Mulch — any material that covers the soil — has several important roles in conserving garden water: it keeps moisture in the soil, moderates the soil temperature, and suppresses weeds that compete with plants for moisture. Organic mulches gradually work their way downward and improve the soil. Mulching also reduces soil erosion and protects the soil from being compacted by foot traffic or heavy rains.
Without mulch, the surface of the soil is exposed to air and sun. It dries out quickly, heats up during the day, and chills at night. Plant roots prefer less extreme conditions and, in dry climates, don’t grow in the top few inches of exposed soil. Water applied to this surface layer evaporates back to the air rather than being taken up by plant roots.
A mulch not only insulates the soil, it prevents moisture from evaporating. Even a thin mulch of organic material reduces evaporation, and an impervious film like polyethylene sheeting stops it altogether.
Mulches also suppress weed growth. Weeds compete with plants for soil moisture, and some weeds give off chemicals from their roots (allelopathic compounds) that slow the growth of plants around them. All mulches slow down weed growth, and deep mulches and impervious films block them altogether.
As organic mulches decompose, they add humus to the soil, enriching it and helping it hold water better.