Tenacity herbicide, which contains the active ingredient mesotrione, is a selective herbicide designed to eliminate certain weeds. Bermuda grass (Cynodon dactylon) exhibits a degree of tolerance to mesotrione, making the product suitable for weed control when applied according to specific guidelines. Successful use relies heavily on precise measurement, proper timing, and strict adherence to label instructions, as improper application can easily cause temporary or sustained injury to the turf.
Understanding Tenacity’s Mechanism and Bermuda Tolerance
Tenacity’s effectiveness stems from its active ingredient, mesotrione, which inhibits the enzyme p-Hydroxyphenylpyruvate Dioxygenase (HPPD). This enzyme is involved in producing carotenoids, which are plant pigments responsible for protecting chlorophyll during photosynthesis. The absence of these pigments disrupts the entire process.
When a susceptible weed absorbs mesotrione, the chemical is distributed throughout the plant, leading to a loss of protective pigments. Without this protection, sunlight destroys the chlorophyll, causing the plant tissue to turn white or translucent before eventual death occurs.
Bermuda grass is considered tolerant because it can metabolize the mesotrione compound quickly enough to prevent catastrophic damage. This tolerance is not absolute, and the turfgrass can still be injured if the application rate is too high or if the grass is under stress. Using the product on established Bermuda grass for general weed control requires the lowest recommended rate to maximize weed impact while minimizing turf injury.
Critical Application Factors for Preventing Damage
The primary safety factor for applying mesotrione to established Bermuda grass is precise adherence to the application rate specified on the product label for warm-season grasses. These rates are significantly lower than those recommended for more tolerant cool-season turf species. Over-application increases the concentration of the herbicide beyond the Bermuda grass’s metabolic capacity, leading to noticeable turf damage.
It is generally required to mix the herbicide with a non-ionic surfactant (NIS) to enhance its effectiveness as a post-emergent treatment. The surfactant helps the spray solution spread more evenly across the leaf surface and improves the absorption of mesotrione. For small areas, a common rate is approximately one-half teaspoon of Tenacity mixed with one gallon of water to treat 1,000 square feet, along with the specified amount of NIS.
Temperature is another variable that directly affects application safety. Applying Tenacity when daytime temperatures are consistently above 85°F to 90°F significantly increases the risk of injury. High heat stresses the turf and reduces its ability to metabolize the chemical, which can result in severe bleaching or turf burn. The best time for safe application is typically in the spring or early summer after the Bermuda grass is actively growing, but before the most intense summer heat arrives.
Applying the herbicide during the active growing season ensures the turf is healthiest and best able to tolerate the chemical. Some application labels may permit use on dormant Bermuda grass for winter weed control. The label must be consulted for specific timing, as the grass must be actively growing to metabolize the chemical effectively, or fully dormant to avoid injury. Avoiding application when the turf is already stressed by drought or disease will help prevent damage.
Expected Results and Managing Temporary Whitening
A successful application of Tenacity will first become apparent through changes in the target weeds, such as clover and annual bluegrass. Susceptible weeds will begin to exhibit symptoms of bleaching, turning white or translucent within several days of treatment. This visual change shows that the herbicide is disrupting the plant’s pigment production. The weeds will then slowly die off, often requiring a second application two to three weeks later for complete post-emergent control.
Even when applied at the correct rate, the Bermuda grass may still show temporary whitening or bleaching, which is a common side effect of the herbicide’s mode of action. This whitening occurs because the mesotrione temporarily interferes with the growth of new, unprotected tissue in the turfgrass. The effect is generally non-lethal to the established Bermuda grass, but it can be visually alarming to the homeowner.
The temporary discoloration typically lasts for seven to fourteen days before the grass begins to green up again. As the grass continues to grow, routine mowing will remove the bleached tips, and the new growth will be a normal green color. To aid recovery, ensure the lawn receives adequate, light watering and avoid heavy fertilization or additional chemical applications immediately following treatment.