Postemergent Herbicides

By Stephen Kelly, regional editor

Postemergence Herbicides. Table prepared by Chuck Burgess, HGIC Information Specialist, Clemson University. This information is no endorsement by the Clemson University Cooperative Extension Service. View full Size Image

Postemergent herbicide is a fancy phrase for killing/controlling unwanted grasses and broadleaf weeds. The ?EUR??,,????'??post?EUR??,,????'?? implies the weed seeds have already sprouted, as opposed to ?EUR??,,????'??pre-emergent?EUR??,,????'?? herbicides that seek to kill weed seed. Broadleaf weeds (dicots or dicotyledons), have seedlings with two seed leaves or cotyledons. Broadleaf plants usually have leaves with pronounced netted or branched venation.

Postemergence herbicides target visible weeds. They are used primarily against broadleaf weeds, perennial grasses, and sedges. The chemicals 2,4-D, dicamba, carfentrazone, triclopyr, clorpyralid, MCPP and MCPA are broadleaf herbicides. They have been combined in many products that control broadleaf weeds, such as Weed-B-Gon Weed Killer for Lawns, Trimec Southern, Bayer Advanced Southern Weed Killer for Lawns and many others.

When to Treat

Most broadleaf weeds are best treated in the spring or fall when air temperatures are between 65 and 85????? F. In hotter temperatures, turf damage is more likely. Weather conditions are a factor.

Treat weeds when no rain is expected for at least 24 hours, for the obvious reason that watering will dilute the herbicide and perhaps spread the herbicide where you don?EUR??,,????'???t want it. However, herbicide treatment works better with moist soil conditions. Drought-stressed turf makes for poor weed control and is more likely to damage the turf. Also avoid treating on windy days because some herbicides can injure ornamental plants.

Other Considerations

Best treatment results occur when weeds are young. Repeat applications, 10 to 14 days apart, may be required to get the control you seed.

Postemergent herbicides can be applied to newly seeded lawns, but at only half the recommended rate and only after the lawn has been mowed four times. If overseeding after a postemergent herbicide treatment, you must wait one to four weeks, depending on the product.

Spray or Granular?

Coverage is easier with spray applications and herbicides that you can spray are generally less expensive. One drawback is that some herbicides (whether liquid or granular) are volatile, i.e., the tendency of the herbicide molecule to become a vapor, which may result in the movement of herbicide to unwanted areas or loss of effectiveness at the application site. Barricade and Ronstar are essentially nonvolatile. If you spray a volatile herbicide, you will need to water it in. Granulars coverage is generally not as uniform as spraying. Mixing granulars with fertilizer defeats the volatility of herbicides. Regardless, the granular herbicides must be watered in to activate them.

Postemergence Schedules

• Feb.-March Before turfgrasses begin to green up for summer growth, apply a postemergence herbicide to control the emerging winter broadleaf weeds or summer broadleaf weeds. With any pesticide, read the label to make sure that it is appropriate for your situation.
• May-June If annual grasses (e.g., crabgrass) or perennial grasses (e.g., dallisgrass) have emerged, apply 2-3 applications (14 to 21 days apart) of a postemergence grass herbicide. For broadleaf weeds, apply a three-way mixture.
• Aug.-Oct. Continue to treat grassy weeds and watch for wild onions and wild garlic.
• Nov.-Jan. On mild days treat winter broadleaf weeds. Nonselective herbicides, such as Roundup, can be used safely on turf that is completely dormant.