- The Washington Times - Tuesday, May 2, 2006

Centreville resident David Yost arms himself with handfuls of yellow granules to avoid a more physical battle with the weeds that, if given the chance, would pop up in his gardens.

Mr. Yost sprinkles the corn gluten, a byproduct of processing cornstarch, on his perennial and vegetable gardens every two months from spring to fall. The organic herbicide takes care of 90 percent of his weeds, he says, and he hand-pulls and uses herbicides to remove the rest.

“Everybody hates weeding, so I rely a lot on weed prevention,” says Mr. Yost, training coordinator and a volunteer Master Gardener for the Fairfax County Extension Office. He is a plant specialist at the Merrifield Garden Center in Fair Oaks.

The first step in weeding is identifying the type of weed, and if it’s unknown, visiting the local extension office to receive assistance, says Monica Lear, horticulture extension agent for the Arlington County Extension Office.

“You need to know what type of weed you have before you can treat it,” says Ms. Lear, who holds a doctorate in plant pathology.

Weeds can be grassy, such as crabgrass, annual blue grass and goose grass. They can be broadleaf, with stems and leaves, including chickweed, dandelions and clover, or they can have hollow stems, such as nut sedge.

Some weeds are invasive, which means they are nonnative plants that suppress native populations and are difficult to control.

The term invasive is interchanged with “noxious” to describe weeds that “infest large areas or cause economic and ecological damage to an area,” according to the Bureau of Land Management’s Web site, www.blm.gov. Some states, including Maryland and Virginia, maintain official lists of noxious weeds, the Web site says.

A weed is a plant that is out of place even if it might be desirable elsewhere, says Adria Bordas, horticulture extension agent for the Fairfax County Extension Office in Fairfax.

Weeds grow and spread rapidly and adapt easily to different conditions, such as a sunny or shady or wet or dry location, metro-area extension agents say.

A variety of organic and chemical herbicides can be used to prevent and control weeds. The herbicides can be selective for particular weeds or nonselective for all types. They can be pre-emergent to prevent weed seeds from germinating, or they can be post-emergent and kill weeds after they germinate and begin to grow.

Roundup, for example, is a nonselective chemical herbicide that does not allow plants to produce chlorophyll, killing the whole plant, including the roots. It is a residual product that dissipates slowly in the soil without harming it, effective for about two months.

Roundup, however, can kill desirable plants if not used carefully, says Jonathan Kavalier, manager of the Merrifield Garden Center in Merrifield.

“You have to be careful when you apply [Roundup] and apply it in a selective manner since the product is not selective,” Mr. Kavalier says.

Preen, a chemical herbicide that prevents weed seeds from germinating, does not prevent every seed that falls into treated soil from germinating, says Michael Zettel, owner of Azalea Acres Landscape Nursery in Fairfax Station.

“Weeds are much more hardy than any other plants. If they find a way, they’re extremely tenacious,” Mr. Zettel says.

For that reason, gardeners must keep a constant watch over their gardens. Metro-area extension agents recommend checking for weeds weekly or, at the least, every 10 days, because weeds can germinate that often.

As they weed, gardeners need to know how to distinguish a weed from a desirable plant, a skill that has to be learned, says David Clement, regional specialist for the University of Maryland Cooperative Extension Home and Garden Information Center in Clarksville.

“The choice is, what do you want to save versus what do you want to kill,” says Mr. Clement, who holds a doctorate in plant pathology. “There are products out there that will make those choices.”

Ms. Lear recommends using herbicides for spot treatment wherever the weeds are growing.

“You don’t want to apply herbicides unless you have to. When you do have to, you need to make sure you follow the directions on the container so you don’t harm other plants growing in the garden,” Ms. Lear says.

An alternative to using herbicides is hand-weeding, which requires time and extensive labor, Mr. Clement says, adding that hand-weeding can be used for areas missed by herbicidal spraying. It’s also an environmentally friendly option because herbicide runoff can travel into streams and rivers.

Another option is using mulch, an organic material such as pine bark, hardwood bark, compost, and shredded or chopped up leaves. Mulch is placed around the plants in a garden bed to shade out weeds and slow their growth as well as prevent moisture evaporation.

“If you mulch thickly — two to three inches — you will inhibit almost 90 percent of weeds that will grow there on the soil surface,” Mr. Clement says. “The problem is weed [seeds] can blow in and land on the surface and grow in the surface of mulch.”

Mr. Clement recommends placing a weed barrier underneath the mulch layer, using a plastic or woven material that allows moisture to penetrate and drain from the soil. The barrier combined with the mulch works especially well with more aggressive weeds, such as Canada thistle and nut sedge, he says.

Checking the soil for pH or acidity level — ideal at 6.5 to 7 pH — and nutrient content is another weed-prevention measure, Ms. Bordas says.

“It’s going to give plants and grass a better chance to grow and out-compete the weeds,” she says.

Local weeds

Maryland and Virginia have a list of noxious weeds posted on the U.S. Department of Agriculture Web site (www.ams.usda.gov/lsg/seed/nox06.pdf)Noxious weeds are unwanted, nonnative plants.

Noxious weeds in Virginia, listed

by common name (excludes lawn

and turf seeds):

Wild onion

Wild mustard


Plumeless thistle

Canada thistle

Field bindweed



Serrated tussock


Giant foxtail

Johnson grass

Noxious weeds in Maryland:

Plumeless thistle

Nodding thistle

Canada thistle

Bull thistle


Johnson grass

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