If you plan to control annual lawn weeds, put it on your calendar

By Larry Williams, UF/IFAS Extension Agent, Okaloosa County

There are basically two times per year to apply a preemergence herbicide in a lawn: late February when targeting summer annual weeds; early October when targeting winter annual weeds.

The majority of visible weeds in lawns now are winter annuals such as annual bluegrass, chickweed, henbit, hop clover, lawn burweed and Carolina geranium. Winter annual weed seeds begin to germinate during fall. The seedlings continue to slowly grow through the colder winter months. As spring approaches, they put on a growth spurt and become very visible.

The majority of calls for advice on winter annual weed control come to me in late February through April when these weeds are the most visible. But by then, it’s too late. The parent plants have grown, produced seed and are about to die. Appling a herbicide at the end of a weed’s life does not accomplish much. A preemergence herbicide should have been applied during early October to prevent these winter annual weeds.

Summer annual weeds such as crabgrass, spotted spurge and Florida pusley don’t exist now. Their seeds begin to germinate during spring. The seedlings continue to grow through the summer. By fall they are well established but eventually die due to frost or freeze.  

The majority of calls for advice on summer annual weed control come to me in late September through early November when these weeds are the most visible. But by then, it’s too late. The parent plants have grown, produced seed and are about to die. Appling a herbicide at the end of a weed’s life does not accomplish much. A preemergence herbicide should have been applied during late February to prevent these summer annual weeds. 

The exception is when dealing with chamberbitter, the summer annual weed that looks like miniature mimosa trees. When targeting chamberbitter, a preemergence herbicide should be applied during late April to early May.

The best way to win the war with any of the annual weeds is to apply a preemergence herbicide just before germination occurs. Otherwise, you’re fighting a losing battle.

If you’ve had problems with annual lawn weeds, consider writing a note on your calendar to remind you when to apply the preemergence herbicide. Otherwise you’re likely to miss the narrow window of opportunity.

Overuse of some preemergence herbicides can result in stubby, weak grass roots. So only apply such a herbicide where there has been a history of annual weeds. It is the user's responsibility to read and follow all label directions and precautions when using any pesticide, including herbicides.

For additional information on lawn weed control, contact your local UF/IFAS Extension Office.

Originally published Feb., 2006