JoAnne Skelly: Be careful with covers; tree roots need to breathe
Trees and shrubs are living creatures that need air, water, nutrients and non-toxic soil to survive. People are living creatures who need many of the same things, but also seem to need weed-free yards. They often lay down landscape cloth under mulches and rocks to prevent weeds. In some cases, people put down plastic instead.
I can see the logic in the assumption that landscape cloth (geotextiles or weed barriers) will help reduce weeds, and maybe for a year or so, it does. After that, our wonderful Nevada breezes (winds, zephyrs, gales, mini-tornadoes, etc.) blow in dust, dirt and silt on top of the fabric and the weeds develop anyway. A 4-inch layer of organic mulch can work just as well as a weed barrier and as it breaks down, it puts organic matter into the soil. Landscape fabric often shreds over time and eventually has to be removed for aesthetic reasons. The downside of organic mulch is its flammability.
However, putting plastic under rocks or organic mulch near plants is a bad idea. Roots need to breathe, just as leaves do. Plastic is impermeable, which means it doesn’t let air (or water) into the soil to reach roots. Plastic creates an anaerobic (low-oxygen) environment. With no soil oxygen, the plant will suffocate and die. In addition, plastic doesn’t degrade. It just leaves a mess as it breaks up, and you end up pulling pieces of black plastic forever.
Tree roots do not grow as deeply as you might think, with the majority of roots in the top 18 inches of soil in Nevada. Many of the smaller roots that do the majority of air and water exchange are in the top 6 to 12 inches of soil. With plastic, roots have a tendency to grow even closer to the soil surface, struggling for air. One gardener described it “almost as if they were gasping for air and saying let me out!” In a clay soil, plastic may hold too much soil moisture in, drowning plant roots.
JoAnne Skelly is the Carson City/Storey County Extension educator for the University of Nevada Cooperative Extension and may be reached at firstname.lastname@example.org or 775-887-2252.