Fighting weeds with herbicides

Synthetic herbicides are not recommended for food gardens. In home ornamental gardens, they should be your last resort, called into play only when other methods have failed. Beyond the risks they may pose to health and the environment, many of these chemicals can damage desirable plants if they drift through the air or run off in irrigation or rainwater. Some persist in the soil for long periods, injuring later plantings. And often, the entire process of herbicide use ― selecting an appropriate product, reading the label, mixing and applying the spray, cleaning up ― takes more effort than simply pulling or digging the weeds.

If you use herbicides, always make sure the product is safe for the desirable plants growing in and near the areas to be treated, and keep in mind that you can be held responsible for any damage to neighboring properties resulting from herbicides you use.

If you have a particularly bad weed problem, consider contacting a professional with a commercial applicator’s license. He or she will have access to a larger arsenal of sprays and will know how to use them correctly.

Herbicides generally fall into the two classes discussed below: pre-emergence and post-emergence. A number of widely used products are described here, listed alphabetically by common name (the name you’ll find on the label under “active ingredients”). Trade names are given in parentheses. You’ll find other herbicides on the market besides those listed.

When applying any herbicide, read the label carefully and follow the directions exactly. Also review safety tips.

PRE-EMERGENCE

Pre-emergence herbicides work by inhibiting the growth of germinating weed seeds and very young seedlings. Before applying these chemicals in ornamental gardens, remove any existing weeds. Some pre-emergence products are formulated to kill germinating weeds in lawns; these may be sold in combination with fertilizers. Follow label directions carefully; some of these products must be watered into the soil, while others are incorporated into it.

• Isoxaben (Snapshot). Sold in combination with oryzalin, this herbicide effectively controls both grasses and broad-leafed weeds.

• Oryzalin (Surflan). Used to control annual grasses and many broad-leafed weeds in warm-season turf grasses and in gardens.

• Pendimethalin (Prowl). Controls many annual grasses and broad-leafed weeds in turf.

• Trifluralin (Treflan). Controls many annual grasses and broad-leafed weeds in ornamental plantings.

POST-EMERGENCE

Two types of herbicides act on growing weeds and other unwanted vegetation. Contact herbicides kill only the plant parts on which they are sprayed; regrowth may still occur from roots or unsprayed buds. Translocated herbicides must be absorbed by the plant, which they then kill by interfering with its metabolism. Many work best if you add a surfactant (spreader-sticker) to the mixture. Check product labels for specific directions.

• Fluazifop-butyl (Fusilade, Grass-B-Gon). Translocated. Controls actively growing grassy weeds. Can be sprayed over many broad-leafed ornamentals; check the label.

• Glufosinate-ammonium (Finale). Translocated. A nonselective herbicide that kills most kinds of weeds. Take care not to apply to desirable plants.

• Glyphosate (Roundup, Kleenup). Translocated. A nonselective herbicide that kills or damages any plant it contacts. Effective on a broad range of troublesome weeds, but must be used with care to avoid damaging desirable plants.

• Herbicidal soap (Superfast). Contact. Made from selected fatty acids, as are insecticidal soaps. Provides quick topkill on many annual weeds; regrowth may occur from the roots.

• MCPA, MCPP, Dicamba (Weed Away). Translocated. This combination of chemicals is used to control broad-leafed weeds growing in turf. Spray drift can injure nearby shrubs and trees.

• Sethoxydim (Poast). Translocated. Controls many grasses growing in ornamental plantings.

• Triclopyr (Brush-B-Gon, Brush Killer). Translocated. A nonselective herbicide that kills or damages any plant it contacts. Effective on hard-to-kill brushy weeds such as blackberry, poison oak, and poison ivy. Use with care to avoid damaging desirable plants.