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