Mulching is an essential practice for gardeners seeking to improve soil health, retain moisture, suppress weeds, and regulate soil temperature. However, improper use of mulch can harm plants rather than help them.

Mulch is a protective layer applied over soil, and gardeners have a wide range of materials to choose from, including inorganic options like gravel, plastic sheeting, and shredded tires, as well as organic materials such as compost, pine needles, wood chips, straw, and shredded bark. Each material offers distinct advantages and drawbacks.

Gravel and stones effectively reduce weed growth and retain some moisture, but they absorb heat, which can damage plant roots by scorching them. In fire-prone areas, gravel is recommended because it is nonflammable, unlike most organic mulches. Plastic sheeting can also suppress weeds but presents environmental concerns, as it eventually breaks down into microplastics, contaminating soil and groundwater over time. Shredded tires are highly discouraged due to their content of volatile organic compounds, lead, and other heavy metals, which persist in the soil even after removal.

Organic mulches are generally preferred due to their ability to enrich the soil as they decompose. Wood chips derived from tree pruning are highlighted as particularly beneficial. These chips are composed mostly of inner wood, which retains moisture better and releases it gradually. This type of mulch also contributes to the soil’s organic matter, supporting plant growth as it breaks down. The National Park Service uses wood chips to preserve the health of the D.C. cherry trees. Shredded bark is another popular choice, although it holds less moisture and can be more easily displaced by heavy rain or wind.

Dyed mulches, commonly found in black, brown, or red, are best avoided. Although their colorants are often nontoxic, the underlying material is usually recycled wood of uncertain origin, sometimes containing harmful substances such as arsenic. Some low-cost dyed mulches may contain toxic pigments, posing additional risks, especially around food crops.

When applying mulch, depth is crucial; three to four inches is generally ideal. Mulch should be kept a few inches away from tree trunks and plant stems to prevent moisture buildup that can lead to rot. The practice of piling mulch against tree bases—known as “volcano mulching”—can suffocate trees and cause long-term damage. Proper mulching leaves the basal flare of the trunk visible and free of covering.

The optimal time to mulch is after the last frost has passed but before weeds emerge. Applying mulch too early can trap cold air in the soil and slow growth, while late application allows weeds to establish and increases soil erosion.

Beyond its functional benefits, mulch also provides a neat, polished appearance to garden spaces.