What is Organic Mulch?

Organic mulch is material that is applied to the surface of garden soil to protect plantings and improve soil structure. Mulch is considered organic if it is made of material that was previously living plant matter. Examples of organic mulch include shredded leaves, compost, and crushed corn cobs.

Gardeners apply organic mulch to help lessen stress on growing plants. Mulch accomplishes this by helping the soil hold moisture by minimizing evaporation. As a result, plants and trees need to be watered less frequently and experience less drought stress. Organic mulch acts as an insulator by maintaining the soil at a relatively constant temperature. This can be especially helpful in the spring, when warmer soil temperatures encourage plant growth.

If organic mulch is applied correctly and the mulch is free of weed seeds, organic mulch will prevent weed germination. Mulch prevents light from reaching the seeds. If the weed manages to germinate, a thick layer of mulch will prevent the weed seedling from reaching the surface.

Organic mulch slowly decomposes into the soil, adding small amounts of nutrients. The decomposition process will attract earthworms and improve the soil structure by improving aeration and drainage. A protective layer of mulch will help prevent compaction from heavy rains or walking on the soil while wet.

There are some problems associated with the use of organic mulch, but many of these can be controlled or easily remedied. In wet climates, organic mulches may hold too much moisture. Wet mulch can compact and mat down, attracting rodents or encouraging growth of fungus and mold. Gardeners can prevent this by fluffing, or stirring up the mulch, periodically. If wet mulch is a continual problem, a thinner layer will allow some of the moisture to evaporate.

Most organic mulches raise the pH level of the soil slightly as materials decompose, resulting in a slightly alkaline soil. The decomposition process of mulches made from woody materials also depletes nitrogen levels in the soil, requiring occasional application of nitrogen-rich fertilizer. Nitrogen deficiency prevents plants from using all of the available nutrients in the soil, even if the soil is otherwise quite fertile.

Mulch should be applied in the spring after the soil has warmed. Otherwise, the mulch might inhibit spring growth by keeping the soil too cool. Mulch should not be applied until new plantings are 4 to 6 inches (10 to 15 cm) high.