Washington State University Extension

Garden Tips


written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published – SEPTEMBER 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 plenty of shrubs and ornamental grasses in landscape beds around the house and the perimeter of yard. Adamantly opposed to using landscape fabric or geotextiles, the beds were mulched with a four inch layer of medium size shredded bark after planting.

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

So why am so I fervently against using landscape fabric? To make landscape fabric more aesthetically pleasing, it is often covered with a thin layer of mulch. Bark or wood chip mulches decompose with 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 can grow. Plus, landscape fabric does break down with time, especially when not covered with a thick (3-4 in) layer of mulch that blocks UV radiation.

Many gardeners who have used landscape fabrics will tell you it is not a permanent solution to weed control. Weeds will invade the beds, and then the fabrics become a nightmare, especially when Bermuda grass and 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 with pulling. If you do try to pull them out, the fabric comes with them. Arrrgh!

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. WSU Extension Horticulturist, Dr. Linda Chalker Scott, prefers wood chips because of some of the problems that bark shredded bark can pose. She favors wood chips because of the possible contamination of bark with salt or weed seeds and because of the sharp, pointed fibers of softwood bark are not “gardener friendly.” Plus bark can resist water penetration because of the waxes they contain.

Whether you use bark or wood chips as a mulch, the layer should be 4-6 in thick. This blocks light from getting to the soil and helps maintain soil moisture. I do advise keeping mulch away from directly around the base of trees and shrubs. It retains moisture and excludes air. A thick layer of mulch touching a plant’s base can lead to collar rot causing serious damage to trees and shrubs.

Chalker-Scott points out that if you are trying to reclaim a site or an area with serious perennial weed problems consider using a 8-12 in thick layer of wood chips. She cautions you to taper down to a very shallow layer close to trees and shrubs.

Mulching with bark or wood chips is not a place-it and forget-it situation. They decompose with time, adding organic matter to the soil. Periodically you will need to add fresh mulch on top of the old, to maintain the weed controlling 4-6 in layer, being careful to keep the mulch away from the base of plants. By the way, fall is a great time to renew the bark or wood chips in your landscape.

Published: 9/19/2014 12:27 PM

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