Mesotrione is a highly effective, selective herbicide used extensively in professional and consumer lawn care products to manage weed populations. The answer to whether it kills crabgrass is a definitive yes, as it is one of the primary targets for this chemistry. Mesotrione is valued because it offers both pre-emergent and post-emergent control of crabgrass, a dual action not commonly found in a single turfgrass herbicide. The product is frequently found under brand names like Tenacity and is a popular choice for comprehensive crabgrass management.
How Mesotrione Selectively Targets Weeds
Mesotrione’s selective nature stems from its unique mode of action, which involves inhibiting a specific enzyme pathway in susceptible plants. It is classified as an HPPD inhibitor, targeting the enzyme p-hydroxyphenylpyruvate dioxygenase. This enzyme plays a crucial role in the biosynthesis of carotenoids, which are pigments essential for protecting chlorophyll.
Carotenoids protect chlorophyll from being destroyed by excess light energy during photosynthesis. By blocking the HPPD enzyme, Mesotrione prevents the formation of these protective pigments. Without carotenoids, chlorophyll molecules are rapidly degraded, leading to a visible “bleaching” effect.
The treated weeds, including crabgrass, first turn white as their chlorophyll is destroyed. This lack of photosynthetic capability causes the plant to die. This mechanism makes the herbicide selective, as many desirable turfgrass species can metabolize the active ingredient quickly enough to avoid damage. The visible whitening effect indicates the herbicide has been successfully absorbed.
Optimizing Application Timing for Crabgrass Control
Mesotrione is effective as both a pre-emergent and a post-emergent treatment for crabgrass, though application timing differs significantly. For pre-emergent control, the herbicide must be applied just before germination, which typically occurs when soil temperatures consistently reach 55 degrees Fahrenheit. This application is unique because Mesotrione is one of the few chemicals that can be safely applied when seeding or overseeding certain cool-season turfgrasses.
As a post-emergent application, Mesotrione targets crabgrass that is actively growing and visible in the lawn. For best control, the herbicide should be applied when the crabgrass is still young, ideally before it has developed more than four tillers. Crabgrass is more difficult to eliminate once it matures past this stage. Because Mesotrione’s residual effect is shorter than traditional pre-emergents, a second application is often necessary two to three weeks later to ensure complete control.
Mixing and Application Safety Guidelines
Successful post-emergent application of Mesotrione requires the addition of a non-ionic surfactant (NIS) to the spray solution. The surfactant acts as a wetting agent, helping the herbicide droplets spread out and adhere better to the waxy leaf surface of the crabgrass. This improved coverage and absorption are vital for the Mesotrione to be effectively translocated throughout the weed’s system.
While commercial application rates vary, a common spot-treatment mixture for homeowners is approximately one teaspoon of Mesotrione concentrate per gallon of water, which is typically enough to cover 1,000 square feet. Precise calibration of the sprayer is necessary to ensure the correct amount of product is applied. Safety precautions must always be followed, including wearing personal protective equipment (PPE) like gloves and eye protection during mixing and application. Care should also be taken to prevent spray drift onto non-target plants, which can also exhibit the temporary bleaching effect.
Turfgrass Tolerance and Sensitivity
The selective nature of Mesotrione means certain desirable turfgrasses can tolerate the chemical without permanent injury, while others are highly sensitive. Cool-season grasses like Kentucky Bluegrass, Tall Fescue, and Perennial Ryegrass generally show high tolerance to Mesotrione and can be safely treated. Even when applied to these species, a temporary, mild whitening or bleaching of the turf blades may occur, but the grass typically recovers within a few weeks without lasting damage.
Conversely, some turf species are highly sensitive and should not be treated with Mesotrione, or should only be treated at much lower rates. Applying the herbicide to these species can result in significant injury, including prolonged whitening or turf loss. Sensitive grasses include:
- Bentgrass
- St. Augustinegrass
- Zoysiagrass
- Fine Fescue
Always consult the product label for specific grass species and application rate restrictions to prevent accidental damage to the lawn.