The Loma Prietan - November/December 2009

Gardening Green

Control Weeds Naturally

story and photo by Arvind Kumar

Woodchip mulch helps control weeds in the native garden at Lake Cunningham Park in San Jose.
Woodchip mulch helps control weeds in the native garden at Lake Cunningham Park in San Jose.

If the plethora of herbicides in the marketplace is any indication, weeds are a major problem in American gardens. The vendors want you to believe that the best way to control weeds is with their synthetic chemical products, such as glyphosate and imazapyr.

Notably lacking from the promotional rhetoric is the impact of these chemicals on the environment. The major sources of pollution in urban creeks are products meant for the home garden, including herbicides, pesticides, and fertilizers. The winter rains wash it all out of our yards, down the storm drains, and into our creeks, rivers, and the Bay.

But there is good news. A recent study shows that when synthetic chemical use stops, the pollution levels in local waterways drop as well.

You need not choose between a weed-free garden and a pollution-free creek; you can have both. In California gardens, we encounter the same set of problem plants over and over — groundsel, oxalis, sonchus, burclover, prickly lettuce — plants that germinate and grow quickly, produce vast quantities of seed, and take over your yard if not checked.

Here are some proven, natural ways to control weeds without harming the environment.

Pulling: The simplest, most effective way to control weeds is to pull them, one by one. The best time to do this is in winter and spring, when the ground is moist and they come out easily, roots and all. In summer, use a weeding tool.

Mulching: What do you do when the area is too large for pulling to be practical? Cover the ground with a thick layer of woodchips, or mulch. Weed seeds simply can't germinate when denied sunlight.

Mulch is good for your garden in other ways, too: it helps conserve moisture in the soil; it prevents soil compaction; and as it decomposes, it returns organic nutrients to the soil in a slow, timed-release fashion. The microbes and worms will mix the nutrients in to produce really good topsoil.

Simply cover exposed soil with woodchips to a depth of four to six inches. Keep the chips away from trunks of trees, shrubs, and wooden structures you want to preserve (like the house). The chips will last three to four years before they need replenishing, but the best gardeners top-dress the beds every year with a thin layer of chips —- it gives the garden a fresh and attractive look.

You can buy woodchip mulch from your local nursery, or get it free from a tree service company. They can deliver a truckload (15 cubic yards) to your driveway; you need to be flexible about delivery time. If you can't use it all yourself, invite a neighbor or two to pick up some free woodchips from your driveway.

You may have heard of weed fabric, but I do not recommend it. Soil is more than just dirt; it is full of life — microbes, fungi, worms, and insects. Covering it up with inert plastic does nothing for soil health or fertility. Little thought has been given to the eventual impacts of these synthetic materials —- when they are no longer wanted, they go into the landfill.

Vinegar: Spraying vinegar on a broad-leafed weed dehydrates the plant and usually kills it within a few days. Adding a little soap and salt makes the spray more effective. Use with caution.

Boiling water: This technique works for very small, hard-to-reach places such as cracks in the concrete. Remove as much of the plant as possible, then pour boiling water over the remainder. You've just cooked it in place. Apply with extreme care.

Cardboard mulching: Bermuda grass (Cynodon dactylon) is an invasive perennial grass that expands by stolons, rhizomes, and seed. Discing only makes it worse, as each piece is capable of resprouting. Control it by digging up as much as you can, then laying cardboard or at least four sheets of newspaper on top, and finally covering it all with woodchips. Pull any stems that find their way to the surface.

Some weeds demand a multi-pronged approach. Bermuda buttercup (Oxalis pes-caprae) is a lemon-yellow spring-flowering perennial that looks harmless, but before you know it will take over your yard and the neighbor's. It may require several treatments of vinegar and pulling out the underground bulbs before you can eradicate it.

Sierra Club life member and California Native Plant Society director Arvind Kumar grows native plants in his Evergreen garden. He can be reached at