Pre-emergent herbicides are foundational for maintaining a weed-free Bermuda grass lawn. These products are not designed to kill existing weeds, but rather to prevent new ones from ever emerging above the soil surface. Successful application relies entirely on precise timing, which is determined by soil temperature rather than the calendar date. Applying pre-emergent at the correct moment ensures a protective barrier is in place before the annual weed seeds begin to germinate.
How Pre-Emergents Work
Pre-emergent herbicides function by creating a thin, protective chemical barrier in the uppermost layer of the soil. This barrier targets weed seeds after they initiate the germination process but before they are visible above ground. The product inhibits cell division in the newly sprouted root and shoot tissues.
When a weed seed begins to sprout and encounters the treated soil zone, its growth is halted. The chemical prevents the rapid cell growth required for the seedling to establish a root system and push through the soil. This disruption causes the seedling to die quickly, resulting in effective weed control. This preventative approach differs distinctly from post-emergent products, which are sprayed directly onto actively growing weeds.
Primary Application Timing for Warm-Season Weeds (Spring)
The primary pre-emergent application for Bermuda grass occurs in the spring to control warm-season annual grasses, such as crabgrass and goosegrass. These grasses begin their germination cycle once the soil warms up consistently. Timing this application relies on monitoring the temperature of the soil, not the air.
The application window opens when soil temperatures consistently reach 50 to 55 degrees Fahrenheit at a depth of four inches. Applying the herbicide before this threshold ensures the barrier is active before the weed seeds sprout. If applied too early, the barrier may degrade before the main flush of germination occurs.
Homeowners can monitor this temperature using a soil thermometer or regional turfgrass extension websites. A second application is often advised eight to ten weeks after the first for extended summer protection. This “split application” maintains the chemical barrier’s density throughout the summer months.
Secondary Timing for Cool-Season Weeds (Fall)
A second pre-emergent application is necessary in the late summer or early fall to control cool-season annual weeds. The primary target is annual bluegrass (Poa annua), which germinates as the soil begins to cool down. If not controlled, these weeds grow throughout the winter and become highly visible in the spring.
This application must be timed when the soil temperature drops to approximately 70 degrees Fahrenheit and is on a downward trend. This signals the start of the germination period for winter annuals. Applying the product during this window establishes a barrier that intercepts the seeds of weeds like Poa annua, chickweed, and henbit.
This fall timing is vital for year-round weed management. Failing to apply pre-emergent allows winter annuals to establish extensive root systems, making them challenging to control later with post-emergent products.
Post-Application Lawn Management
Once the pre-emergent herbicide is applied, it must be activated to create the protective barrier. This requires watering in the product immediately, typically with about a half-inch of irrigation or rainfall within 24 hours. Water moves the active ingredient into the top layer of the soil where the weed seeds will germinate.
A primary consideration is the conflict between pre-emergent herbicides and any plans for seeding or sodding. Because these chemicals stop all new seed development, they prevent new Bermuda grass seed from sprouting. If seeding is planned, a waiting period of 60 to 90 days after application is required for the product to dissipate.
It is recommended to avoid disturbing the soil barrier after application, including core aeration. The common practice is to aerate before applying pre-emergent or to wait until the application window has passed. Disturbing the soil, especially with core aeration, can create small gaps in the protective layer, allowing weed seeds to germinate successfully.