Mulching

“In agriculture and gardening, mulch is a protective cover placed over the soil, primarily to modify the effects of the local climate. A wide variety of natural and synthetic materials are used.”

“Mulches are used for various agricultural and gardening purposes. Properly used, they benefit plant growth and minimize garden labor. The main functions of mulches are the conservation of soil moisture and the moderation of soil temperature. They moderate internal soil temperatures by retaining heat from the day and radiating it to the soil at night during spring and fall; in the winter mulches moderate soil warming during the day, limiting the stress plants undergo during soil freezing and thawing cycles and they can prevent frost heaving of unestablished plants.”

http://en.wikipedia.org/wiki/Mulch