Mulches protect the soil from erosion. They hold in moisture, keeping plants hydrated and healthier while requiring less water. Mulches also suppress the growth of weeds. Any weeds that do manage to germinate and grow through the mulch material will be weak, spindly and easily pulled. Finally, mulches can help stop the spread of disease. Soil without mulch can splash on garden plants when raindrops fall thereby splashing disease organisms onto the plants.
Hay or Straw
Straw is comprised of the stalks of cereal grasses leftover after the grains have been threshed out. Hay is dried legumes, such as clover, or grasses, such as orchard grass. It is grown for animal fodder and carries much more nutrition than straw. Although straw is used more commonly for mulch, it does contain weed and grain seeds while hay does not, as the clover or grass is usually harvested before it has set seed. Either hay or straw make an effective mulch for flower and vegetable gardens when put down in a layer at least 3-inches deep.
Wood Chips/Shredded Bark
Available as rough-cut or smooth-cut, wood chips are widely used to mulch the area immediately around the trunks of trees and sometimes out to the trees' drip lines. They are also frequently used to mulch around shrubs or within the row of a hedge or shrub border. Shredded bark is more decorative and generally used as a mulch in perennial or annual flower beds, formal gardens and high-visibility shrub borders, such as foundation plantings. Wood chips or shredded bark should be applied in a layer at least 3 to 4 inches deep.
Buckwheat or Cocoa Hulls
Highly decorative, buckwheat or cocoa hulls are most often used in garden beds featuring flowers or specimen shrubbery in high-traffic areas. They are lightweight, however, and tend to blow away in high winds. Cocoa hulls smell slightly of chocolate. Both buckwheat hulls and cocoa hulls are more expensive than other types of mulch and must be reapplied frequently to make up for that which has blown away. They are most effective when applied in a layer 3- to 4-inches deep.
Fallen autumn leaves, grass clippings, pine needles and other garden wastes can be used as mulch. They break down more quickly than other types of mulch materials and enrich the soil as they do. If using fallen autumn leaves, it's best to shred them first as they have a tendency to mat down if used whole. Do not apply grass clippings as mulch to a vegetable garden if they have been sprayed with chemical herbicides, insecticides or fertilizer, as these preparations can leach out of the mulch and adversely affect garden vegetables. Pine needles change the pH of the soil they mulch, causing it to become more acid. They should only be used on plants which prefer acid soil, such as rhododendrons, azaleas, blueberries and most types of evergreens.