Crabgrass (Digitaria sanguinalis or Digitaria ischaemum) is an opportunistic summer annual weed common in lawns and gardens. It is classified as an annual because its entire life cycle, from seed to seed production and death, occurs within a single growing season. Crabgrass thrives in the hot, dry conditions that often stress cultivated turfgrasses. Understanding its life cycle is the best way to determine when and how to interrupt its growth and prevent its spread.
Germination: The Timing of Initial Growth
The start of the crabgrass life cycle depends on soil temperature. Seeds require the soil surface temperature to consistently reach between 55°F and 60°F for four to five consecutive days to signal germination. This thermal requirement usually corresponds with the time when native trees like lilacs begin to bloom. The window of initial sprouting often begins in early spring and can continue through late summer, although the largest flush of seedlings occurs early in the season.
The timing of this growth phase is the plant’s main vulnerability and the period for pre-emergent control measures. Seeds that germinate later in the summer still contribute to the seed bank for the following year. Crabgrass seeds that are buried deeper than two inches in the soil are less likely to sprout, but the seeds can remain viable in the soil for up to three years.
The Aggressive Maturation and Spreading Habit
Once the seedlings emerge, the plant quickly establishes itself. Crabgrass is a warm-season grass, meaning it performs best during the summer heat when most cool-season lawn grasses are under environmental stress. The plant grows in a prostrate habit, with stems radiating outward from the center like the legs of a crab.
This spreading habit allows the plant to outcompete surrounding turf by shading the soil and monopolizing sunlight and moisture. The plant also has a prolific tillering ability, which means it produces numerous side shoots that increase its horizontal spread across the lawn surface. As the stems, called culms, creep along the ground, they are capable of rooting at the nodes, which are the joints along the stem. This enables the plant to develop an extensive fibrous root system, sometimes penetrating up to six feet deep, granting it drought tolerance.
The low growth profile and ability to root at the nodes make crabgrass highly resistant to typical mowing practices. Even when cut short, the plant retains its growth points near the soil surface and can still produce seeds. The establishment of this wide, low mat effectively chokes out the desirable turfgrass.
Seed Production and Life Cycle Completion
The final stage of the annual life cycle is dedicated to reproduction. As the days shorten in mid-to-late summer, the plant shifts its energy from vegetative growth to forming characteristic seed heads. These seed heads are often described as finger-like spikes that appear at the tips of the stems.
A single crabgrass plant is capable of producing thousands of seeds, with estimates up to 150,000 under ideal conditions. The seeds are shed onto the soil surface, where they lie dormant until the temperature conditions are right for germination the following spring.
The life cycle concludes abruptly with the first hard frost of autumn. Because crabgrass is a summer annual, the parent plant is killed completely. The dead, brown patches of crabgrass remain, but the next generation is already resting in the soil, awaiting the warmth of the next spring to begin the cycle anew.