Allen Wilson: Mulch reduces weeds

Question: My neighbor has fewer weed problems than I do. He tells me it is because he uses mulch. Is mulch really that effective in reducing weed problems?

Answer: Yes, indeed. Using various types of mulch can reduce weed growth by 80 to 100 percent.

Most weed seeds need light to germinate. They lie dormant in the soil for years until soil is cultivated and they are brought to the surface. Placing mulch over the soil reduces light and can reduce weed seed germination by up to 90 percent. It requires a two-inch layer of mulch to produce this result, but smaller amounts are also helpful.

The most widely used mulch material in east Idaho is bark. Bark is available in a range of sizes from chunks down to fine bark dust, which is often referred to as “soil pep” the trade name of the largest bark distributor in our area. It is available in bags but is much cheaper purchased in bulk. You can pick up bulk quantities at local distributors or have it delivered to your driveway.

Although the larger sizes can be quite attractive around trees and shrubs, I prefer the finer sizes for most uses. Bark is normally applied about one inch deep around plants. One cubic yard of bark will cover about 324 square feet.

Compost, lava rock, pea gravel and river rock are other materials used for mulching around plants.

Black and colored polyethylene plastic sheets are useful in mulching around larger vegetables. They completely shade the soil underneath and prevent all weed growth. In addition to weed prevention, they increase soil temperature which speeds vegetable growth. They are normally applied in three- to four-foot widths with open soil strips between.

The most effective mulch around trees and shrubs is a combination of weed barrier fabric with granular mulch. Weed barrier fabric is a black, woven plastic cloth which is porous so water and air can pass through. Polyethylene plastic should not be used as a weed barrier around trees and shrubs. Larger plants do not get adequate irrigation. Plant roots grow close to the surface in order to get enough oxygen. Trees and shrubs need deep, anchor roots for support.

Another advantage of organic mulches such as bark and compost is that they are converted by worms and micro-organisms into humus and eventually release nutrients to the plants growing underneath them. Typically, bark and compost mulches are added to every year or two. Organic mulches also improve water penetration and reduce water evaporation.

When organic mulches are used around annual flowers and vegetables, they are typically incorporated into the soil in the fall or early the next spring. This improves the soil and adds nutrients.

Two other readily available materials are sometimes used as mulches. Grass clippings make excellent mulch, especially for vegetables. I used to use grass clippings around all my flowers, vegetables, and shrubs until I began leaving the clippings on the lawn. Multiple layers of newspaper make very good mulch for vegetables. However, it is necessary to put some soil on the edges to hold it in place.

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