The existing crabgrass plant dies in the fall. Classified as a warm-season annual weed, crabgrass completes its entire life cycle within a single growing season. It thrives during the hot summer months, but its existence ends abruptly with the arrival of cold weather. The first hard frost typically delivers the final blow, causing the plant’s foliage and roots to cease function and decay.
The Annual Life Cycle and Seasonal Death
Crabgrass is a summer annual, a significant distinction in lawn management. Unlike perennial weeds, which survive winter via established root systems, the individual crabgrass plant does not survive the cold season. Its biological design dictates that it must germinate, grow, set seed, and die within the span of a few months.
The mechanism of its death is straightforward: it lacks the physiological defenses to withstand freezing temperatures. When the first frost occurs, water within the plant’s cells freezes and expands, rupturing the cell walls. This destruction causes the rapid collapse and death of the entire plant, leaving behind a brown, dead patch of grass. Since the root system is also dead, the same plant cannot return the next year.
The Crucial Fall Activity: Seed Production
While the mature plant dies in the fall, this seasonal death does not signal the end of the crabgrass problem. Throughout late summer and early fall, as days shorten, the plant enters its reproductive stage. Its primary focus shifts to producing and dropping a massive number of seeds before succumbing to the cold.
A single, healthy crabgrass plant is capable of producing an estimated 150,000 seeds in one season. These seeds are scattered onto the soil surface, where they lie dormant over the winter, forming what is known as the “seed bank.” Killing the plant in September or October is often too late to prevent this seed drop, as the plant has likely already completed its mission to ensure the next generation. The problem for the next year is not the survival of the current plant, but the dormancy of thousands of new seeds ready to sprout in the spring.
Preventing Next Year’s Growth with Pre-Emergents
The presence of a massive seed bank means that next year’s control strategy must focus on prevention rather than killing existing plants. Pre-emergent herbicides are designed to stop dormant seeds from successfully germinating and establishing themselves in the spring. This product works by forming a chemical barrier at the soil surface that disrupts the emerging seedling’s root or shoot development, effectively killing it shortly after germination.
Timing the application of a pre-emergent herbicide is important for its effectiveness. Crabgrass seeds begin to germinate when the soil temperature at a depth of one to two inches consistently reaches 55 degrees Fahrenheit for four to five consecutive days. Applying the product too early may result in the chemical barrier degrading before the main germination period, while applying it too late means the seeds have already sprouted.
Split Application
To ensure season-long protection, a split application is often recommended. A second, smaller application is made 30 to 45 days after the first, extending the barrier’s protection into the warmer summer months.
Long-Term Cultural Management
Controlling crabgrass long term involves modifying the lawn environment to make it less hospitable to the weed. Crabgrass requires heat and sunlight to germinate and thrive, so a dense, tall turf canopy is an effective non-chemical deterrent. Homeowners should adjust their mower height to at least 2.5 to 3.5 inches. Taller blades shade the soil, keeping the surface temperature cooler and inhibiting seed germination.
Proper watering practices also play a significant role in crabgrass management. Shallow, frequent watering encourages shallow root systems in turfgrasses and keeps the soil surface moist and warm, favoring crabgrass germination. Instead, watering should be deep and infrequent. This encourages turfgrass to develop deep roots that compete better for resources and creates a drier soil surface less suitable for weed seeds. Maintaining a healthy, thick stand of turf through appropriate fertilization and fall overseeding helps crowd out any crabgrass seedlings.