"The cornerstone of a landscape weed-control program is the use of mulches," report University of Florida researchers. Studies have shown that mulch can reduce weeds by 40 to 60 percent. It's also effective in container gardening, where it has been shown to reduce weeds by up to 90 percent, depending on the type of weed.
Mulch keeps weeds at bay by limiting the amount of light, air, and water that weeds receive. Mulch also creates a barrier to keep weeds away from the soil. There are two kinds of mulch: Organic (plant-based) and inorganic (man-made, which doesn't decompose).
Types of organic mulch include pine bark, hardwood bark, cedar chips, shortleaf pine needles, longleaf pine needles, pine straw, pine bark nuggets, cypress mulch, and cocoa shells. (Be careful, cocoa shells can be poisonous to dogs.) Hay and straw can also make up mulch, but Martin says to be sure they are weed- and seed-free or you'll just cause more problems.
Inorganic mulch varieties include gravel, quarry dust, sand, rubber chips, and landscape fabrics (both permeable and impermeable). Unlike organic mulches, they do not breakdown over time. While some of these mulches can be effective, they do have some pretty hefty downsides (see "What to Avoid," below).
"There are two cardinal rules for using organic mulches to combat weeds," says Martin. "First, be sure to lay the mulch down on soil that is already weeded, and second, lay down a thick enough layer to discourage new weeds from coming up through it." She recommends laying down 4 to 6 inches of mulch in sunny garden spaces, 2 to 3 inches in shady spaces. She also suggests using several sheets of newspaper, kraft paper (like grocery bags), or corrugated cardboard under your mulch for extra weed protection.
"Mulched gardens are healthier, more weed free, and more drought resistant than unmulched gardens, so you'll spend less time watering, weeding, and fighting pest problems," Martin adds. Other benefits of mulch include keeping your soil hydrated (especially important during a summer drought), reducing soil erosion and compaction, maintaining optimal soil temperature, and enhancing plant growth.
Natural Mulch Mistakes to Avoid:
• Don't confuse mulch and compost, note the University of Florida researchers. "Fine-textured and nutrient-dense materials such as composts are not suitable for weed control," they explain. Compost is made up of biodegradable materials (like food and yard waste) with the intent to create richer soil. If you spread compost around expecting to kill weeds, you'll be sorely surprised to find that it could have the opposite effect.
• Wet mulch against the stems of your plant can cause rot, so be sure to keep it pulled back 1 inch from flowers and vegetables and 6 to 12 inches from shrub and tree trunks, Martin advises.
Reasons to Nix Man-made Mulches:
• The researchers do not recommend landscape plastics because they don't just hurt the weeds; they can also hurt your plants and soil by causing nutrient stress and restricting root growth, water, and air. Martin also points out that there are few places to recycle these plastics.
• Martin advises against using plastic mulch under shrubs. "Although it keeps out weeds and can be camouflaged with decorative mulch, black plastic destroys the shrubs' long-term health," she explains.
• Black paper mulches, while biodegradable, are often treated with synthetic antimicrobial chemicals, says Martin, which isn't something you want in your garden.
• Permeable landscape fabrics allow for more air and water movement and do keep the weeds away, but they aren't effective long term nor do they protect against perennial weeds, and they are costly to replace. Martin also says that shrub roots commonly grow into the fabric, which causes issues when you want to remove them. Some are also treated with harmful UV-protective chemicals.
• Recycled rubber chips are not only less effective than regular wood chips, but there's also the added risk that they may leach unhealthy zinc, selenium, lead, and cadmium into the soil.
More: 8 Weeds You Can Eat
Grow Resistant Species
While the researchers say that there are no true "weed-resistant plants," some plants are more susceptible to weeds than others.
Plant-resistant species are able to resist parasitic weeds. Parasitic weeds, such as dodder and mistletoes, can attach themselves to a host plant to steal nutrients from the host. Resistant plants include scaevola, begonia, gomphrena, ornamental sweet potato, and verbena.
What to Avoid
• Plants that are the most susceptible to dodder are petunias, vincas, impatiens, and coleus.
Grow Plants That Make Their Own Shade
If your plants create their own canopies, the weeds won't have the sun they need to invade your space. Plants that create a thick, weed-suppressing ground cover include lady's mantle, catmint, creeping phlox, autumn goldenrod, Acaena inermis and Muehlenbeckia axillaris.
What to Avoid
• Keep in mind that some plants that are thick enough to crush weeds may in fact become weeds themselves. Creeping lilyturf can provide weed suppression, but can become invasive. Opt for native plants whenever possible.
More: How to Naturally Kill Midsummer Weeds
Weeding by Hand
Yes, it can be a little backbreaking, but it is certainly effective. "Sounds like a lot of work, we know," says Martin. "But pulling out a few weeds every day, or at least every week, keeps them from getting out of control and brings you close to your garden so you can inspect your plants for problems."
As you'd expect, hand-weeding is going through your garden and manually ripping out plants by the roots. To make it easier, Martin recommends weeding after rain. "Weeds' roots come free more easily from moist soil, and a cloudy day can seem downright inviting compared with pulling weeds under the blazing summer sun," she says.
What to Avoid
• Tilling or cultivating is effective, but it can be damaging to nearby plants and it can degrade the soil structure, so avoid going overboard with an electric or gas rotary tiller. "Too many tiller turns around the garden pulverizes the soil and destroys its structure," says Martin. "Tilling also reduces the organic matter content of your soil by speeding up decomposition of the soil's organic reserves." And tilling kills the important garden helper, earthworms.
"Use a rotary tiller for big soil-turning tasks, but avoid the temptation to apply those whirling blades to every soil-related chore," she says. "A sharp hoe is the tool of choice for routine weed removal and cultivation."
• Ditch the square-headed garden hoe. Martin recommends a stirrup-shaped oscillating or swan-neck hoe; hold the hoe as you would a broom.
Thermal Weed Control
Turn up the heat on your weeds. Thermal weed control uses heat—either from the sun, steam, or flames—to kill weeds. "A quick pass with a flame weeder (a propane tank with a long wand that lets you stand upright while putting the flame to weeds on the ground) boils, then bursts, the water cells in the leaves, wilting the plant," says Martin. "No need to set it on fire, no matter how enjoyable that may be."
Thermal weed control from the sun is called solarization, and it's effective against pigweeds, common purslane, and henbit, among others. After you've done your best hand-weeding, moisten the soil and cover it with a tight layer of clear plastic, explains Martin. " Leave the plastic in place for six weeks so the sun cooks any remaining weed seeds," she says. "This method also "cooks" many beneficial soil organisms, so it's helpful to amend a solarized bed with compost after you remove the plastic covering."
What to Avoid
• The researchers say not to use an open flame to kill weeds during dry conditions, as this can be dangerous.
More: Scientists Prove Goats Are Better Than Chemical Weedkillers
Smart Gardening Techniques
An ounce of prevention is worth a pound of cure in your garden. Here are some preventative gardening techniques to keep weeds under control.
• Fertilize by putting compost near the root zone so the nutrients are less available for weeds to steal. Avoid broadcasting fertilizer over the entire garden area—you'll be feeding the weeds, too, which is counterproductive.
• Deliver water directly to your plants and keep the weeds thirsty. Microirrigation, drip, or sub-irrigation systems work well for this. Overhead or surface watering systems can lead to more weeds.
• Sow vegetable seeds in rows to make it easy to separate out the veggies from the weeds, recommends Martin. "Planting in defined rows makes it simpler to mulch in between the rows, too, for additional weed suppression," she adds.
• Leave no ground naked, says Martin. "Choose plants vigorous enough to outcompete your most troublesome weeds, and plant them close enough so that once mature, their foliage touches, shading out competition."