Mulches have many positive effects on soils. In general, organic mulches conserve water, reduce weeds, improve soil quality and enhance plant growth, according to the Oregon State University Extension Service.
Mulch is any material that is spread over the soil surface and influences soil characteristics and sometimes plant growth. Organic mulches accumulate naturally in forests, in gardens where leaves and organic debris are not removed, and in lawns that are “mulch mowed.” The most common organic mulches used in the Pacific Northwest include bark products, yard waste compost, wood chips (arborist mulch), mint hay and deciduous tree leaves.
Mulching reduces irrigation needs by reducing surface evaporation and runoff. Mulches also protect the soil from compaction by rain and foot traffic. By preventing the loss of soil structure caused by compaction, mulches promote water infiltration into the soil. Also, mulched plants can better use water stored in the soil because they have larger root systems than do unmulched plants.
March or April is a good time to mulch woody ornamentals for water conservation. Mulch applied in summer, after soil has dried out, is not as beneficial, OSU Extension says. Fall mulch applications can smother winter annual weeds, decrease runoff and increase soil water retention for the following summer.