The cooler temperatures 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 pre-emergent herbicide to prevent winter weeds in home lawns. Applications must be made before weed emergence or poor control will result. Recommended dates for fall pre-emergence 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, mouse-ear 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 pre-emergence herbicides (chemicals that prevent weeds from growing) applied in the early fall before winter annual weed germination. If you miss the window for applying a fall pre-emergent herbicide you can use postemergence 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 pre-emergent program:
• Timing: Most pre-emergence herbicides will not control weeds that have germinated before 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 pre-emergence the third week of August to mid-September. Specific dates for applications of pre-emergence herbicides are difficult to give due to varying environmental conditions for each location and year.
• Turf preparation: To ensure the pre-emergence herbicide of getting into the soil where weed seed is located, remove excessive layers (thicker than ½ 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 foot.
• Activation: Water the pre-emergence herbicide area if 0.5 inch of rain does not occur within 24 to 48 hours following application. All pre-emergence herbicides are soil applied and must be “washed” into the soil where weed seeds are located.
• Second activation: A second application may be required for season-long control. This will depend on the particular herbicide and environmental conditions, but pre-emergence herbicides generally remain effective for 60 to 110 days.
Ray Ridlen is an agriculture-horticulture educator for the Oklahoma County Extension Service. His column addresses frequently asked horticulture questions. For more information, call 713-1125.
• Third Thursday Gardening: Bulbs, 5:30-6:30 p.m., Sept. 19, 930 N Portland.