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Mulch is any material, such as wood chips, grass clippings, leaves or compost, even rock, newspaper or shredded tires, that is spread over the surface of soil to retain water, inhibit weed growth and keep temperature moderate. Organic mulches are preferred as they break down overtime and encourage earthworms and beneficial insects to live in your garden.

Mulches and where to use them

  • Chipped or shredded woody waste: Spread three to six inches deep to make a good-looking, long-lasting mulch or path cover.
  • Deciduous tree leaves: Spread as mulch in the fall.
  • Evergreen leaves: Use these on pathways because they take longer to decompose.
  • Grass clippings: When grass is too long to mulch mow, you can save and spread thin layers of clippings over vegetable and flower beds.
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Mulch conserves water, prevents weeds and feeds the soil.

What does mulch do?

  • Retains water.
  • Inhibits weed growth.
  • Keeps soil temperatures from becoming too hot or too cold.
  • Protects sloping ground from soil erosion.

Applying mulch

  • Weed the area first, then spread mulch around plants to the drip line or cover the entire garden bed.
  • Three inches of mulch can be safely used for any woody plant, and up to eight inches can be used for large trees. However, thick mulches are harmful to shallow-rooted plants such as rhododendrons and azaleas. Mulch should be kept a few inches away from the trunks of trees and shrubs to prevent damage to the plants from disease and rodents. Be sure water can penetrate it to reach the soil and it doesn't smother the roots of the plants being mulched.
  • Replace mulches annually, or as needed when weeds poke through or the mulch has decomposed into the soil.

Sources of mulch

Every yard has grass, leaves or other green or woody materials that can be made into mulch. Lawn and garden centers sell bark of all sizes, compost and other materials that can be used as mulch. Sometimes tree services offer mulch left over from tree cuttings. Also, King County's GroCo compost can be used as mulch around plantings.
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