Crabgrass is a warm-season annual grassy weed that presents a significant challenge to homeowners seeking a uniform lawn. It thrives particularly well in hot weather, thin turf, and areas where the existing lawn is stressed or sparse. The weed is notoriously difficult to manage because it germinates later than desirable turfgrass species and spreads aggressively throughout the summer months. A single plant is capable of producing tens of thousands of seeds, ensuring a returning problem in subsequent years. Successfully controlling this invader requires a targeted strategy, focusing on selective methods that eliminate the weed while leaving the established turfgrass unharmed.
Preventing Crabgrass Sprouting (Pre-Emergent Strategy)
The most effective approach to managing crabgrass is preventing its emergence before it is visible in the lawn. Pre-emergent herbicides function by establishing a chemical barrier in the top layer of the soil. As the crabgrass seed begins to germinate, its root or shoot comes into contact with this barrier, which inhibits cell division and prevents the seedling from establishing itself.
The application timing is highly dependent on soil temperature, not air temperature or the calendar date. Crabgrass seed germination begins when the soil temperature at a four-inch depth consistently reaches 55 degrees Fahrenheit for four to five consecutive days. Applying the pre-emergent herbicide just prior to this threshold is necessary for the treatment to be effective.
Active Ingredients for Pre-Emergence
Prodiamine is known for its low solubility and long residual effect, offering extended protection throughout the season. Pendimethalin is another widely available ingredient, although it may require follow-up applications due to a shorter residual period.
Dithiopyr offers a distinct advantage, as it provides both pre-emergent control and limited early post-emergent control. This means it can terminate young crabgrass seedlings that have already sprouted, typically before the two- or three-leaf stage. These products are designed for prevention and will not control mature, actively growing crabgrass plants.
Selective Killing of Mature Crabgrass (Post-Emergent Strategy)
Once crabgrass has emerged and is actively growing, selective post-emergent herbicides are necessary to terminate the existing plants without damaging the surrounding turf. These products are formulated to target the physiology of annual grassy weeds while being tolerated by established lawn grasses. The active ingredients work systemically, moving throughout the plant after being absorbed by the foliage.
Systemic Post-Emergent Options
Quinclorac is one of the most reliable active ingredients for controlling existing crabgrass, even in its more mature stages. It is effective on plants with multiple tillers, which are the side shoots that form as the plant spreads. For optimal absorption, quinclorac products often require the addition of a non-ionic surfactant to help the herbicide adhere to the crabgrass leaves.
Fenoxaprop p-ethyl, commonly found in products like Acclaim Extra, is another selective post-emergent option. This compound is slowly translocated within the weed and is often safe for most cool-season turfgrasses. Its application rate may need to be increased as the crabgrass plants develop more tillers, reflecting the difficulty of controlling larger weeds.
Mesotrione is particularly useful for newly emerged crabgrass in new seedings of certain turfgrasses, such as Kentucky bluegrass. This herbicide causes a visible whitening of the weed tissue before it dies, a result of its mechanism of action that interferes with pigment production. Control is most successful when the crabgrass is still young, ideally before it develops more than four tillers.
Non-Chemical Control and Long-Term Prevention
Creating a dense, healthy turf is the most sustainable long-term defense against crabgrass, as a thick lawn naturally crowds out the weed. Crabgrass seeds require light to germinate, so maintaining a higher mowing height shades the soil surface and suppresses sprouting. Setting the mower deck to three to four inches for most turfgrasses helps achieve this shading effect.
Proper watering practices contribute significantly to lawn health and discourage crabgrass growth. Watering deeply and infrequently encourages the turfgrass to develop a deep, robust root system, making it more resilient to heat stress. Shallow, frequent watering, by contrast, favors the shallow-rooted crabgrass and weakens the desirable lawn.
A responsible fertilization schedule also assists in thickening the turf, reducing the available space and sunlight for weeds. If small, isolated patches of crabgrass appear, manual removal is an option, ensuring the entire root system is extracted. Horticultural vinegar, which contains acetic acid, can be used as a non-selective spot treatment to desiccate the weed. Caution is necessary, however, as this substance will also terminate any desirable turfgrass it contacts.