Use Organic Mulch-Not Landscape Fabric-To Control Weeds

In preparing to launch our latest innovation, I'll be sprinkling in landscaping tips into our blog.  This post comes courtesy of the Tri-City Herald, in Columbia, Washington.

Marianne C. Ophardt, WSU Benton County ExtensionSeptember 19, 2014

When we moved to our new house seven years ago, I had the enjoyable task of starting our landscape from scratch. My plan included a variety of trees in the lawn, and shrubs and ornamental grasses in landscape beds around the house and the yard perimeter.

Opposed to using landscape fabrics, or geotextiles, I mulched the beds with only a 4-inch layer of medium-sized shredded bark.

Admittedly, landscape fabrics are better than the black plastic used for mulching landscape beds in the '70s and '80s. The fabrics allow for the movement of air and water, where black plastic does not. Even though they break down with time, fabrics covered with mulch last longer than black plastic in the same situations.

So why am I fervently against using landscape fabrics? To make landscape fabrics more aesthetically pleasing, they are often covered with a thin layer of mulch. Bark or wood chip mulches decompose over time, creating material in which weeds can grow. While rock mulches do not break down, blown-in dust can create a layer of soil where weeds will also grow. Plus, landscape fabrics do eventually degrade, especially when covered with only a thin layer of rock or bark mulch that does not block UV radiation well.

Many gardeners who have used landscape fabrics will tell you they are not a good solution to long-term weed control. Weeds will invade the beds, and then the fabric becomes a nightmare, especially when Bermuda grass or field bindweed (wild morning glory) are involved. The roots, or rhizomes, of these trailing pernicious weeds find their way into the fabric fibers, making it impossible to rogue them out with pulling. If you do pull them, the fabric comes with them.

The best way to control weeds in landscape beds is with organic mulches. I prefer the look of shredded bark, but wood chips are just as effective and less expensive.

Dr. Linda Chalker Scott, Washington State University's Extension Horticulturist, prefers wood chips because of the possible contamination of bark with salts or weed seeds, and because the sharp, pointed fibers of softwood bark cause tiny slivers in gardeners' hands. In addition, bark mulch may resist water penetration because of the waxes and lignins contained in bark.

Whether you use bark or wood chips as a mulch, the layer should be 4 to 6 inches in thick. This blocks light from getting to the soil, preventing weed growth. The layer also helps retain soil moisture.

Keep mulch away from direct contact with the base of trees and shrubs. A thick layer of mulch touching a plant base can lead to collar rot, causing serious damage to trees and shrubs.

Chalker-Scott recommends applying a 8- to 12-inch thick layer of wood chip mulch if trying to reclaim a landscape site with a serious perennial weed problem. She cautions to taper down to a shallow layer close to trees and shrubs.

Mulching with bark or wood chips is not a place-it and forget-it situation. They gradually decompose, adding organic matter to the soil. This is good. Periodically, add fresh mulch on top of the old to maintain the weed controlling 4- to 6-inch layer, being careful to keep the mulch away from the base of plants.

-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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