The pros and cons of landscape mulches

Mulches have long been considered an important component of aesthetically pleasing and environmental friendly gardens. However, not all mulches are created equal.

The best mulches (when properly applied) can conserve water, suppress weeds, regulate soil temperature, control erosion, and contribute to the health of both plants and soil. Conversely, inferior materials can be harmful and create problems in the garden.

There are two categories of mulch: (1) organic, made from once-living, carbon-based materials; and (2) inorganic, made from synthetic materials or non-living natural materials. Organic mulches are superior to inorganic ones for several reasons. They eventually decompose, and this decay supplies nutrients to plant roots and improves soil texture (especially important for clay). They don’t introduce foreign, non-biodegradable materials into the soil. And, although they need to be replenished periodically, they’re the most effective at cooling the soil.

Organic

Bark mulches. These come in many forms—shredded, large chunks, small nuggets—each with distinct qualities. Mulches of cedar and redwood bark decompose slowly and have natural insect repelling properties. Shredded bark products knit together in mats that resist shifting but can repel water. Bark chunks are the longest lasting and most water permeable. All are attractive and beneficial to the garden ecosystem.

Organic wood mulch in a thriving, low water use garden (Kathy Ikeda)

Recycled wood. Black- or reddish-dyed wood mulches have become popular lately, and they’re often cheaper than other wood products. They’re generally made from chipped scrap lumber, so it’s important to use a source that doesn’t incorporate painted or chemically treated wood. Avoid heat-retaining, black colored mulch in sunny areas.

Wood chippings. Some tree trimmers, electrical utilities, and landfills with green waste disposal facilities offer wood chippings free of charge to homeowners. Use caution with such sources, and do your best to verify that the material is free of diseases and weed seeds. As an alternative, rent a chipper and reuse your own pruned branches.

Other. Weed-free straw, grass clippings, shredded leaves, and pine needles can be used as mulch, but they decompose rapidly and are most appropriate for small areas or vegetable gardens.

Inorganic

Stone mulches. Natural materials such as rocks, pebbles, and gravel are long lasting, moisture conserving, and attractive, but they have limited usefulness in San Joaquin County’s broiling summer climate. Stone absorbs and retains the sun’s energy, then transfers heat to the soil and radiates it to nearby plants, even after air temperatures drop. This stresses plants, damages their bark and/or foliage, and leads to a need for excessive irrigation. Stone mulches are also difficult to clean of fallen leaves and other organic matter, and drip irrigation systems are very difficult to maintain and repair when covered with rock. Use stone mulches only in cactus and succulent gardens, dry streambeds and pathways, or shady areas with few plants.

Rubber mulch. This is made from recycled tires, and although it’s very water-permeable and comes in a variety of colors, it’s better suited for playgrounds than landscapes. It retains heat, is more flammable than wood, and can leach potentially toxic chemicals into the ground and water. Also, the durable rubber chunks eventually settle into the soil, creating an unfavorable environment for plants and their roots.

Black plastic sheeting. Once widely used for weed control, black plastic is impermeable and severely restricts air and water penetration into the soil. It also tears easily, leaving an unsightly and difficult-to-clean mess. It’s best used as a short-term soil covering for vegetable gardens, where it can reduce weed growth and warm the soil early in the planting season.

Geotextiles or landscape fabrics. These porous materials do allow pass-through of air and water. They’re typically used as underlayment beneath inorganic mulches, but they’re entirely unnecessary under organic mulches and are detrimental to soil health. Those containing UV inhibitors are more durable, but all will eventually deteriorate and leave non-biodegradable pieces of polypropylene in the soil.

Once you’ve selected your preferred mulch, here are a few important considerations:

  • Irrigate the soil thoroughly before covering it with mulch.
  • Apply mulches on top of (not incorporated into) the soil in a layer deep enough to retain moisture and minimize light penetration: 3 to 4 inches for coarsely texture mulches, 2 to 3 inches for finer ones.
  • Keep mulch several inches away from plants. Mulches placed directly against tree trunks or up to the “crown” of shrubs can encourage rot or pest infestation.
  • Leave a sizeable patch of bare, sunny, well-drained soil for ground-nesting native bees. These beneficial pollinators won’t tunnel through mulch, and they need areas undisturbed by cultivation or pesticides.
  • For the most efficient irrigation, convert overhead sprinklers to drip systems placed under the mulch. Water from sprinklers must be applied longer and in greater quantities in order to penetrate a layer of organic mulch and reach plant roots.
  • Don’t use mulch in areas where it could be washed off curbs and into storm drains during heavy rains.

Improper and damaging "mulch volcanoes" at the bases of recently planted trees (Mississippi State University)

For more information, see these resources: “Types of Mulch” and “Types and Uses of Mulch in the Landscape

For gardening-related questions, call the UC Master Gardener office at 209-953-6112, or use our website.

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    Lee Miller

    Lee Miller is a University of Delaware graduate and retired fisheries biologist, he gardens on 10 acres and makes wine each year with the help of a cadre of friends. However, his first love is gardening and he grows various fruit trees, heirloom ... Read Full

    Marcy Sousa

    Marcy Sousa is the San Joaquin County UC Master Gardener Program Coordinator. She is a Stockton native and enjoys teaching others about gardening. She has her bachelors from Stanislaus State in Permaculture. She has been with the program since 2007. Read Full

    Nadia Zane

    Nadia Zane is a UC Master Gardener, a landscape designer and Stockton native. She has a fondness for California native plants and sustainable landscaping, which she utilizes in her work for Native Beauty Garden Design. She is a member of the CA ... Read Full
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