Dicamba is a common, widely used herbicide, and crabgrass is a pervasive, warm-season annual weed that plagues lawns across the country. This article addresses the specific relationship between dicamba and crabgrass, explaining how dicamba works, why it is generally ineffective against this type of weed, and what alternatives provide genuine control.
Understanding Dicamba’s Selectivity
Dicamba is classified as a selective, systemic herbicide, meaning it targets specific types of plants without harming turfgrass. Its chemical structure is that of a synthetic auxin, mimicking natural plant hormones that regulate growth. This mimicry causes uncontrolled, rapid cell growth, which eventually destroys the plant’s vascular tissue, resulting in death.
Dicamba is absorbed through the leaves and roots, traveling throughout the plant’s system. This systemic action makes it effective against broadleaf weeds like dandelions, clover, and thistle. The chemical’s mechanism is tailored to disrupt the hormone pathways in broadleaf plants (dicots).
Dicamba’s Efficacy Against Crabgrass
The direct answer is that dicamba is not effective against crabgrass when used alone. The selective nature of dicamba is precisely why it fails against crabgrass, which is a grassy weed (monocot). The herbicide’s synthetic auxin action is designed to overwhelm the growth regulators of broadleaf plants.
Because crabgrass is a grass, its internal physiology allows it to tolerate the dose of dicamba. While dicamba is sometimes included in multi-herbicide blends, its purpose in those mixtures is to control the broadleaf weeds, not the crabgrass. Crabgrass control in such combination products is typically provided by a different active ingredient, such as quinclorac.
Effective Control Strategies for Crabgrass
Since dicamba does not provide a solution, effective crabgrass control requires a multi-pronged approach utilizing both chemical and cultural practices.
Chemical Control
The most effective chemical strategy involves applying pre-emergent herbicides, which create a chemical barrier in the soil to prevent crabgrass seeds from germinating. Timing is important, as the application must occur before the seeds sprout, which generally happens when soil temperatures reach 55 to 60 degrees Fahrenheit for several consecutive days. Common active ingredients for this preventative step include prodiamine and pendimethalin.
If the pre-emergent window is missed, a post-emergent herbicide is necessary to kill the actively growing plants. Herbicides containing the active ingredient quinclorac are highly effective for post-emergent control, even for mature plants that have developed tillers. Another active ingredient, mesotrione, is also effective, particularly on younger crabgrass plants. For optimal results, post-emergent treatments are usually best when crabgrass plants are small, typically having two to four leaves.
Cultural Practices
Complementing chemical control, cultural practices are the long-term foundation for a healthy, crabgrass-resistant lawn. Mowing the turf at a higher setting (usually between two and three inches) helps shade the soil, keeping it cooler and inhibiting crabgrass seed germination. Proper watering involves deep, infrequent irrigation to encourage deep root growth in the desirable turfgrass. Maintaining a dense, healthy lawn through appropriate fertilization helps the turf naturally crowd out weed seedlings.