Timing, rate, uniform coverage, and activation are the keys to effective pre-emergence herbicide weed control. Valentine's Day generally provides perfect timing and is an easy day to remember for applying pre-emergence herbicides.
Pre-emerge herbicides must be applied prior to weed seed germination. Seeds of weeds such as crabgrass will germinate when soil temperatures reach around 58 degrees and even though it may feel awful cold now, it only takse a few sunny, warm days to raise the soil temperature to this level. A couple of weeks early are much better than a day late missing those early germinating weeds.
The herbicide label will provide a list of the weed species the active ingredient will control and at what rate. If there are several weed species that you are targeting use a rate that controls the most difficult weed while staying within recommended rates labeled for your specific turf species.
Pre-emergence herbicides create a thin weed control barrier on the soil surface. This barrier must provide complete coverage at a uniform rate to prevent any voids that would allow weed escapes. Liquid or granular application equipment should be accurately calibrated prior to making an application to ensure proper rates and coverage.
Finally, once the herbicide is applied it must be activated. This will require approximately one-half inch of rainfall or irrigation shortly after application to move the herbicide down through the leaf canopy to form the thin herbicide barrier at the soil surface. With any pesticide you should always READ THE LABEL carefully and completely before applying.
Published February 2, 2009
Dr. Wayne Wells is an Extension Professor and Turfgrass Specialist. His mailing address is Department of Plant and Soil Sciences, Mail Stop 9555, Mississippi State, MS 39762. email@example.com