Crabgrass, scientifically known as Digitaria, is an annual grassy weed that aggressively invades lawns, thriving in warm weather. Unlike perennial weeds, crabgrass completes its life cycle within a single growing season. It germinates in the spring and dies with the first hard frost, but not before producing a massive number of seeds. A single mature plant can generate tens of thousands of seeds, which remain viable in the soil for years, making control a persistent challenge. Chemical management involves two distinct strategies: a preventative approach that stops germination and an elimination approach that kills established plants.
Preventing Crabgrass Seedlings from Sprouting
The most effective chemical strategy against crabgrass involves pre-emergent herbicides. These products create a chemical barrier on the soil surface to prevent the emergence of new seedlings by disrupting the germination process. They do not kill established, actively growing plants. Timing the application is highly important for success. The critical window occurs just before the soil temperature consistently reaches 55 degrees Fahrenheit at a depth of two to three inches.
The active ingredients commonly utilized include Prodiamine and Dithiopyr. These chemicals are mitotic inhibitors, meaning they disrupt cell division in the emerging root or shoot of the germinating seed. This interference causes the seedling to become stunted and deformed, preventing it from establishing a viable root system. The chemical barrier must be intact and properly watered into the soil before the seeds sprout.
Dithiopyr offers a slight advantage over Prodiamine because it possesses limited early post-emergent activity, controlling seedlings that have just begun to emerge (typically up to the one-tiller stage). The effectiveness of the barrier is reduced if the soil is disturbed after application, such as through heavy raking or aeration, as this breaks the continuous chemical layer. A second application six to eight weeks later is often required to maintain protection through the summer germination period.
Eliminating Mature Crabgrass Plants
Once crabgrass has successfully emerged and established itself in the lawn, post-emergent herbicides are necessary for elimination. These chemicals are systemic, meaning they are absorbed through the plant’s foliage and translocated throughout the entire weed, including the roots, to achieve a complete kill. The most recognized and effective active ingredient is Quinclorac, which is selective and safe for use on most common turfgrass species.
Quinclorac functions as a synthetic auxin, mimicking the plant’s natural growth hormones but at toxic levels. This over-stimulation of the plant’s hormonal system leads to abnormal growth patterns. In susceptible grassy weeds like crabgrass, this mechanism triggers the rapid induction of ethylene biosynthesis. Ethylene production results in the accumulation of cyanide, which poisons the plant’s tissues and causes symptoms of stunted growth, chlorosis, and eventual necrosis.
The effectiveness of Quinclorac depends on the growth stage of the crabgrass. Optimal control is achieved when the plants are young, generally before they have developed more than three or four tillers (secondary shoots). Mature plants are significantly more difficult to eliminate and may require a repeat application two to three weeks after the initial treatment.
Quinclorac is frequently formulated with other herbicides, such as 2,4-D and Dicamba, to provide broader weed control, targeting both grassy and common broadleaf weeds. Application should be performed when temperatures are moderate (60 to 90 degrees Fahrenheit) to ensure the plant is actively growing and can efficiently absorb the chemical. Applications during periods of heat stress, drought, or extreme cold can reduce the herbicide’s effectiveness.
Non-Chemical Methods of Control
While chemical solutions are highly effective, cultural practices and natural alternatives offer viable non-synthetic control of crabgrass. The most important cultural practice involves maintaining a dense, healthy lawn, which naturally outcompetes the weed.
Raising the mowing height to three inches or more is highly recommended, as the taller turfgrass shades the soil surface. Crabgrass seeds require light and warmth to germinate, and the shade keeps the soil temperature below the optimal 55-degree threshold. Proper watering techniques also play a significant role in prevention; deep, infrequent watering encourages the turfgrass to develop deep, robust roots that better resist weed invasion. Shallow watering, conversely, creates a surface environment that favors the shallow-rooted crabgrass.
For existing individual plants, mechanical removal through hand-pulling is effective, particularly before the plants begin to set seed. Organic alternatives exist for chemical-free prevention and elimination:
- Corn gluten meal is a natural product that acts as a pre-emergent, releasing peptides that inhibit root development in germinating seeds. However, the application rate required for effective control can be substantial for homeowners.
- Concentrated horticultural vinegar, containing high levels of acetic acid, can be used as a non-selective spot treatment to kill small, emerged weeds by rapidly desiccating the foliage. This method typically only burns the top growth, and the weed may regrow if the growing point is not fully saturated.