Crabgrass is a common warm-season grass that often invades lawns. It is classified as a summer annual weed, meaning the individual plant completes its entire life cycle within a single growing season. The visible plant does not enter dormancy like a perennial; instead, it dies completely when cold weather arrives.
The Annual Life Cycle of Crabgrass
The life cycle of crabgrass is dictated by soil temperature. Germination begins in late spring or early summer when the soil temperature consistently reaches between 55°F and 60°F for several consecutive days. The seedlings emerge and begin a period of rapid vegetative growth throughout the summer months, thriving in the heat where cool-season turfgrass may struggle. Crabgrass is a C4 plant, giving it a photosynthetic advantage over many lawn grasses in high temperatures. Once the days of summer begin to shorten, the plant shifts its focus from spreading foliage to reproduction. This reproductive phase occurs in the late summer and early fall, resulting in the formation of seedheads. The entire plant, including the roots, then turns brown and collapses.
Winter Survival Strategy
Unlike perennial weeds that use underground structures to survive the cold, the crabgrass plant itself is killed by the first hard frost or freezing temperatures of autumn. The coarse, dead remnants are visible in the lawn until they decompose. The survival of the species is entirely dependent on the vast number of seeds produced during the summer. A single mature crabgrass plant is capable of producing thousands of seeds, with some estimates ranging up to 150,000 seeds per plant. These seeds fall to the soil surface and are incorporated into the top layer of the ground. The seeds then enter a state of protective dormancy, remaining viable in the soil seed bank. Cold temperatures are beneficial to the seeds, helping to break down their protective coating and preparing them for germination in the spring.
Timing Control Measures
Since the existing plant dies every year, the only effective way to control crabgrass is by preventing the next generation from germinating. This is achieved by applying a pre-emergent herbicide to the lawn in the spring. The treatment creates a chemical barrier at the soil surface that disrupts the development of any seedling that attempts to sprout. The timing of this application is specific and must occur before the soil temperature reaches the 55°F threshold that triggers germination. Applying the herbicide too late, after the seeds have already begun to sprout, renders the control measure ineffective. Monitoring soil temperatures, rather than relying on calendar dates, is the most reliable method for accurate timing.