by Susan H. Barrott, Horticulture Technician

"Mulching" describes the time-honored practice of covering soil with a layer of material that will provide a variety of beneficial gardening results. Mulch can limit weeds, conserve soil moisture, moderate soil temperature, decrease soil compaction and may also reduce the spread of some soil-born diseases. Mulching materials may be organic, from living sources such as wood chips, or inorganic, such as plastic sheeting. Over time, organic mulches can help build a better soil structure that pays off in healthy, vigorous plants that may be better able to live with insect and disease infestations.

No single material is the best "all-purpose" mulch for every job. The locations most often mulched include landscaped areas with trees and shrubs, perennial borders, annual flower beds and vegetable gardens. Matching materials, mulch depth and timing of application to the specific areas in your yard can ensure you get the most benefits from mulching.

The most effective landscape mulches should not require annual replacement or extensive maintenance. Trees and shrubs in landscaped areas need mulches that can reduce weeds and are easy to maintain, conserve soil moisture, reduce compaction and moderate soil temperature. Mulching can help keep equipment such as weed whippers and mowers away from trunks and stems, which in turn reduces bark injuries.

Because of their permanent nature and high visibility, more expensive materials that are slow to break down are often used in landscaped areas. Wood chips or shredded bark work well. Apply finely shredded wood chips 2 to 3 inches deep. Coarse textured bark and wood chips can be applied to a maximum depth of 6 inches; exceeding that depth will begin to block the flow of oxygen in and out of the soil. Keep mulches a few inches away from all trunks and stems so you don't provide a place for insects or diseases to begin attacking the plants.

Wood chips or shredded bark are often used on top of landscape fabric to achieve better weed control. Woven or layered landscape fabrics are made of various combinations of synthetics, such as plastics or vinyls, and allow air and water through while keeping light and weeds out. Rock can also be used to top-dress fabric, but may absorb enough sunlight to alter soil temperature unfavorably. Do not use solid plastic sheeting because it creates a barrier to the movement of air and water in and out of the soil, and bark has a tendency to slide off it in heavy rains.

Perennial borders are often located within or adjacent to landscaped areas; many of the same materials are suitable for both areas. Spread mulch 2-3 inches deep--enough cover to limit soil from splashing up onto to the undersides of the plants in order to reduce the spread of soil-borne diseases.

Annual flower beds do best with organic mulches that will break down rapidly when tilled into the soil at the end of the season. This adds organic matter to the soil, provides food for earthworms and helps builds a healthier soil structure. If annuals are directly seeded into the bed, wait until they are several inches high and you have weeded at least once before applying 1 to 2 inches of mulch. Shredded leaves, partially decomposed compost, grass clippings or cocoa bean hulls are all easy to turn in at the end of the season.

Vegetable gardens often produce larger and healthier crops when mulched at the correct time. Wait until the soil has warmed thoroughly for heat loving crops, but mulch cool season crops early in spring. Two to four inches of chipped leaves, partially decomposed compost, dry grass clippings and straw are all excellent choices for vegetable gardens. Mulched soils are less likely to compact and will stay evenly moist, which encourages root system development and can help contribute to increased yields.

Black plastic mulch is excellent for pre-warming cool spring soil for heat loving crops such as eggplant, melons, peppers, squash and tomatoes. Put the plastic in place one or two weeks before planting. Covering the plastic with a reflective material such as straw later in the season will ensure the temperatures don't get too high and "cook" the roots. You must punch holes in the plastic to allow water penetration or water adequately through the planting holes.

Mulching materials are available at your local yard and garden center and through some catalogs. Many municipalities also have compost available to the public. Compost can be made easily in your own backyard. See the Extension fact sheet AG-FS-3899 Backyard Composting, available from the Extension Distribution Center (612-625-8173), or search under "backyard composting" on our Extension website http://www.extension.umn.edu for further information on composting.

	Advantages					Disadvantages
Mulching material	Controls weeds effectively	Conserves moisture	Moderates soil temperature	Blocks soil splash	Adds organic matter	Blocks rainfall	High cost	High annual maintenance	Unsightly
Partially Decomposed compost
	Adds plant nutrients and builds soil structure. Excellent mulch material. Highly recommended for use on annual, vegetable and perennial gardens.
Shredded leaves						*
	Provides food for earthworms and builds soil structure. Excellent mulch material. Highly recommended for use on annual and perennial gardens.					*May mat and block rainfall if shredded too fine.
Cocoa bean hulls						*
	Has a chocolate aroma when wet. Recommended for annual and perennial beds.					*May mat and interfere with water penetration. Can blow away in exposed areas. May develop an unsightly but harmless white mold.
Grass clippings
	Completely dry clippings can be applied at 1-2 inches. Can be used on annual, vegetable and perennial beds.					Herbicide residues in clippings may injure mulched plants. Use clippings from non-treated lawns or wait three mowings before using clippings.
Pine Needles
	Will not cause a measurable change in soil pH. Best used on perennial beds as they will last for two or three years.					Removing needles annually from underneath evergreens may eventually cause nutrient deficiency in evergreens.
Mulching material	Controls weeds effectively	Conserves moisture	Moderates soil temperature	Blocks soil splash	Adds organic matter	Blocks rainfall	High cost	High annual maintenance	Unsightly
Wood Chips
	Appearance of mushrooms or other fungal growths is common, but not harmful. Has little effect on soil nitrogen when on the soil surface. Best for permanent landscape plantings. Can be used on perennial beds.					Once worked into the soil, chips will soak up high amounts of available nitrogen. Compensate by adding a high nitrogen fertilizer when you incorporate it.
Shredded bark
	Additional bark must be added every two or three years. Has little effect on soil nitrogen when on the soil surface. Best for perennial and landscape plantings.					Will soak up large amounts of nitrogen if worked into the soil. Compensate by adding high nitrogen fertilizer when you incorporate it.
Sawdust
	Good mulch if composted before it is applied.					If applied fresh, it will cause nitrogen deficiency in the soil, which can affect plant growth. Adding excessive nitrogen to compensate may burn plants.
Straw
	Keeps soil splash off vegetables which may decrease soil-borne diseases. Chopped straw is easier to handle and turn in at end of the season.					Must be free of weeds so it doesn't introduce weed seeds into the garden.
Landscape fabric
	Highly effective with low maintenance for several years after installation.					Needs top dressing with more attractive material. Deteriorates after several years.
Mulching material	Controls weeds effectively	Conserves moisture	Moderates soil temperature	Blocks soil splash	Adds organic matter	Blocks rainfall	High cost	High annual maintenance	Unsightly
Landscape paper
	Somewhat expensive, but very effective and easy to use. Can be tilled in at end of the growing season.					Expensive to use in large areas.
Newspaper						*
	Strips of paper allow better water penetration and don't blow away as easily as large sheets.					*Becomes slippery when wet for extended periods. Not recommended for paths.
Black plastic
	Excellent when used to raise spring soil temperature for heat-loving crops like melons, tomatoes, peppers and eggplant. Punch holes in the plastic to allow water and air penetration.					Soil temperature may be too high for cool-season crops like peas, cabbages, spinach and lettuce. Should be covered with a reflective material like straw mid-season to keep the soil temperature from damaging roots of all crops, including warm- season ones.
Clear plastic
	Works well to raise soil temperature in spring. (See disadvantages.)					Not recommended as weeds will grow under the plastic and soil temperatures are likely to rise high enough to damage plant roots.

Copyright  ©  2003  Regents of the University of Minnesota.
All rights reserved.

References:
Sears, E. 1996 Step-by-step Gardening Techniques Illustrated, Storey Communications, Inc. Pownal, VT, pp.187-192 Ball, J. 1988 Rhodale's Garden Problem Solver Vegetables, Fruits, and Herbs, Rhodale Press, Emmaus, PE, pp. 482-485