Do You Need Weed Barrier If You Put Down Mulch?

Many landscape architects and garden professionals view tree bark and other mulches as decorative "natural look" additions to the landscape, so using weed-barrier cloth underneath mulch to control weeds makes perfect sense. Gardeners committed to developing and maintaining dynamic living soil, however, believe organic mulch is the only weed barrier anyone could ever need.

One of the reasons to apply mulch is to prevent weeds. Mulch does that by blocking light. Plants, roots or seeds that might otherwise grow or germinate cannot do so without sunlight. Inorganic mulches include plastics and woven weed-barrier fabric. Most plastic mulches are covered with another material for aesthetic reasons, but also to delay the break-down of plastic from sun exposure.

Organic mulches include tree bark, straw, compost and grass clippings, usually applied in layers several inches thick. They need to be replenished periodically because of natural decomposition. Natural mulches also help prevent soil erosion, conserve water in soils and "feed" soil by releasing nutrients through steady decay. Some gardeners use thick layers of cardboard or newspaper mulch beneath other organic mulches to more effectively beat weeds and also build their soil.

There’s no question that weed-barrier cloth has its fans. Unlike black plastic, which is impermeable, woven landscape or weed-barrier cloth allows at least some air, water and soil nutrients to reach the soil, yet blocks light and restricts growth, therefore preventing weeds. Garden professionals and some homeowners like to use it because they can block weeds without drying out the underlying ground or completely killing soil microorganisms. They also don’t need to add tree bark or other decorative landscape mulches as frequently, because the barrier cloth prevents mulch contact with soil and slows its decomposition.

Don Engebretson, a Minnesota garden writer and designer, argues that weed barrier cloth is worse than useless. It’s insidious, he says, how popular its use has become: "What began years ago as landscape cloth—designed for vertical use behind boulder walls, to keep soil from eroding through the wall—has evolved into any number of ‘weed barrier’ products that homeowners are encouraged to lay flat on the ground before planting trees, shrubs, even perennials." Such products are superfluous, Engebretson writes, and they damage soil by limiting soil absorption of moisture, oxygen and nutrients. Thick natural mulch itself will block weeds just as well, and it costs very little in time or money to replenish it when needed.

Even advocates of weed-barrier cloth admit it has limitations, especially when you add inches of mulch on top for a natural look. Weeds seeds often get trapped in the surface material and sprout above the barrier cloth, thereby defeating the cloth's main purpose. Sometimes shrub or plant roots will also start growing above the cloth instead of beneath it, or get hopelessly entangled in it, ultimately damaging the permanent plants you meant to help. But there are some very good uses for weed barrier cloth, including rolling it beneath gravel or mulched walkways and under permeable patio paving to prevent weed growth and to stabilize the walking surfaces. Whether you use it in landscaping beds ultimately depends on you own choice.

In addition to mulching—whether with organic, natural mulches or inorganic mulches such as wee- barrier fabric—prevent weeds in the first place by making sure you don’t inadvertently introduce them with topsoil or "uncooked" compost. Also pull annual and perennial weeds as soon as you see them, pulling them out by the roots before they go to seed.