If you’ve had a history of winter annual weeds in your lawn, the best option is to apply a preemergence herbicide during October. But, winter annual lawn weeds don’t exist yet. They are in the form of seeds now.
Understanding this timing is critical.
During late February to mid-April, homeowners bring in bags of winter annual weeds from their lawns wanting advice on how to control these weeds. They usually seem disappointed and confused when I tell them that it’s too late to bother with controlling winter annual weeds during late winter to early spring. They usually follow with a statement and a question such as, “Well, my lawn is full of these weeds now. What can I do?” I suggest to just mow them and explain that these weeds will completely die as a result of the warmer temperatures of late spring and early summer.
At this point, looking frustrated, the homeowner may ask, “Is there something I can spray on my lawn to kill these weeds now?” To which I reply, “Yes, but it’s mostly a waste of time and money this late.” I then try to explain the winter annual weed lifecycle. The weed seed remain dormant during the hot summer months and begin to germinate in fall. The new seedlings continue to grow through winter, mature, produce flowers and countless seed. By late winter/early spring (when the homeowner is asking about control), these winter weeds are at the end of their lives. They are almost done with producing seed and will begin to die with the coming warmer temperatures of mid spring. Control should have been attempted just ahead of the beginning of their lives, in October; not at the end of their lives during late February to mid-April.
Perhaps part of the problem with understanding the importance of timing when applying a preemergence herbicide is due to the fact that the herbicide is applied before there are any weeds. The preemergence herbicide needs to be applied during October when nighttime temperatures drop to 55° to 60°F for several consecutive nights. This will be just before the winter annual weeds emerge. Done correctly, the application of a preemergence herbicide forms a chemical barrier along the soil surface preventing the winter annual weeds from emerging. Hence the name preemergence.
A second application may be required six to nine weeks after the initial application to achieve season-long control, based on the product’s label directions.
Follow label directions and precautions when using any pesticide, including herbicides.
Common winter annual weeds include chickweed, henbit, Carolina geranium, annual bluegrass, hop clover and lawn burweed.