Lawn weed control timing can be confusing

NORTH FLORIDA — I am not sure that most homeowners will ever understand the importance of timing in controlling lawn weeds. I will admit that it can be confusing.

The majority of weeds that are in lawns now are warm season weeds. They should have been controlled way back in February with a preemergence herbicide or in early spring with a postemergence herbicide.

Most of these warm season weeds that are large and dominant in North Florida lawns now will be killed when we receive our first killing frost or freeze.

After the warm season (summer weeds) have been killed by cold weather, the cool season (winter weeds) begin to grow. These winter weeds are in the form of seeds now awaiting cooler temperatures and shorter days to germinate.

By December and January, they will be growing all over North Florida lawns. By late winter and early spring (February-April), these same winter weeds will be large and very visible.

They will be killed by the warmer temperatures of April and May.

By late winter/early spring (when homeowners are asking about control), the winter lawn weeds will again be at the end of their lives. They will be almost done with producing seed and will begin to die with the coming warmer temperatures of mid spring.

Control should have been attempted a little before the beginning of their lives, in October; not at the end of their lives during late February to mid-April.

Understanding this cycle of life for both the summer and winter weeds is important.

If your lawn has a history of winter annual weeds, the best option is to apply a preemergence herbicide during October to early November.

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 the weeds you’re targeting exist.

The preemergence herbicide needs to be applied during October to early November 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.

More information on Florida lawns is available at https://edis.ifas.ufl.edu/entity/topic/lawns or from the UF/IFAS Extension office in your County.

Larry Williams is the Extension horticulture agent with the Okaloosa County Cooperative Extension Service, University of Florida. Contact Larry at 689-5850 or email lwilliams@myokaloosa.com.