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Fall preemergent weed controlIt may not feel like it now, but cooler temperatures will soon mark the end of another growing season. Perennials are preparing to go dormant, oaks are becoming heavy with acorns, and summer annuals are preparing to set seed. For the gardener, the heavy work of the summer is over but there are chores yet to be done. One of the fall gardening chores that must be undertaken soon is the application of a fall preemergent herbicide to prevent winter weeds from germinating in home lawns. Applications must be made prior to weed emergence or poor control will result. Recommended dates for fall preemergence herbicide applications to prevent annual winter weeds are the third week of August here in Central Oklahoma. Winter annual weeds begin to emerge from seed when warm season turfgrass begins to enter dormancy in the fall. Annual bluegrass, smooth brome, henbit, mouseear chickweed, common chickweed, cranesbill and shepherd's purse are the most common winter annual weeds that plague Oklahoma Gardeners during the cooler months. These weeds emerge in the fall when temperatures begin to cool from seeds that have been dormant since late spring. Annual winter weeds grow slowly throughout the winter and begin rapid growth when temperatures rise in the late winter. Winter weeds are unsightly and heavy infestations can be extremely damaging to warm-season turfgrasses during spring green-up. Weeds compete for sunlight, soil moisture and plant nutrients during this time period. When winter weeds die, the warm-season turfgrasses can be severely stunted or have large bare spots that can easily be infested by summer annual weeds such as crabgrass. There are many herbicides that may be used to control winter annual weeds. The best control technique is prevention and this is achieved through the use of preemergence herbicides (chemicals that prevent weeds from germinating) applied in the early fall prior to winter annual weed germination. If you miss the window for applying a fall preemergent herbicide you can use postemergent herbicides (chemicals that are applied to the plant itself after germination from seed) on an as needed basis. Here are the basics to a successful preemergent program: --Timing: Most preemergent herbicides will not control weeds that have germinated prior to application. Therefore, try to apply these herbicides several weeks before germination. If they are applied too soon before germination, the herbicide may lose its effectiveness. For the fall a good rule of thumb is to apply a preemergent the third week of August to mid-September. Specific dates for applications of preemergent herbicides are difficult to give due to varying environmental conditions for each location and year. --Turf Preparation: To ensure that the herbicide reaches the soil where weed seeds are located, remove excessive layers (thicker than 0.5 inch) of thatch, and also remove debris such as leaves and grass clipping before you apply the herbicide. --Amount: Always read the label and apply the recommended amount on your lawn. Check the label to see that the herbicide is safe for use on your lawn. --Coverage: Achieve a complete, uniform coverage by dividing the recommended amount of granular herbicide into two equal portions and spreading each in opposite directions. For adequate coverage, make spray applications at approximately 3 quarts per 1000 ft. --Activation: Water the target area if 0.5 inch of rain does not occur within 24 to 48 hours following application. All preemergent herbicides are soil applied and must be "washed" into the soil where weed seeds are located. --Second Application: A second application may be required for season-long control. This will depend on the particular herbicide and environmental conditions, but preemergent herbicides generally remain effective for 60 to 110 days. Date: 8/20/07
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