COLUMBIA, Mo. – Mulches can help garden soil stay cool during the heat of summer.

Maintain 2-4 inches of an organic mulch to keep the soil cool, says University of Missouri Extension horticulturist David Trinklein. This promotes root growth and curbs soil moisture loss. By blocking sunlight, mulch also prevents weeds from germinating. Finally, organic mulches improve soil structure as they decompose and add nutrients to the soil.

Mulch materials include dried grass clippings, shredded leaves, pine needles and ground softwood tree bark.

A recent trend has been commercially bagged mulch made from recycled shipping pallets that have been ground into “wood chip” mulch. But shipping pallets are made from less valuable hardwoods such as cottonwood or sycamore, Trinklein said. “Wood chip mulches tend to break down more rapidly than softwood mulches such as pine bark or shredded cypress.”

For best results, Trinklein recommends pine bark or shredded cypress over hardwood mulches, especially for annual flowerbeds. Dried grass clippings also work well.

Gardeners may see problems with nitrogen deficiency when they plant into existing mulched areas. Mulch can fall into the planting holes, where soil microbes will break down organic matter in the mulch, consuming nitrogen in the process. Nitrogen deficiency often results in lighter green leaf color, weak or slow growth, and even yellowing and loss of lower leaves.

To counter this, spread a fertilizer high in nitrogen on the soil surface before applying mulch. For established mulch, add more nitrogen fertilizer during the growing season as mulch decomposes.

Sour mulch has become a problem in recent years, Trinklein said. “Basically, sour mulch is mulch that has decomposed improperly. The result is a foul-smelling mulch that often gives rise to compounds toxic to plants. Symptoms such as leaf scorch, defoliation or death of the plant can result from sour mulch.”

While sour mulch is not common, hardwood mulches tend to break down more rapidly, which makes them more likely to become sour than softwood mulches. Good mulch should have the aroma of freshly cut wood or good garden soil. Sour mulches smell of ammonia, sulfur, vinegar or, perhaps, silage.

“The benefits of mulching greatly outweigh the concern of plant damage from sour mulch,” Trinklein said. “However, be aware of the problem and check mulches before applying.”

Once mulch is established, it may not be necessary to add new mulch every year. This especially is true in areas where it initially is applied more thickly, such as around trees and shrubs. However, adding a thin layer of new mulch often improves the appearance of the landscape. In most cases, a depth of new mulch equal to the amount of decomposition during the past season is adequate.

Mulches help to make gardening more sustainable, said Trinklein. “Their ability to conserve water, retard weed growth and eventually add nutrients to the soil make their use a logical way to reduce the inputs needed to grow an attractive garden.”

For more information, see the MU Extension publication “Mulches” (G6960), available for free download at extension.missouri.edu/G6960.