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Six Easy Steps for Success with Preemergence Herbicides This Spring

Tim Murphy
University of Georgia
Retired

Preemergence herbicides form the base of a chemical weed control program for summer annual weeds in turfgrasses and are used primarily to control annual grasses and certain annual broadleaf weeds. They persist in the soil and control susceptible weeds for two to six months. The length of control depends on the specific chemical being used, soil, physical and chemical properties, soil moisture levels, and soil temperatures.

 Each year there are instances where for some reason preemergence herbicides fail to control weeds or injury occurs to turfgrasses. Maximum control of summer annual weeds with preemergence herbicides can be achieved by following these basic guidelines:

 1. Apply the product at the recommended time and rate. Weather varies from year to year and it may be necessary to apply earlier than normal. Reference to 30 day weather forecasts can help with this decision. (Visit www.georgiaweather.net )

The various species of crabgrass and goosegrass are among the most troublesome annual grass weeds in turf. Crabgrass initiates spring germination when soil temperatures at a 4-inch depth reach 53 to 58oF . This can occur from mid-February to late April in most areas of the southeastern United States. Goosegrass germinates at soil temperatures of 60 to 65oF. Because of higher temperature requirements for germination, goosegrass normally germinates two-to-eight weeks later in spring than crabgrass.

 The old rule of thumb is to apply the preemergence herbicide two weeks before crabgrass seed germination. However, recent research has shown that most preemergence herbicides can be applied in December and January and still provide high levels of crabgrass control the following summer months. The low activity of soil microorganisms involved in herbicide decomposition during the cold, winter months is probably the reason preemergence herbicides can be applied in December and January several weeks in advance of crabgrass and goosegrass seed germination and still provide high levels of control the following summer. Early is always better than late with respect to preemergence application for summer annual grass weeds!


2. Apply the product before rain is expected or water it in with ½ inch of irrigation water. Numerous instances of poor weed control occur each year because of the lack of rain or an irrigation event within 7 days of preemergence application. Additionally, irrigating-in the herbicide is an excellent method to prevent losses due to volatility and lateral herbicide leaching. Turfgrass preemergence herbicides essentially do not leach in downward direction beyond a depth of 2 to 3 inches due to binding to soil colloids and organic matter. But they can move laterally, particularly if heavy rainfall occurs shortly after application. Irrigation will usually improve weed control and will help to prevent lateral movement.

 3. Calibrate all application equipment. Uniform application is critical to achieving good weed control.

 4. If fertilizer/herbicide formulations are to be used, select a product that has uniform particle size and a sufficient number of particles that will ensure even, uniform application. Also, be sure that the herbicide load is sufficient to apply the recommended rate of the product. There is good data that indicates that dithiopyr rates can be reduced if applied on a dry granular carrier. However, with most other preemergence herbicides the amount of active ingredient applied per acre should be the same either for sprayable or dry formulations.

 5. Delay mowing until after rainfall or irrigation. Studies have shown that mowing and bagging operations can remove significant quantities of a preemergence herbicide if conducted before the herbicide is moved into the soil by rain or irrigation water.

 6. Properly maintain the turfgrass. Following recommended cultural practices that promote normal turfgrass growth and development will enable the turfgrass to compete with weeds. The first line of defense against weed infestations has been, and probably always will be, a thick, healthy, properly maintained turfgrass. Adherence to recommended soil fertility and pH levels, proper irrigation, controlling other pests, and mowing at the correct height and frequency will improve the effectiveness of most chemical weed control programs. The use of herbicides in the absence of proper turfgrass maintenance practices may provide weed control but the eventual goal of a high quality, aesthetically-appealing turfgrass will not be achieved.

 

This information is adapted from this publication, find more information here - http://www.commodities.caes.uga.edu/turfgrass/georgiaturf/WeedMngt/weedcontrol/turfprea.htm

 

 

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