Crabgrass, a warm-season annual weed, is a persistent problem for many homeowners who desire a uniform lawn. The plant is an aggressive invader that spreads rapidly due to its prolific seeding nature. A single, mature crabgrass plant can produce as many as 150,000 seeds in one growing season, which remain viable in the soil for several years. Successfully managing this weed requires a two-pronged strategy involving both preventative measures and direct control methods.
Stopping Crabgrass Before It Starts
The most effective strategy for controlling crabgrass is prevention, which involves the precise application of pre-emergent herbicides. These products create an invisible chemical barrier near the soil surface that disrupts the germination process. The herbicide works by inhibiting cell division in the emerging seedling, preventing it from developing a root system and establishing itself.
Timing the application correctly is important, as the product must be in place before the crabgrass seeds sprout. Crabgrass begins to germinate when the soil temperature consistently reaches 55 to 60 degrees Fahrenheit at a depth of three inches for several consecutive days. A common indicator for this window is the blooming of certain spring-flowering trees, such as forsythia.
Common active ingredients in these preventative products include prodiamine, dithiopyr, and pendimethalin. Dithiopyr is valued because it offers a small window of post-emergence control, meaning it can kill very young crabgrass seedlings that have just sprouted. Applying the product too late, after germination has fully begun, will render the pre-emergent ineffective, as it does not control established plants.
Chemical Solutions for Established Weeds
When crabgrass has already emerged and is visible in the lawn, post-emergent herbicides are required to kill the growing plant. These products are absorbed through the leaves and stems, then translocated down to the roots to eliminate the entire weed. For best results, these chemicals must be applied when the crabgrass is still young, ideally before it develops more than two tillers, or side shoots.
The most common and effective active ingredient for selective post-emergent control in turfgrass is quinclorac. Quinclorac is designed to target grassy weeds like crabgrass without causing significant harm to the surrounding turfgrass. It is effective on crabgrass that is still relatively immature.
Another effective active ingredient is fenoxaprop-P-ethyl, which provides reliable control for crabgrass at various growth stages. Some products also contain mesotrione, often used to control newly emerged crabgrass, especially when seeding new turf. Multiple applications spaced according to the product label are necessary to achieve complete control, especially as the plants mature.
Non-Chemical and Manual Removal Techniques
For those seeking to avoid synthetic chemicals or for small infestations, manual removal is a practical option. Hand-pulling is best done when the soil is moist, allowing the root system and crown of the plant to be removed completely. Since crabgrass does not have a deep taproot, the entire plant must be extracted to prevent re-sprouting.
If the infestation is extensive or the plants have already set seed, pulling them may scatter thousands of seeds, exacerbating the problem next season. An organic preventative option is corn gluten meal, a byproduct of corn milling that acts as a natural pre-emergent. It interferes with the newly germinated seed’s ability to form roots.
For spot treatment, horticultural vinegar or boiling water can be used to kill individual plants. Both methods are non-selective and will kill any desirable turfgrass they contact, leaving bare spots that require reseeding. These treatments are most effective on very small, young crabgrass seedlings.
Lawn Maintenance to Prevent Recurrence
Maintaining a healthy, dense turf is the most reliable long-term defense against crabgrass, as it crowds out the weed and makes the environment less favorable for germination. Crabgrass seeds require sunlight and warm soil temperatures to sprout, so a taller, thicker lawn acts as a natural shade barrier. Setting the mower height to at least three inches will help shade the soil, reducing the ground temperature and inhibiting seed sprouting.
Proper watering techniques also favor turfgrass over the weed. Deep, infrequent watering encourages the lawn to develop a robust, deep root system, making it more resilient to stresses that favor crabgrass growth. Watering lightly and often, conversely, encourages shallow roots in the turf and creates the moist, warm surface conditions that crabgrass prefers.
Fertilization should be focused on promoting the health and density of the desirable turfgrass. A healthy lawn that is regularly fed will be better able to outcompete crabgrass for light, water, and nutrients. Overseeding thin or bare patches is also an important practice, as crabgrass readily colonizes any open, sun-exposed soil.