Mulch reduces evaporation from the soil surface, moderates soil temperature, and insulates roots from summer heat (and winter cold). Mulch suppresses weeds, reduces soil compaction, prevents erosion, and adds organic matter to the soil.
Mulch is any material that protects the soil surface and allows air and water through. Organic mulches–mulches derived from plant materials–not only protect the soil but add nutrients over time while enriching overall soil composition.
Summer mulch. Apply three to four inches of dried leaves or compost or grass clippings or straw around plants at midsummer to protect plants from hot weather and reduce the soil temperature by 10°F or more. Soil temperatures of greater than 85°F can slow plant growth. The temperature of soil under mulch changes much more slowly.
Mulch protects bare soil from being compacted over time by the drying of soil particles in the sun and the beating of rain or irrigation. A thin layer of mulch will protect soil microorganisms and other beneficial soil borne organisms from cooking in the summer heat.
Almost all plants benefit from the protection of mulch. Vegetables that most benefit from summer mulching are eggplants, tomatoes, cauliflower, celery, potatoes, currants, and gooseberries. These crops especially appreciate cool, loose, well-drained soil.
Applying mulch. Two cubic feet of compost mulch can cover an area of 8 square feet to 3 inches deep; one cubic yard of compost mulch covers an area of 108 square feet to 3 inches deep.
Mulch should be applied loosely and not compacted. Mulch should be kept back a few inches from the stems or crowns of plants and trunks of trees. If placed too closely, mulch can retain moisture and cause plant stems and trunks to rot.
Water can be delivered to plant roots through mulch into the soil. Drips systems set under mulch will more directly apply water to the root zone.
Mulch can control insects and diseases. Straw mulch can reduce the number of adult cucumber beetles laying eggs at the base of the plants. Mulch reduces or eliminated the spread of fungal spores often spread by the splashing of irrigation water or rain.
Types of mulch for hot weather vegetable gardens include:
• Aged compost. Loose, aged compost applied as a 2- or 3-inch sheet across the soil will slow soil moisture evaporation and keep roots cool in hot weather. Aged compost adds a wide-range of nutrients to the soil and improves soil structure and the retention of soil moisture.
• Shredded or chopped leaves. A 4-inch layer of dried leaves can cool the soil by as much as 18°F. Raked dried leaves can be shredded by the passing of a lawn mower set at 3-inches high.
• Grass clippings. Dried and aged grass clippings free of seed or weeds will protect the soil and add nitrogen to the soil as it decomposes. Avoid fresh grass clippings more than ½ inch thick which can mat and decay from the center causing a sour smell and too much heat. Lawn clippings used as mulch should be herbicide- and pesticide-free.
• Straw. Set loosely around plants to 4 inches thick, straw will protect the soil from summer heat while allowing water to easily reach planting beds. Straw set under strawberries or summer fruiting vegetables will protect crops from insects, soil splashing, and rots.
• Pine needles. Pine needles are effective in keeping rain or irrigation from washing away furrows and raised beds and lightly applied will protect newly seeded rows. Pines needles break down slowly.
• Newspaper. Four to six sheets of damp black and white newsprint paper will protect the soil and suppress weeds. Newspaper should be applied loosely. Paper is best topped with grass clippings or straw to keep it from drying out. Check to make sure the paper does not absorb irrigation or rain water.
• Living mulch. Vegetable crops closely spaced can form a leafy canopy that acts as living mulch protecting the soil from the heat of the sun and slowing the evaporation of soil moisture. Crops should be planted so that their leaves just touch or slightly overlap at maturity. Intensively planted crops require fewer square feet. Less unproductive soil is watered. Moisture transpiring from leaves is trapped as humidity below the leafy canopy.
Other mulches. Plastic mulches and plastic sheeting or films are not good choices for mulch in hot summer regions. They are likely to make the soil to warm. Black plastic absorbs solar radiation and radiates heat to the soil. Clear plastic transmits sunlight directly into the soil and holds the heat there. Plastic mulches are best used to help warm cold soil in early spring.
Aluminum foil or white plastic film can be used as mulch when the summer is not too hot. Light-colored mulch seems to blind aphids and keep them from landing on plants. Aluminum foil also will deter potato aphids, bean beetles, and squash borers.
Mulch problems. Rough-textured mulch can harbor slugs, snails, or sow bugs and should be kept back from vegetable crops.
End of season. When the weather cools, remove mulch from vegetable beds to minimize the drop in soil temperature. At the end of the growing season, turn the mulch under to add organic matter to the soil.