How to Manage Pests

Pests in Gardens and Landscapes

An organic mulch
An organic mulch


A mulch is any material placed on the soil to cover it. Mulches suppress annual weeds by limiting the light, moisture, and gas exchange required for weed establishment. They can improve water penetration, regulate soil temperature, and prevent soil erosion.

For best weed control, use a coarse-textured mulch with a low water-holding capacity. When used alone, mulches rarely provide 100% weed control. To improve the level of weed control, apply preemergence herbicides at the same time as the mulch. Supplemental hand-weeding or spot spraying may also be needed. There can be problems associated with mulches. Some perennial weeds such as nutsedge often have sufficient root reserves to enable them to penetrate some mulches, such as a black plastic. Some annual weeds will grow through mulches; others may germinate on top of them. Applying mulches at depths of greater than 4 inches may injure plants by keeping the soil too wet and limiting oxygen to the plant's roots. However, lesser depths may have less weed control benefits. Disease incidence may increase when deep mulches are maintained.

Organic mulches

Organic mulches can conserve moisture, prevent surface crusting, improve water penetration, and cool the soil, but also can harbor invertebrate pests. Mulches include compost, very fine wood chips, grass clippings, sawdust, leaves, clippings, chipped and shredded prunings, wood products, and hardwood or softwood bark chips or nuggets. Plan to replenish landscape mulches periodically because of decomposition, movement, or settling.

Inorganic mulches

Natural inorganic mulches include sand, gravel, and pebbles. They do not provide organic matter for soil, but do conserve moisture. If using a rock mulch, consider placing a landscape fabric underneath to create a layer between the mulch and the soil and prevent rock pieces from sinking into the soil. Black plastic has been used to improve weed control, but it restricts air and water movement. Synthetic mulches, which are manufactured materials that are called geotextile or landscape fabrics, have been developed to replace black plastic in the landscape. Geotextiles are porous and allow water and air to pass through them, overcoming the major disadvantage of black plastic.


Statewide IPM Program, Agriculture and Natural Resources, University of California
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