Until recently, gardeners had two choices when it came to getting rid of weeds around their trees and shrubs: Wipe them out with chemical products or get down on your hands and knees and do the dirty work yourself. Now there's an easier, more eco-friendly alternative: landscape fabrics.
Landscape fabrics eliminate weed growth around trees and shrubs for years, saving the time and energy you'll spend weeding and spraying herbicides. The fabrics are woven, nonwoven or spunbonded synthetic petroleum products that, when laid on the ground around plantings, provide a physical barrier against weeds and tree roots. Using a barrier for weed prevention is not new, of course. But the standard barrier for weed prevention -- plastic mulch -- prevents roots from getting essential water, oxygen and nutrients. It can also keep roots too wet, leading to an increase in root-rot diseases. Unlike plastic, landscape fabrics breathe, letting air and water through, and don't adversely affect the health of plants. In fact, research has shown that trees and shrubs with landscape fabrics around them grow just as well as ones without fabric, yet without the same weed competition.
"The best place to use landscape fabrics is around trees and shrubs or under decks and patios where you don't want weeds to grow," notes Bonnie Lee Appleton, nursery specialist at Virginia Tech University's Hampton Roads Agricultural Research and Extension Center in Virginia Beach. Appleton has been testing various brands of landscape fabrics since 1987. "I don't recommend using them for annual and perennial flowers," she advises.
When planting and transplanting annual flowers, gardeners tend to make many holes, opening the fabric to weed invasion. And most perennial flowers don't expand and fill in properly when grown with landscape fabrics. Also, don't get high expectations that these fabrics will control all weeds, says Appleton. Landscape fabrics control annual weeds better than perennials. "We've seen tough perennial weeds such as Bermuda grass and nutsedge eventually make their way through even the best landscape fabric," reports Appleton.
When installing landscape fabrics, first clear the area of all existing weeds. "It's best to install them after planting so you can carefully fit the fabric around the plant," advises Appleton. Anchor the fabric with metal pins with at least a six-inch overlap between pieces. Then cover it with mulch, if desired.
"Most brands on the market are now UV-stabilized and can last for more than five years, even in direct sun, but gardeners still like to cover their fabric with mulch for aesthetic reasons," notes Appleton. "Our research has shown that no more than a one-inch depth of organic mulch, such as shredded bark chips, is recommended over the fabric." Organic mulch applied any deeper allows weed seeds to germinate easily in the mulch. Once growing, weed roots will eventually break apart the fabric when you pull them, allowing for more weed penetration.
"The best mulch for weed control is decorative stone or rock," says Appleton. "Weeds don't tend to grow in it no matter what depth the mulch is. However, the soil under the fabric will dry out more than [when using] other mulches." In general, inorganic mulches are less weedy and keep the soil drier than organic mulches. Size has an effect, too. Large-sized organic and inorganic mulches, such as bark nuggets and rock, keep the soil cooler than smaller-sized mulches, such as crushed marble or shredded bark. If soil moisture and temperatures are a concern, choose the mulch appropriate for your soil.
There are many landscape fabric brands available to homeowners. In Appleton's studies, the best brands for suppressing weed growth were the ones with the smallest-sized pores (spaces between the fabric fibers). "We feel the small pore spaces didn't allow weed or tree roots to penetrate and ge a foothold in the fabric," says Appleton. Short of asking the manufacturer for its fabric's pore space size, a simple test a home gardener can use to rate landscape fabrics is to raise the different brands up to the light and see how much light shines through. "The less light, the smaller the pore spaces and probably the less weed growth with that fabric," notes Appleton. "We found the Weed-X brand to have the smallest-sized pore spaces of all the fabrics tested. It also had the least amount of root penetration from weeds and nearby trees," she reports. "Another brand that performed well was Dewitt's Weed Barrier."
The latest research in landscape fabrics is creating a new generation of treated fabrics that are even more effective in controlling weeds regardless of the types and depth of the mulch. "The fabric I'm most excited about is a nonwoven landscape fabric impregnated with copper," says Appleton. "Whenever tree or weed roots come in contact with the copper, they're essentially stunted, thus making the fabrics last even longer." Look for this new product this summer.