A pre-emergent herbicide prevents weed seeds from successfully establishing themselves in a Bermuda grass lawn. Instead of killing existing weeds, this herbicide forms a chemical barrier in the top layer of the soil. When a weed seed sprouts, the emerging root or shoot absorbs the herbicide, inhibiting cell division and preventing the seedling from developing further. This proactive approach is a powerful tool for maintaining a healthy turf, but its success relies entirely on precise timing.
The Critical Timing Signal: Soil Temperature
Relying on a calendar date for pre-emergent application is unreliable because weed germination is governed by environmental conditions. The true indicator for application timing is the temperature of the soil, which dictates when weed seeds become active. Warm-season annual weeds, such as crabgrass and foxtail, begin to germinate when the soil temperature consistently reaches 50 to 55 degrees Fahrenheit for several consecutive days.
Applying the herbicide within this narrow window—before the temperature hits 60°F—ensures the protective barrier is in place before the target weeds begin to sprout. Homeowners can monitor this threshold using a simple soil thermometer probe inserted one to two inches deep into the ground, or by checking local agricultural extension services and online tools that provide current soil temperature data, often focusing on the five-day average.
Spring Application Strategy for Warm-Season Weeds
The primary application window targets warm-season annual weeds that emerge in the spring, notably crabgrass and goosegrass. These weeds start their germination cycle before the Bermuda grass breaks dormancy, which typically occurs when soil temperatures reach 60 to 65°F. Applying the pre-emergent when the soil temperature is consistently 50 to 55°F places the barrier down during this vulnerable period for the weeds, giving the Bermuda grass a significant advantage.
A single application often will not provide season-long control, especially in southern and transition zones where the growing season is extensive. Most pre-emergent products remain effective for about 6 to 12 weeks, but the germination window for summer weeds can last four to five months. For this reason, a “split application” strategy is recommended for Bermuda grass.
The split application involves applying the full annual rate of the product in two parts. The first half is applied when the soil temperature hits the initial 50 to 55°F trigger, and the second application is made eight to ten weeks later. This second application helps restore the chemical concentration in the soil, maintaining the protective barrier throughout the summer and preventing late-germinating weeds from establishing themselves.
Fall Application Strategy for Cool-Season Weeds
A second application window occurs in the fall to control cool-season annual weeds. These weeds, like Poa annua (annual bluegrass), henbit, and chickweed, germinate as the weather cools down. The correct timing for this fall application is when soil temperatures drop consistently below 70 degrees Fahrenheit and are trending downward.
This fall application is important because these winter weeds grow slowly through the cool months, becoming highly visible and disruptive in late winter and early spring. By applying the pre-emergent in early to mid-fall (typically September or October depending on location), the barrier is established before the cool-season weed seeds begin germination. This preventive measure significantly reduces the weed pressure the Bermuda grass will face when it emerges from dormancy the following spring.
Application Constraints and Restrictions
Proper application requires the herbicide to be activated immediately after it is spread across the lawn. The chemical barrier is formed only after the product is watered into the soil, which can be accomplished with light rainfall or irrigation within 24 hours of application. Failing to water the product in promptly leaves the herbicide sitting on the turf blades, where it cannot form the subsurface barrier.
A restriction for pre-emergent use is its effect on any form of seed. Since these herbicides stop all seeds from germinating, they must not be applied if the homeowner plans to overseed or lay down new sod soon after. Pre-emergent chemicals can prevent Bermuda grass seeds or stolons from establishing new roots, and a waiting period of several months is required before successful seeding can occur. Using pre-emergent on newly established sod should also be delayed until the turf is fully rooted, generally for a full growing season, to prevent damage.