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:
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