The main purpose of applying a mulch to your garden is to reduce water evaporation, raise or lower the soil temperature and restrict weed growth. We do not use mulches to fertilize the soil, prevent erosion, minimize transmission of plant diseases, prevent soil compaction by heavy rains, or minimize harmful soil fungi and nematodes, although it is claimed that mulches also do these things. To us, saving water, warming the soil in spring, and preventing weed growth are by far the most important features of using mulches.
Mulches can be either inorganic or organic. Inorganic mulches, also called synthetic mulches, consist of plastic sheeting and sheets, synthetic (indoor/outdoor) carpeting, aluminum foil and other materials that basically arent made by nature and dont decompose readily in the natural environment. These mulches warm the soil beneath them, provide better aeration, reduce fertilizer leaching and moisture evaporation, enhance bacterial activity, control weeds and keeps fruit clean and more free of disease. Plants tend to grow faster as well, the reason being that the plastic traps carbon dioxide, one of the most essential nutrients for plants, and allows the gas to escape around the base of the plant, thus providing it with an enriched growth medium. At night, the plastic radiates the heat back, that it collected during the day, to the plant, resulting in less frost injury and thermal shock. Organic mulches dont do this.
Reducing the evaporation rate of water is the single most important use of a mulch. Water is becoming both scare and expensive, and every drop you save is money in your pocket. In our garden we use drip hoses under a plastic mulch (black plastic sheeting) and find it is the best way to prevent unwanted evaporation and practice good water conservation.
During normal rains, the water collects on the plant leaves and quickly travels throughout the plant, and enough water travels down the stem of the plant and into the ground to satisfy the roots and worms. We have found that during a period of normal rains, we can turn off our water computer and stop watering the garden via the underground drip hoses. The soil remains moist and soft during this time period and we dont have to add additional water. However, during the times of drought the water computer adds enough water to keep the plants going, and the black plastic mulch prevents unwanted evaporation. This saves money and allows us to produce a nice crop of vegetables during difficult water deficient times.
A plastic mulch can also prevent flooding of your garden. In spring it tends to rain too much here in Wisconsin, and the garden can quickly become water-logged, rotting out the young seedlings. A plastic mulch prevents the heavy rains from penetrating the garden soil, keeping it reasonably warm and dry. If you have the good fortune of building your garden on a slight angle (the side of a gently sloping hill) as we did, it will allow the excess spring rains to flow off of the garden, instead of forming puddles and soaking the daylights out of it, as what will happen on flat ground. When the rains come, they quickly flow off of the garden plastic thus preventing the soil from becoming water-logged. Beginning gardeners think that the only way they will lose their crop is due to drought, but spring rains can ruin the crop just as easily, by drowning it. Remember this!