A pre-emergent herbicide is a specialized product designed to prevent weed seeds from sprouting rather than killing weeds that have already emerged. This proactive weed control method works by establishing a thin, protective chemical barrier in the top layer of the soil. When weed seeds begin to germinate, they absorb the herbicide, which interrupts cell division and inhibits root and shoot development, effectively stopping the weed before it breaks the soil surface. Successful weed prevention relies entirely on applying this barrier at the precise moment before germination begins.
Understanding Soil Temperature
Determining the correct application time hinges on soil temperature, which is a far more reliable indicator than any date on a calendar. Air temperature fluctuates daily and does not accurately reflect the conditions at the depth where weed seeds germinate. The soil must reach a specific temperature threshold for several consecutive days to trigger germination.
The critical temperature for applying pre-emergent is just before the soil consistently reaches 55°F at a depth of 2 to 4 inches. This temperature signals the start of the growing season for warm-season annual weeds, most notably crabgrass. Tracking local soil temperatures is the most accurate way to ensure the herbicide barrier is in place before the seeds begin to stir.
Spring Application: Targeting Crabgrass and Warm-Season Weeds
The spring application is primarily aimed at controlling crabgrass and other summer annual weeds like goosegrass. Since Georgia spans several climate zones, the application window varies significantly from south to north. South Georgia, such as the Valdosta and Savannah areas, typically requires application in late February or the first week of March.
Central Georgia, including the Atlanta metro area, usually applies between early and mid-March. North Georgia, with its cooler climate, experiences the latest soil warming, making the ideal time closer to mid-March or early April.
To ensure season-long protection, a second, lighter application is often necessary six to eight weeks after the initial one. This booster application renews the chemical barrier, which naturally degrades over time, preventing late-season germination of weeds like goosegrass, which germinates at 65°F.
Fall Application: Targeting Poa Annua and Cool-Season Weeds
The second major application window targets cool-season weeds, primarily Poa annua (annual bluegrass). Unlike the spring application, the fall timing is determined by cooling soil temperatures. Poa annua seeds begin to germinate when the soil temperature drops below 70°F, often coupled with increased moisture from fall rains.
This application must occur in the late summer or early fall to prevent the weed from establishing itself throughout the winter months. For most of Georgia, this window is typically from early September to mid-October, depending on the region. Applying the pre-emergent before the soil consistently dips below the 70°F mark prevents the establishment of winter weeds like henbit and chickweed.
Successful Application Techniques
The effectiveness of any pre-emergent herbicide is dependent on proper mechanical application, regardless of the season. Accurate coverage begins with correctly calibrating your spreader to ensure the product is distributed evenly across the lawn. Uneven application results in visible patches of weeds where the barrier was too thin or completely missed.
After spreading the product, it must be activated by moisture to work correctly. Granular products require a light watering of about a half-inch from rain or irrigation to dissolve the granules and move the chemical barrier into the top layer of the soil.
Pre-emergent herbicides cannot distinguish between weed seeds and grass seeds. Any plans to aerate the lawn or sow new grass seed must be postponed for several months after application to avoid inhibiting the desired grass’s germination and root development.