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Postemergence Herbicides

By Tanya Khan ; Updated September 21, 2017

Lawn weeds are every gardener’s bane. Homeowners and commercial gardeners strive to develop a healthy and thick grassy lawn. The emergence of weeds, however, captures their attention, and they resort to measures to prevent them from competing with surrounding grass and desirable plants for moisture and nutrients. While organic control methods exist, herbicides or chemicals specifically formulated to kill undesirable plants complete the task in a shorter time.


There are two main types of postemergence herbicides: selective and non-selective. Selective herbicides control perennial, biennial and annual broadleaved weeds without damaging surrounding or nearby grassy plants, but they can severely damage nearby flowers, shrubs and trees. Nonselective postemergence herbicides kill all vegetation they come in contact with. These herbicides are used to spot treat perennial grassy weeds not affected by selective herbicides. Postemergence herbicides are systemic or contact. Systemic or translocated herbicides are absorbed by the foliage and move throughout the plant, including stems, roots and storage tissues, to control perennial weeds. Contact herbicides kill only the portion of the weed they come in contact with. Contact herbicides burn large annual weeds or perennials but kill small annual weeds.


Postemergence herbicides are applied to established broadleaved and grassy weeds that have germinated in the soil and are clearly visible, according to the University of Arkansas. It is imperative that the weed over which the herbicide application is to be made is actively growing and not just green.


For best results, apply postemergence herbicides when the weed is young (two- to four-leaved) and actively growing, the air temperature is consistent between 60 to 80 degrees Fahrenheit and there is no forecast of rain for at least 24 hours. The most effective control is achieved on non-windy days in spring or early fall. In some cases, repeated application every 20 to 30 days is required.


Spray the undesirable weed plant with the postemergence herbicide for best results. Direct the nozzle of the spray bottle or spray tank directly over the plant for maximum contact and coverage, especially if using a contact herbicide. Wear protective clothing, gloves and a face mask and cover nearby desirable plants with a tarp to prevent accidental sprays.


For best results, the area over which the herbicide is applied should not be irrigated one to eight hours after application. In situations where herbicide applications prove futile, consider hand pulling or cultivating the weeds. Refrain from applying postemergence herbicide during spring green up, when the weed or undesirable plant ends its dormancy and begins to grow actively again. Do not mow the area three days before and after herbicide application. Weeds stressed because of environmental factors, such as the heat and dry weather, are difficult to control with a postemergence herbicide.


About the Author


Tanya Khan is a freelance author and consultant, having written numerous articles for various online and print sources. She has a Master of Business Administration in marketing but her passion lies in writing.