastic will raise the soil temperature to approximately five degrees higher than uncovered soil. Clear plastic will warm the soil even more, to 10 degrees higher than bare soil.

Plastic is an effective mulch material for some crops, such as strawberries or tomatoes, where the plastic is removed each year. Plastic is not an effective mulch, however, for landscape plantings containing trees or shrubs, as it restricts movement of gases between the soil and the atmosphere. An adequate root system to support vegetative growth will not develop under plastic, which prevents adequate root intake of oxygen. Regulation of the proper level of soil moisture is also difficult under plastic. A drip irrigation system located under this mulch is necessary to efficiently and effectively apply water.

Unlike plastic, landscape fabrics (geotextiles), are porous. Oxygen and other gases and water enter and exit these fabrics, making them a suitable mulch for trees and shrubs. There are, however, several disadvantages to using landscape fabrics as mulch--when the pores in this fabric become filled with soil or other organic material, for example, weed seed can germinate above the fabric. If the fabric is covered with an organic mulch to hide its appearance and extend its life, the roots of woody plants can also become intertwined with the fabric, and considerable root loss can occur when these plants are transplanted. Always cover landscape fabrics with an inorganic mulch such as pebbles. Another disadvantage of landscape fabric is its tendency to slide off of sloped surfaces.

When to Apply Mulches:

Growing Season Mulches
Some mulches are intended to function for only a brief time. A mulch applied to a newly seeded lawn, for example, is designed to reduce loss of soil and plant moisture until the turf is established. On steep slopes, mulch can prevent erosion until a new planting is established.

In spring, allow the soil to reach the optimum temperature before applying mulch. For warm-season crops, such as tomatoes, apply mulch when soil temperatures reach 70 degrees F. Cool-season annuals such as pansies are mulched in early spring as soon as new growth begins.

In summer, mulching materials low in nitrogen, such as straw or coarse sawdust, should be fortified with a nitrogen fertilizer. One tablespoon of ammonium sulfate per one bushel of mulch, applied once or twice in spring, will prevent yellowing of plant material. Scatter fertilizer uniformly on surface of mulch, then slowly water in.

Dormant Season Mulches
Dormant season or winter mulches reduce injury by moderating temperature fluctuation and reducing foliar moisture loss. These mulches reduce frost heaving of the crown and upper roots caused by repeated thawing and freezing of soil water. In the fall, apply a loose mulch after several hard freezes (temperatures below 20-30 degrees F). If applied too early, the mulch will delay the plant processes that allow the plant to tolerate cold temperatures. After mulching, avoid pruning, adding nitrogen or any activities that stimulate new growth. Rose bushes and strawberries especially need winter mulch. Remove mulch in spring when new growth is visible. If a late freeze threatens, re-cover plants.

Thickness of Mulch Layer:

Over-mulching (maintaining mulch at depths greater than three to four inches) has reached epidemic proportions in managed landscapes. With fine organic mulches, such as compost or shredded leaves, maintain a two to three inch layer. For coarse materials, such as wood chips, maintain a three to four inch layer. Mulches decompose in time; their rate of decomposition depends on particle size and composition. Plan to add more mulch occasionally, but do not exceed the recommended layer depths.

Precautions for Special Cases:

Use very little, if any, mulch on poorly drained soils, for the mulch will keep the soil too wet, fostering both root rot and growth of toxic compounds injurious to the plants. If mulch must be used in a wet environment, use a coarse, textured mulch only.

Grass clippings are an effective mulch but require some preparations before use. Allow the clippings to dry before application; grass clippings are high in water and nitrogen and readily ferment. The heat and ammonia released in fermentation will injure or kill other plants. Before applying the dried clippings, mix with compost or an organic material low in nitrogen. Do not mulch with grass clippings that have been treated with an herbicide.

Do not use reflective mulches, such as white rock, close to a building. The reflected heat warms the building in summer, increasing the cost of air conditioning and also causing winter injury to plants from unseasonable rapid warming. Be cautious about using a wood mulch near the foundation area of a home. This provides an environment attractive to termites from which the pest can gain access to the home. Use a crushed, non-white rock border to keep wood two feet away from the home.

Table 1. Mulches and their characteristics

Adapted from Donald H. Steinegger and Amy Greving, Nebraska Extension, 2000