PHOTO BY DEBRA BURROWS Pictured is an example of “volcano mulching,” which is very harmful to trees and is not recommended.

When used properly, organic mulch can provide numerous benefits for garden beds and landscapes. When used improperly, mulch can cause serious damage. With the growing season fast approaching, learning how to apply organic mulch properly can help gardeners maximize the advantages and avoid costly mistakes.

Let’s begin by looking at the advantages.

A layer of organic mulch can help the soil retain moisture, which is especially important during heat waves and periods of drought. Mulch also prevents weeds from sprouting by blocking sunlight from reaching weed seeds. Mulch moderates the temperature of the soil, allowing it to rise and fall more gradually, thereby reducing damage from frost-heave in winter and helping to keep roots cool in summer. In areas where soil borne disease organisms are often a problem, such as vegetable gardens, mulch can keep soil from spattering up onto plant leaves during rainstorms or overhead watering.

In addition, mulch adds a measure of eye-appeal to the landscape by contrasting with plants and giving beds a neat appearance.

Organic mulch options for landscaping include shredded bark, wood chips, pine needles (sometimes called pine straw), nut shells, rice hulls and shredded leaves. Organic mulch options for vegetable gardens include pine needles, shredded leaves and straw.

Although hay can be used as a mulch, it is not recommended because it contains a much higher volume of weed seeds than straw. While shredded leaves work well as mulch, using whole leaves is not recommended because they tend to mat down when wet and prevent water and air from reaching the ground. Organic mulches are made from plant-based materials so they decompose over time and improve the soil as they are incorporated into it.

Mulches can be purchased in bulk (by the truckload) as well as in individual bags. Mulch comes from a variety of sources, and those sources matter.

Mulch made from recycled lumber may contain toxins or other chemicals. Mulch made from grinding trees, shrubs and other organic materials that were treated with pesticides prior to harvest may contain residues. If purchasing in bulk, ask the supplier about the source of the materials. If purchasing bagged mulch, carefully read the label.

While using mulch in the landscape is usually a good practice, sometimes there can be too much of a good thing, and mulch is one of those things. In the case of mulch, more is not necessarily better. A two-to-four-inch layer of mulch is recommended for most applications. Applying more than 4 inches can prevent water from penetrating and cause roots to suffocate due to lack of air.

Piling mulch up the trunk of a tree in the shape of a cone, often referred to as “volcano mulching,” can lead to a variety of problems. When wet, layers of mulch against the bark provide opportunities for pests and diseases to take hold. The cone of mulch invites rodents to take shelter and begin eating away at the trunk of the tree. While cones of mulch might look appealing to some, they can be very harmful and even fatal for trees. Mulch should not touch the trunk of a tree, base of a shrub, or crown of a plant.

Another common mistake occurs when the layer of mulch is not thick enough, allowing sunlight to penetrate and reach weed seeds. A layer of mulch that is too thin will be less effective at retaining moisture and moderating soil temperature. Most recommendations are for a minimum depth of two inches.

Do not place landscape fabric or plastic under organic mulch. Not only is it unnecessary, it will interfere with the incorporation of decayed organic mulch into the soil. The soil surface should be weed-free before applying mulch. Weeds can be tenacious survivors and are unlikely to be killed by placing mulch on top of them. Once mulch has been applied, try not to disturb it and bring soil to the surface, which will then provide a location for weed seeds to sprout.

Finally, remember that organic mulch will eventually decompose and it will be necessary to add new mulch to replace it. While this may seem like additional work, it also has benefits. I’ve used organic mulch by the truckload in my landscape for many years and have been very pleased with the results. Not only did it decrease weeds and help keep my plants healthy, it improved the heavy clay soil found throughout our property as it decomposed.

Additional information on organic mulching is available from Iowa State University Extension at https://yardandgarden.extension.iastate.edu/how-to/using-mulch-garden. Penn State Extension has information on organic mulch, as well as synthetic and living mulches at https://extension.psu.edu/mulch-a-survey-of-available-options

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Debra C. Burrows, PhD is a retired Penn State Extension Educator and a certified Master Gardener. She can be reached at dcb3@psu.edu.