Black plastic kills weeds by blocking sunlight from reaching the soil. Plastic mulch also helps prevent soil from drying out when water is scarce, and it sheds water during long periods of rain, preventing water-soaked roots from developing fungal diseases. Agronomists at Mississippi State University say cucumbers, eggplant, melons, peppers, squash, tomatoes and other warm season vegetables protected from weeds by black plastic mulch will have larger yields than if they are grown in bare soil.
Plastic mulch warms the soil in the spring, and depending on the weather, it can accelerate the first harvest by as much as seven to 14 days. White plastic much reflects the sun, keeping the soil cooler. Plastic mulch helps prevent soil from compacting, keeping it crumbly and loose. When cultivation equipment is used, plastic mulch helps protect the roots of crops.
Plastic mulch can be expensive, and because it is non-degradable, it has to be removed. Agronomists at Oklahoma State University estimate about eight hours of labor are needed to remove plastic from one acre. When removed, plastic mulch is dirty and may be contaminated by pesticides, a barrier to it being recycled. Some weeds still manage to grow through the holes intended for vegetables and along the edge of plastic mulch. Clear plastic mulch lets in the sun and can produce a humid growing environment that encourages weeds. Agronomists at Mississippi State University recommend using clear plastic only if the soil has been fumigated to kill weeds first. Plastic mulch can cause plant roots to get too hot in the summer; to prevent this, straw, hay, pine needles or other organic material is placed on top of the plastic. Another solution is to spray black plastic mulch with white latex paint to reflect the heat.
Plastic mulch is most useful in controlling weeds when used with drip irrigation. Drips systems and soaker hoses should be put in place before the mulch is applied. Plants can be irrigated with furrows along the edge of the plastic, but that doesn't work as well. When plastic mulch is in place to control weeds, irrigation by sprinkler is of little value.
Laying Plastic Mulch
Black plastic mulch should be laid one to three weeks before planting. Large sheets of plastic should be used to cover the entire area of a small garden rather than individual rows. The soil should be prepared completely before plastic mulch is applied; fertilizer and lime should be worked in, large clods broken up, weeds removed and the soil raked.
Planting and Fertilizing
When plastic mulch is used to control weeds, plants can either be planted manually by cutting slits in the plastic or with machines that punch holes. Excessive rain can leach fertilizer from the soil around the roots, especially in sandy soils. Since plastic mulch protects soil from rain, less fertilizer is required. Additional fertilizer can be applied through the planting holes or slits.