A mulch is any kind of material applied to the soil surface for protection or aesthetic improvement of the area covered. The mulch material may be organic, such as bark or straw, or inert, such as stones or polyethylene cloth.

A mulch helps maintain favorable soil conditions under it. Increased plant growth is due primarily to conditions resulting from the use of a given material rather than to any growth-promoting substances present in the mulch itself.

Mulches have many beneficial effects on the soil, plants and the area surrounding the plants. A mulch:

Conserves soil moisture by reducing the evaporation of water from the soil.

Prevents crusting of the soil surface, thus improving absorption and percolation of water to the soil areas where roots are growing.

Maintains more uniform soil temperatures by insulating the soil, keeping it warm during cool spells and cool during the warm months of the year.

Reduces weed problems when the mulch material is weed free and is applied deeply enough to prevent weed seed germination or to smother existing small weeds. Proper use of mulches considerably reduces time and labor needed to weed garden areas.

Adds to the beauty of the landscape by providing a uniformly colored ground cover that may add an interesting texture to an otherwise drab surface.

Prevents plants and fruit from becoming mud splashed and so reduces losses from soil-borne diseases.

Despite these beneficial effects, mulches have limitations. They cannot smother large weeds or diminish plant pest problems and may increase slug or rodent infestations. Mulches have no marked influence on the vitamin or mineral content of the plant material.

Organic mulches

Because organic mulches are derived from plant material, they will decompose and affect both the soil and the plants in several ways.

Physical effects

If mixed in the upper soil layer, the mulch dilutes the soil and usually increases root growth. When a mulch such as sphagnum peat moss, bark or aged sawdust is used, it affects the soil almost immediately. On clay soils, bark or aged sawdust increases aeration. Sphagnum peat moss increases the water-holding capacity of sandy soil.

If the mulch is not well decomposed, it will promote granulation, or clinging together, of the soil particles. During decomposition of the organic material, soil micro-organisms secrete a sticky material that glues soil particles together to form granules. This is especially true of heavy soil types. Materials that decompose slowly, such as sphagnum peat moss, have little effect on granulation. Straw, fresh leaves or aged manure, which decompose rather rapidly, do promote granulation.

Mulches improve and stabilize soil structure, the arrangement of the soil particles. Because of the mulch layer, the soil structure is not disturbed by pelting rain or coarse streams or drops of water from irrigation devices. Some gardeners do not realize that cultivating when the soil is wet destroys good soil structure. Using mulches eliminates the danger of cultivation at the wrong time because very little, if any, cultivation is necessary. Another way to harm the soil structure is to walk on the soil when it is wet. A mulch on the soil serves as a cushion and reduced soil compaction.

Chemical effects

The pH, the degree of acidity or alkalinity of the soil, may be slightly affected. Acid sphagnum peat moss will gradually lower the pH--make the soil more acid--while most other organic materials will raise the pH slightly--make the soil reaction more alkaline. Oak leaves may be acid when reasonably fresh, but as they finally decompose, the net result is a slightly alkaline reaction. Continuous use of oak leaves, however, will keep the soil acid.

Small amounts of plant nutrients become available during the decomposition of the mulch because it, too, was once plant material containing these essential elements. The amount of fertilizer added is usually very small, however, and may not have much effect on the nutrients available in the soil. As an example, manure has iron in it but not enough to correct iron deficiency in an azalea or a pin oak. Reasonably fresh animal manure could supply nitrogen, but the ammonia in it could damage the foliage and/or roots. Dried animal manures that are packaged for sale are relatively high in nitrogen. If used at rates higher than those recommended on the container, these products can cause problems from too much nitrogen. Broadcasting gypsum or superphosphate will reduce the problem from ammonia fumes.

Nitrogen starvation or deficiency may become apparent in plants mulched with certain materials. If organic mulches such as crushed corncobs, sawdust, wood chips or straw are used, micro-organisms decomposing the organic mulch remove appreciable amounts of nitrogen from the soil. The removal of this nitrogen, which is used by the micro-organisms, reduces the nitrogen reserve in the root zone of the growing plant. If additional amounts of nitrogen fertilizers are not applied regularly, nitrogen starvation occurs in the mulched plants, which become yellowish green and grow slowly.

Biological effects

Organic mulches serve as food for many micro-organisms that maintain and promote soil granulation. The mulch also helps keep the temperature fairly constant so that activity of the micro-organisms can occur at a uniform rate.

Sometimes undesirable organisms--disease-causing fungi, bacteria and nematodes--may be added to the soil when mulches are applied. Molds often develop on cocoa bean or buckwheat hulls when these materials are kept too moist or are used in shady spots. Stirring them occasionally so the surface of the mulch will dry eliminates the mold. Insects and rodents may overwinter in some organic mulch. Poison baits can be used to eliminate rodents which may damage plants.

Weed seeds may be introduced into the garden with hay or straw or strawy manure. If you use various kinds of hulls or corncobs, be sure they are free of the grain or seed. Grain mixed in with crushed corncobs can attract rodents. To control weeds, mulch needs to be sufficiently deep, or weeds from the soil below will grow through it.

Organic mulch materials

Grass clippings

Grass clippings are readily available when the lawn is actively growing. Using grass clippings has several disadvantages, however. Weeds from the lawn can be introduced into the mulched bed. If the clipping are applied too deeply and packed too tightly, they will release heat and a foul odor as they decay. Clippings from lawns that have been treated with weedkillers can cause herbicide injury in tender plants.

Leafmold

Leafmold can be made by composting leaves in the fall. The material should be partially decomposed by the following spring. It is a good mulch but difficult to apply evenly and may not be particularly neat looking. Leafmold collected from wooded areas may contain nematodes.

Leaves

Leaves, probably the most inexpensive material available, are used extensively in areas with many trees. Avoid leaves collected from streets that have been sprayed with oil or treated with calcium chloride--both oil and calcium chloride are toxic to plants. Leaves may be difficult to keep in place in windy locations. Very dry leaves can be a fire hazard, and packed, wet leaves interfere with air and water movement into the soil. Leaves should be mixed with some other light material, such as straw, to prevent their becoming a compacted, soggy mass.

Peat

Peat is often called "peat moss," but this is a misnomer. Moss peat comes from mosses, while other types of peat originate from cattails, reeds, sedges and other similar water plants. Sphagnum peat moss is acid and useful for mulching and amending the soil around azaleas, rhododendrons and other plants that grow best in acid soil. For other plants, apply limestone at 3 to 5 pounds per 100 square feet on top of a 2-inch sphagnum peat mulch to counteract the acidity. Sphagnum peat is very resistant to decomposition and has many fibers into which roots often grow profusely. Hypnum peat is usually neutral or slightly alkaline, breaks down more rapidly and may have few fibers.

Most types of peat are brown and serve as a neutral background in the landscape. If the surface of a peat mulch becomes very dry, it will shed water and require stirring. Carelessly tossed cigarettes can ignite peat, but it smolders rather than flames. Fine peat will often be blown away in windy weather, but chunky peat will hold. Self-seeding of some plants may occur in peat.

Pine boughs or needles

These are usually recommended for winter protection of newly set or somewhat tender ornamental plants but can serve as a mulch as well. Any needle-type evergreen is a satisfactory source.

Sawdust

Sawdust is very commonly used in areas where it is readily available. Nitrogen deficiency is almost certain if fertilizer is not applied regularly. The alleged toxicity of sawdust is usually nitrogen deficiency in the soil from the action of micro-organisms decomposing the material. Sawdust from walnut logs can stunt plants because of toxic material in it.

Bark--shredded, chunked or chipped

Bark has become popular in recent years. It makes an excellent mulch and is very attractive in landscape plantings. Shredded, chunked or chipped bark lasts as long or somewhat longer than peat moss and adds organic matter to the soil. Bark may also be used for walks or paths in the garden area. To prevent nitrogen deficiency in plants mulched with bark, apply nitrogen fertilizer.

Straw

Straw is used for winter protection and as a summer mulch.

It is highly flammable, so do not use it where cigarettes or matches could be carelessly flipped into the material. Additional nitrogen must be applied to prevent starvation of the mulched plants. Weed seeds may be introduced with this mulch. Unless the straw is chopped into short lengths, it will be difficult to apply between plants growing close together.

Wood chips and shavings

Wood chips and shavings have become available in large quantities from utility companies. These materials vary in coarseness. They are sometimes rather fine or may be largely broken twigs or branches. Wood chips decompose slowly and may cause nitrogen deficiency if additional fertilizer is not applied. They make a very rustic-looking, satisfactory mulch. Wood chips should not be used in foundation plantings next to the house where termites are or could be a problem.

Inert mulches

These materials do not have plant material as their origin, but in many respects they may be similar to organic materials as mulches.

Physical effects

Using inert materials sharply limits evaporation of water from the soil surface, and overly wet conditions could develop in heavy, poorly drained soils. Mulching with inert materials helps maintain good structure on the soil surface by protecting the soil from the dispersing action of water from rain or irrigation. Cultivation, of course, is unnecessary when inert mulches are used. The soil temperature under inert mulches is more uniform than it would be if no mulch were applied. When clear or translucent plastic materials are used, the soil warms rapidly. Such materials may raise the temperature higher than opaque materials.

Chemical effects

Limestone or acid-derived gravel affect soil acidity, but there are no other chemical effects.

Biological effects

The fairly uniform moisture and temperature conditions under the mulch are conducive to development of various micro-organisms in the soil. This could favorable or undesirable, depending on the nature of the effect of the organism. As the temperature increases, organic matter decomposes more rapidly, releasing nitrogen that is available for plants. Soil granulation could be promoted, but disease-causing organisms would also flourish. Weeds or weed seeds are not introduced with inert mulches.

Inert mulch materials

Asphalt paper

Asphalt paper can be difficult to keep in place and can become unsightly. Tar is toxic to plants, so paper treated with tar should not be used as a mulch.

Crushed stone, gravel, pebbles

Various types of stone may be used for the effects they create. The material may be colored to blend in with the features of the home, patio or landscape. Stones used near lawn areas may get into the grass where rotary mowers could throw them and cause injury or property damage. The stones may also be handy ammunition for vandals. Stone mulches areas are difficult to keep free of debris, such as leaves and stems.

Fiberglass

Fiberglass is available as building insulation or in a woven or pressed mat. The insulation type is rather loose when dry, but it packs down when wet. When you handle the dry material, small pieces may break off and irritate the skin or eyes. The mats are preferable for mulch but must be fastened down to prevent their being pushed up by perennial weeds. Mats may be cut and placed around plants in the landscape or containers. They are effective in suppressing weeds.

Polyethylene film (black plastic), polyethylene-coated paper or plastic cloth

These black materials are popular as mulch not only in the garden but also around the home, where they are often used under stones, wood chips or other such materials, though organic mulches on top of plastic may float away during heavy rains. These materials are quire effective in preventing weed growth. (Clear or translucent materials applied over the soil surface will not suppress weed growth because light penetrates and undesirable plants will grow beneath the plastic.) Prepare the planting bed and apply fertilizer before applying plastic mulch. Future applications of fertilizers or herbicides will requite your pulling the plastic up so you can distribute the chemicals evenly. Soil can be used to hold down the edges of the plastic and prevent the wind's blowing it away. Set plants in slits cut in the material with a knife. If you use strips wider than 3 feet, additional slits may be needed to permit water to enter the soil. The plastic cloth allows good moisture penetration. Rodents can become a problem, particularly if the plastic is laid in the fall. Poisoned baits offer the most effective method of control. These baits are available from garden centers and retail nurseries.

When to apply mulch

Apply an organic mulch on most established ornamental plants in midspring when the soil has warmed sufficiently for active root growth. If applied before this time, the mulch will keep the ground cool and root growth of ornamental plants may be slow. Apply an organic mulch around newly set ornamental plants after they are put into place and thoroughly watered.

How deep to apply organic mulches

For best results, apply mulch at least 2 to 4 inches deep over the whole area during spring, summer and early fall, but avoid covering the crowns of very low-growing ornamental plants.

Tender ornamentals that need winter protection may require an additional 1 to 2 inches of mulch around the crowns or bases of the plants during the winter. In the spring, this added mulch should be fanned out or away from the stems or crowns of the plants before more material is added for a summer mulch.

Fertilizing organically mulched plant material

As indicated previously, mulching with many organic materials--including wood chips, sawdust, straw or shredded bark--means you must apply extra fertilizer around the plants to reduce the chance of nitrogen deficiency or starvation. Apply 1/4 pound of ammonium nitrate or ammonium sulfate for each bushel of mulch material used on the bed, or 2 pounds per 100 square feet of a complete fertilizer such as 5-10-5, 12-12-12 or similar analysis. This fertilizer should be applied before the mulch is placed on the soil, or in early spring before more material is added to plants already mulched. If the lower foliage yellows and the plants lack vigor during early summer, apply additional fertilizer.

Do not overmulch

The roots of plants need a constant supply of oxygen at all times. Overmulching kills the roots of shallow-rooted plants by suffocation.

Symptoms of too much mulch include chlorotic foliage (symptoms often resemble those of iron deficiency), abnormally small leaves, poor growth and dieback of older branches. Disease organisms that are active under conditions of low oxygen and excessive moisture can become active and attack the roots.

Sometimes the old root system will be rotted as the plant tries to send out new roots into the mulch layer. Excessive amounts of mulch applied around tree trunks can lead to cankers on susceptible species.