Have you been outside lately? It is easy to get confused about what season it is due to the warm weather. This has sparked a few questions about applying pre-emerge herbicides to lawns. Pre-emergence herbicides should be applied before weed emergence. Recommended dates of application for crabgrass and other annual grasses are February 15 to March 5. Apply herbicides to control summer annual weeds such as crabgrass and goosegrass prior to soil temperatures reaching 55° F. We need to monitor soil temperatures. The chart below shows the 2-inch and 4-inch soil temps for the period January 9 to February 9 from the Sunbelt Expo location of the UGA Weather Network. The 2-inch is highlighted in green, and 4-inch is highlighted in blue. Source: UGA Weather Network, Sunbelt Expo.
The monthly weather forecast for February 2025 shows the southeast will be warm and dry.
What about weed and feeds? Preemergence and postemergence herbicides are formulated with dry fertilizers. These mixtures enable a weed-and-feed treatment in the same application or trip over the lawn. Although applying weed-and-feed treatment is convenient, consider certain factors before application. Depending on the turfgrass, the time of year an herbicide should be applied may not coincide with the time a year a fertilizer should be applied. The nutrient of greatest concern is nitrogen. Weed-and-feed products containing nitrogen should be avoided until the 4-inch soil temperature is consistently 65° F. Some combination products contain potassium as the only nutrient, which is satisfactory for late winter and early spring applications.
For example, centipedegrass should not receive spring fertilization until complete green-up or until it has recovered from winter dormancy. When centipedegrass has fully greened-up, it is generally too late to apply a preemergence herbicide since many weeds have already emerged.
Atrazine is sold with a dry fertilizer carrier under the brand name Bonus S. Atrazine has both preemergence and postemergence activity on a wide variety of annual broadleaf weeds. On centipedegrass or St. Augustinegrass, atrazine-fertilizer products can be applied after green-up and provide control of many annual broadleaf weeds.
Postemergence herbicides are applied after annual weeds emerge or when new growth or regrowth of perennial weeds appears. Follow these guidelines for better weed control and improved turfgrass tolerance:
- Apply postemergence herbicides in the fall and late spring months. Air temperatures are cooler at this time of year, resulting in better turfgrass tolerance to herbicides. Also, perennial and many annual weeds are actively growing during these times of year and are easier to control with postemergence herbicides.
- Do not apply postemergence herbicides to turfgrasses and weeds that are stressed due to high temperatures or drought. Turfgrass tolerance to postemergence herbicides decreases at air temperatures greater than 90° F or when turfgrasses are drought-stressed. Also, weed control is poorer when herbicides are applied to weeds in a stressed condition than when applied to actively growing weeds.
- Do not apply postemergence herbicides during the green-up (transition from winter dormancy to active growth) process of warm-season turfgrasses. The risk of injury from postemergence herbicides is greater during the green-up process than when the turfgrass is fully dormant or actively growing.
If you want more information about weed management in lawns contact your local county Extension agent or look over some of these resources.
UGA Pest Management Handbook — Home Turf
Have a great day.
