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Purdue University
Consumer Horticulture

Department of Horticulture and Landscape Architecture

Mulching Conserves Soil Moisture

Released June 1992
by B. Rosie Lerner, Extension Consumer Horticulture Specialist

Mulching your garden is a good practice for any gardening year but is particularly important in times of drought. Applying the right type of mulch not only conserves soil moisture, but can cool the soil as well. And, a mulch will help keep weeds under control so that your garden plants wont have to compete for what little water there is.

Most flowers, vegetables, trees, and shrubs will benefit from mulches in all but the wettest weather. In extremely dry conditions, mulching can make the difference between life and death for some plants. Mulching dramatically reduces water that is otherwise lost through evaporation from the soil surface. Sites that are exposed to heat, sun, and wind dry rapidly, but even shady areas could use some mulch this year. Because the mulch prevents light from reaching the soil surface, the ground stays cooler and most weed seeds will not be able to germinate. A 2-4 inch layer of mulch should be sufficient to conserve moisture and keep weeds controlled.

Many materials can serve as garden mulch, the most common being hardwood bark, straw, and rocks but many other materials work just as well. A good mulch is one that is clean of weed seeds, insects and other pests, easily applied, and economical. So use your imagination to make use of materials around the home.

Unwatered lawns probably haven't generated any clippings in recent weeks, but if you have been watering, you could bag the clippings as you mow and spread them in your garden. As long as the grass has not been treated with weed killers, the clippings can make an excellent short term mulch.

Other common materials that can be used include pine needles, sawdust, and newspaper. The pine needles do tend to have a slight acidifying affect on soil, but should not be excessively so if used in limited quantities. Sawdust, as well as some other organic mulches, may cause a nitrogen deficiency in the soil. You may need to add a little extra nitrogen fertilizer to the garden to offset that which is used up by the microorganisms in the soil as they break down the materials. Newspaper can be used either shredded or in sheets, but be sure to weight down the paper if sheets are used.

Black plastic mulches also work well in combatting weeds and conserving soil moisture but they cause the soil to heat up rather than cool. This warming effect is a great advantage in spring to get a head start on the season, but is less desirable in hot, sunny weather. Most spring plantings should have sufficient foliage cover to shade the plastic, but if not, a shallow layer of other mulch might be helpful in cooling the soil.

The most recent development in mulches is the fabric mulch that is most commonly used in the landscaping business. The fabric is made of woven or spun plastic cloth, allowing water and air to penetrate. Unfortunately, these fabrics also allow some light to penetrate as well, giving weeds a chance to germinate and break through the barrier. The fabric is best used where another, perhaps decorative, mulch will be used on top to prevent light from reaching the seeds.

Site last updated: 10 April 2006

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