CORVALLIS, Ore. – If you're considering applying bark products or other mulches to your home landscape, you might wonder what kinds are best, how much to use and how to maintain the benefits over time. A new publication by Oregon State University Extension Service guides the way.
What is mulch? It's any material that is spread over the soil surface and influences soil characteristics and sometimes plant growth, according to the publication. Called "Mulching Woody Ornamentals with Organic Materials," the publication (EC 1629-E) is in a question-and-answer format and can be found online.
Mulching helps conserve water, suppresses weeds, improves soil quality and enhances plant growth. The most common organic mulches in the Pacific Northwest include bark products, yard waste compost, wood chips (arborist mulch), mint hay and deciduous tree leaves.
Spring is a good time to mulch woody ornamentals (trees and shrubs) to conserve water. Mulch applied in summer, after the soil has dried, is not as beneficial. Fall mulching also is effective; it can smother winter annual weeds, decrease runoff and increase soil water retention for the following summer.
"Woody" mulches such as bark mulch or wood chips control weeds best because weed seeds have a hard time germinating in them. Composted mulches, on the other hand, often create a good seedbed for weeds because they contain more fine particles and nutrients.
To conserve water and control weeds around a young tree, place a circle of mulch four feet in diameter. The mulch will cover the developing root system and prevent mower damage to the trunk. The mulch layer should be three to four inches deep. Avoid piling it right next to the trunk.
Bark mulch usually needs to be reapplied every two to four years and wood-chip mulch every one to two years. Bark usually decomposes more slowly than wood chips because it is higher in lignin, which is highly resistant to decomposition. Wood-chip mulches are less dense and undergo greater settling.
Several types of mulches are described in the publication with color photos. A table compares their characteristics: ease of application, longevity, nutrient release, weed control, water conservation, erosion control and cost.