How to Manage Pests

Pests in Gardens and Landscapes

An organic mulch
An organic mulch
Rock mulch on top landscape fabric
Rock mulch on top landscape fabric


A mulch is any material placed on the soil to cover and protect it. Mulches suppress annual weeds by limiting light required for weed establishment. Many types of landscape mulches are available. The most common are bark and other wood products and black plastic or landscape fabric materials. Other products include paper (e.g., in rolls, or sheet mulch), yard compost, hulls from nuts (almonds) or cereals (rice), municipal composts, and stones.

In addition to good weed control, mulch conserves soil mois­ture by reducing evaporation and reducing water use by weeds. Mulch moderates the soil wetting and drying cycle between irri­gations and moderates soil temperatures around roots, improving plant growth. Mulch also reduces compaction and ero­sion from irrigation, rainfall, and foot traffic.

When mulch is promptly applied to an adequately pre­pared site, especially after removing peren­nial weeds and their propagules, occasion­al hand-pulling or shallow hoeing of weed seedlings and periodic reapplication of organic mulch may be the only weed management activities necessary. If perennials become established, other practices such as herbicide use may be necessary.

Mulch can also be combined with a preemergence herbicide. The mulch type and thickness, herbicide placement on top versus beneath mulch, and preemergence herbicide efficacy are among the important considerations when combining mulch and preemergence herbicide. See Pest Notes: Weed Management in Landscapes for how to use mulches effectively and specific herbicide recommendations for use with mulch.

Mulching Problems

If not properly selected or used, mulches have disadvan­tages, such as favoring root diseases and certain weeds and interfering with irrigation. Keep organic mulch and waterproof synthetics 6 inches or more back from trunks to avoid promoting root and crown diseas­es. Regularly inspect mulch and remove any weeds soon after they appear.

Before applying mulch, properly grade the soil, control perennial weeds, and install any needed irrigation equip­ment. Be sure your type of irrigation (e.g., drip versus overhead sprinklers) and how you operate it provide appropriate root zone moisture and do not favor water-­related root diseases. How much water to apply, how often to irrigate, and the best method vary by growing situation and the water penetration and moisture retention characteristics of your mulched landscape.

Certain types of pathogens can be spread in some types of mulch. Depending on the type of plant pathogen and which plant parts will be used, you can apply the material as mulch around hosts after you chip or grind and properly handle it (e.g., by adequately heating or thoroughly composting).

Slime mold, sometimes called dog vomit fungus, commonly grows in mulch. It is not harmful and will dry out within 1 or 2 days.

Mushrooms sometimes grow in decay­ing mulch, and their appearance can alarm people or pose a hazard if children are tempted to eat them. Unless consumed, mushrooms are generally harmless to peo­ple, and many are beneficial, as discussed in Pest Notes: Mushrooms and Other Nuisance Fungi in Lawns.

Mulch types

The benefits and drawbacks of mulch vary by their type.

Organic mulches

Organic mulches can improve soil conditions as they decompose and are often inexpensive or free. Organic mulches include compost, grass clippings, greenwaste, leaves, and chipped, ground, or shredded bark or wood. Plan to replenish organic mulches periodically because they decompose, move, and settle.

Inorganic mulches

Inorganic mulches include gravel and crushed stones. They do not decompose to improve soil organic matter and usually must be purchased. If using a rock mulch, apply landscape fabric underneath to prevent soil and mulch from mixing, which favors weed growth and contaminates soil with rocks.

Synthetic mulches

Black plastic controls weeds, but restricts air and water movement. Manufactured, synthetic mulches called geotextiles, or landscape fabrics, allow water and air to pass through, avoiding a major disadvantage of black plastic.

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