The aggressive growth of Bermuda grass requires proactive weed control to maintain a dense, attractive turf. Pre-emergent herbicides are an important tool, preventing weeds before they become established competitors for water, nutrients, and sunlight. Since Bermuda grass thrives in warm conditions, it is susceptible to invasion by summer annual weeds as the turf greens up. Preventative treatment is far more efficient than eradicating mature weeds later in the season.
How Pre-Emergents Work
Pre-emergent herbicides establish a chemical barrier near the soil surface, rather than killing existing weeds. As weed seeds germinate, developing seedlings absorb the chemical upon contact with this protective layer. The active ingredients interfere with the plant’s growth processes, specifically inhibiting cell division and root development.
This disruption prevents the seedling from forming a stable root system, killing the plant before it can break through the soil surface. The herbicide targets the vulnerable seedling stage, not the actual weed seed. Pre-emergent products are distinct from post-emergent herbicides, which kill weeds that have already sprouted.
Recommended Active Ingredients for Bermuda Grass
Effective pre-emergent programs for established Bermuda grass rely on specific active ingredients known to be safe for this warm-season turf.
Prodiamine
Prodiamine is widely recommended for its long residual control. It forms a durable barrier that protects the lawn for several months, making it suitable for early spring application to cover the summer weed season. Prodiamine is effective against a broad spectrum of annual grasses and some broadleaf weeds.
Dithiopyr
Dithiopyr is another effective option, often preferred for its dual-action capability. While primarily a pre-emergent, Dithiopyr offers early post-emergent control of crabgrass seedlings that have just begun to sprout. This makes it a flexible choice if the initial application timing is slightly missed. Unlike Prodiamine, Dithiopyr is more soluble, setting up faster in the soil but may not offer the same extended residual protection in areas with heavy rainfall.
Isoxaben
For targeting specific broadleaf weeds, Isoxaben can be integrated into the program. It is highly selective for broadleaf weeds like chickweed and henbit, providing extended control, sometimes lasting up to eight months. It functions as a cellulose biosynthesis inhibitor, disrupting the formation of the plant’s cell walls. Isoxaben is often tank-mixed with other herbicides to broaden the overall spectrum of weed control, as it is less effective against grassy weeds.
Critical Application Timing and Technique
Successful weed prevention depends on applying the pre-emergent at the correct time, determined by soil temperature rather than the calendar date. The spring application must occur before the soil temperature consistently reaches 55°F, the point at which summer weeds like crabgrass begin to germinate. Applying too late means the weeds will have already begun to sprout, rendering the pre-emergent ineffective.
Many experts recommend a split application approach, especially in warmer climates where the herbicide barrier breaks down faster. This involves applying half the maximum rate early in the spring (around 55°F soil temperature) and the second half 8 to 12 weeks later. This technique extends the protective barrier throughout the long Bermuda grass growing season.
The fall application window is equally important for controlling winter annual weeds, such as annual bluegrass (Poa annua). The fall treatment should be applied when soil temperatures begin to drop below 70°F, typically in late summer or early fall, when these cool-season weeds start to germinate.
Proper application technique is paramount for efficacy. The product must be distributed evenly across the lawn using a calibrated spreader or sprayer to ensure a uniform chemical barrier. After application, the product requires activation by water (about 0.5 inches of irrigation or rainfall) to dissolve the granules and incorporate the chemical into the top layer of the soil.
Specific Weed Control and Safety Considerations
The primary target for spring pre-emergent application is annual grassy weeds, most notably crabgrass and goosegrass. The fall application aims to prevent the germination of winter annuals, including annual bluegrass (Poa annua), henbit, and chickweed. Preventing these weeds allows Bermuda grass to develop a denser turf canopy, which naturally suppresses future weed emergence.
A safety consideration is that pre-emergent herbicides inhibit all seed germination, not just weed seeds. Therefore, these products must be avoided when seeding, overseeding, or laying new sod. Applying a pre-emergent during the establishment of new turf will prevent the desirable grass seed from sprouting. A waiting period of several months is necessary after application before planting new grass seed. If new sod is laid, wait until the turf is fully rooted, often after one full growing season, before beginning a pre-emergent program.