We are quickly approaching the window of opportunity to use a pre-emergence herbicide in lawns in Northwest Florida. But not every lawn needs an application of pre-emergence herbicide – only those lawns that have a history of summer annual weeds.
When used correctly, pre-emergence herbicides form a chemical barrier on the soil surface that prevents annual weeds from pushing through or emerging. When you see annual weeds, it’s too late to use a pre-emergence herbicide.
The key to good control with this kind of chemical is to apply it to the lawn before summer annual weeds emerge, hence the name pre-emergence. Germination can vary from year to year. However, if you intend to use a pre-emergence herbicide, you need to apply it during mid-February to early March when day temperatures reach 65°F to 70°F for 4 to five consecutive days.
This will be just before summer annual weeds emerge. Applying the herbicide early in this narrow window is better than doing it late.
Use pre-emergence herbicides primarily to control summer annual weeds such as crabgrass, goosegrass, spurge, Florida pusley, doveweed, sandbur and poorjoe. There are others.
Pre-emergence treatments offer some advantages:
- Since you apply the chemical before annual weeds emerge, susceptible weeds never show up in the lawn.
- There are few to no post-emergence herbicides to control grassy weeds in centipedegrass or St. Augustinegrass lawns.
- Except for herbicides that contain atrazine, most ornamental trees, shrubs and flowers are tolerant to pre-emergence herbicides. Some pre-emergence herbicides that contain DCPA, oryzalin, benefin and pendimethalin are labeled for use on some ornamentals. Many post-emergence herbicides can injure ornamentals.
- Most post-emergence herbicides can cause slight injury or temporary yellowing in lawns. Pre-emergence herbicides typically don’t cause noticeable injury to established lawns.
Use pre-emergence herbicides only on lawns that have been established for at least a year. They can severely injure freshly seeded or sprigged lawns.
Don’t apply a pre-emergence herbicide if the lawn will be seeded, sprigged or sodded in the spring or early summer. These herbicides persist in the soil for 2 to 4 months and can interfere with turfgrass establishment.
A second application may be needed 6 to 9 weeks after the initial application for season-long control, especially in lawns with a history of heavy annual weed populations.
Pre-emergence herbicides for lawns are sold under a number of trade names. The UF/IFAS Extension Office in your County or lawn and garden stores can help you select a pre-emergence herbicide.
Be sure to select one that’s labeled for use on the lawn species you have. And, always follow the label directions and precautions when using any pesticide, including herbicides.
Larry Williams is the Extension horticulture agent with the Okaloosa County Cooperative Extension Service, University of Florida. Contact Larry at 689-5850 or email [email protected].
