Atrazine is a selective herbicide used widely in warm-season turfgrass management, particularly on St. Augustine grass, a common turf found in southern climates. The active ingredient in Atrazine works by interfering with the photosynthesis process in susceptible plants, causing them to die. This chemical is specifically formulated to eliminate weeds without causing significant harm to tolerant grasses like St. Augustine. Whether Atrazine will kill St. Augustine grass depends entirely on the application method and environmental conditions. This article details how Atrazine interacts with this turf, which weeds it targets, and the essential steps for safe and effective use.
Atrazine’s Specific Effect on St. Augustine Grass
Atrazine is generally considered safe for use on established St. Augustine grass because the turf has a natural tolerance, metabolizing the chemical quickly enough to prevent the interruption of photosynthesis that kills weeds. This selectivity makes it a primary choice for controlling broadleaf weeds without damaging the lawn.
However, the safety margin is narrow, and the grass can suffer damage if conditions are not ideal or if the herbicide is misused. Over-application, such as using double the recommended rate, severely increases the risk of turf injury, leading to yellowing or “burning” of the grass blades. Applying Atrazine when St. Augustine grass is under stress, particularly from drought or high heat, can also reduce the turf’s natural tolerance, potentially resulting in damage.
The grass is also highly sensitive during certain phases of its growth cycle. Applications should be avoided during the “green-up” phase in the spring, as shifting physiology increases sensitivity. Newly sodded or recently established areas should not be treated until the grass is fully rooted and mature. Cultivars of St. Augustine grass can vary in tolerance, so checking the product label for restrictions is important before application.
Target Weeds Controlled by Atrazine
Atrazine is valued for its dual-action capability, functioning as both a pre-emergent and post-emergent herbicide against a wide spectrum of weeds common in southern lawns. As a post-emergent, it effectively controls weeds that have already sprouted and are actively growing. The chemical enters the plant through the leaves and roots, leading to plant death.
The herbicide is particularly effective on many common broadleaf weeds that invade St. Augustine lawns. Susceptible weeds include dollarweed, clover, henbit, spurge, and chickweed. When applied as a pre-emergent, Atrazine forms a chemical barrier in the soil, preventing weed seeds from germinating.
This pre-emergent action is beneficial for controlling winter annual weeds, such as annual bluegrass (Poa annua), before they emerge. Effectiveness is highest when applied to weeds that are small and actively growing, or just before they are expected to emerge. Atrazine is also a labeled control option for tough weeds like sand spur.
Essential Application Guidelines
To ensure the safety of St. Augustine grass and the efficacy of weed control, precise application timing and conditions are necessary. The optimal time for application is during the dormant or semi-dormant season, typically in the late fall or early spring, which targets winter weeds when the turf is most stable. Applying the product when the grass is not actively recovering from a stressful period reduces the risk of injury.
Temperature is a major constraint, and Atrazine should not be applied when the ambient air temperature exceeds 90°F. High temperatures decrease the selectivity of the herbicide, increasing the likelihood of damage. The product should only be used on turf that is healthy, not under heat or drought stress, and free of disease or insect infestations.
Accurate measurement and application rate are necessary to prevent turf injury. A common rate for liquid products is around 4.3 fluid ounces mixed with one gallon of water to treat 500 square feet of turf. To maintain consistency and avoid damaging overlaps, the product should be applied with a coarse, low-pressure spray.
It is important to avoid applying Atrazine under windy conditions to prevent spray drift onto non-target plants, such as nearby ornamentals or vegetables. After application, the herbicide needs a period to dry. It should not be watered in immediately, as this can wash the chemical away and reduce its post-emergent efficacy.