A two- to three-inch layer of mulch spread around your beds and borders once or twice a year shields plants from temperature extremes, prevents soil erosion, helps retain water, and keeps weeds from sprouting. Natural mulch also adds nutrients to the soil as it decomposes. While inorganic mulch—pebbles, tumbled glass, recycled rubber—doesn't improve soil quality (though you can make up for this by spreading it over a layer of aged compost), it is long-lasting. (Note: Some mulches are only sold in certain locales; retail prices may vary and shipping costs apply. Check with companies for details.)
Determine how much mulch you need to sufficiently cover your gardens.
For more information on mulch.