Crabgrass is a pervasive summer annual weed that invades lawns across the world. The two most common species causing trouble in turf are large crabgrass and smooth crabgrass. As an annual plant, crabgrass completes its entire life cycle from seed to seed production within a single growing season before dying off with the first hard frost. This rapid life cycle, combined with specific biological advantages, explains its ability to germinate and take over healthy turf. Understanding the environmental conditions that trigger its growth is the first step in combating its relentless spread.
The Critical Temperature for Germination
The primary factor dictating when crabgrass first appears is not the calendar date but the soil temperature. Crabgrass seeds remain dormant through the winter, waiting for a sustained period of warmth to signal the start of the growing season. Germination begins when the soil temperature consistently reaches 55 to 60 degrees Fahrenheit for several consecutive days.
This warming period often coincides with the first signs of spring, just as desirable cool-season turfgrasses are beginning to emerge from dormancy. The timing gives crabgrass an advantage, as it is ready to sprout and colonize bare soil the moment conditions are right. Not all seeds germinate at once, as variations in temperature across a lawn cause staggered germination. This means new seedlings can continue to emerge throughout the late spring and early summer, presenting a persistent problem.
Thriving in Adverse Conditions
Once established, crabgrass possesses specific physiological traits that allow it to outcompete and overwhelm typical cool-season lawn grasses. This competitive edge stems from its classification as a C4 plant, which utilizes a highly efficient form of photosynthesis. C4 plants are better adapted to high temperatures and intense sunlight than C3 cool-season turfgrasses, which struggle in the summer heat.
The efficiency of the C4 pathway allows crabgrass to thrive when temperatures exceed 90 degrees Fahrenheit, maintaining vigorous growth and efficient water use in conditions that cause turfgrasses to wilt and go dormant. Crabgrass also tolerates dry weather and compacted, poor-quality soil, which further stresses the surrounding turf. A weak or thin stand of turf allows sunlight to reach the soil surface, creating the perfect environment for crabgrass to germinate and rapidly expand. Maintaining a dense lawn and proper mowing heights are important cultural practices to reduce the amount of open soil available to the weed.
Aggressive Growth and Seed Production
The physical growth habit of crabgrass is perfectly suited for survival in a regularly mowed lawn. It exhibits a low, spreading growth pattern, known as tillering, which causes the plant to hug the ground. This prostrate habit allows the plant to effectively evade the blades of most lawnmowers, leaving its leaves and reproductive structures intact.
The most significant factor in its persistence is its prolific seed production, which guarantees re-infestation year after year. A single, mature crabgrass plant is capable of producing tens of thousands of seeds. These seeds fall into the soil, creating a large reservoir called the “seed bank,” where they can remain viable and dormant for several years. Although the original plant dies in the fall, the massive quantity of viable seeds ensures that the problem will return the following spring, often worse than before.