Crabgrass (Digitaria species) is a common annual weed that appears in lawns during the warm summer months. As a summer annual, it completes its entire life cycle—from germination to seed production—within a single growing season before dying off with the first hard frost. Identifying the plant when it reaches its reproductive, or seeding, stage is necessary for preventing its return and proliferation the following year.
Identifying Mature Crabgrass (The Vegetative Mat)
Crabgrass develops a distinctive vegetative structure before the seed heads become visible. It gets its common name from its growth habit, as its stems and leaves sprawl outward from a central point, forming a low, dense mat that resembles the legs of a crab. The leaves are typically a lighter, yellowish-green color than most turfgrasses and have a coarse texture.
The individual leaf blades are generally wider than lawn grass, often ranging from one-half to three-quarters of an inch across, and they have a conspicuous midvein. A technical identifying characteristic is the presence of a ligule, a small, thin, membranous structure found where the leaf blade meets the stem. Large crabgrass (Digitaria sanguinalis) has leaf sheaths and blades covered with stiff, sparse hairs. Smooth crabgrass (Digitaria ischaemum) is mostly hairless but may show a reddish-purple hue at the base of the stem.
The Appearance of the Seed Heads
The clearest indication that crabgrass is “going to seed” is the emergence of its inflorescence. Crabgrass seed heads are terminal panicles that emerge from the top of the stem and consist of several slender, finger-like branches called racemes. These racemes radiate outward from a central stalk, often giving the structure the appearance of a bird’s foot or an open fork.
A single plant typically produces between three to seven spikes, though some large plants can produce up to 13. Along each thin spike, tiny seeds, called spikelets, are arranged in two alternating rows. The seed heads initially emerge as silvery-green but mature to a greenish-purple or reddish-brown color as they ripen. These reproductive structures stand noticeably upright and taller than the rest of the sprawling plant, making them visible above the surrounding grass canopy.
When Seeding Occurs
The reproductive cycle of crabgrass is tied to the summer season and the accumulation of heat. Crabgrass plants begin to produce seed heads in mid-to-late summer, generally starting around July and continuing through September. This timing coincides with the plant shifting its energy from vegetative growth to reproduction.
A single crabgrass plant continues to set seed until it is killed by the season’s first hard frost in autumn. A single, healthy plant can potentially generate up to 150,000 seeds. This late-summer seeding window is the final stage of the plant’s life cycle for the year.
The Impact of Seed Dispersal
Once the seed heads mature and dry out, the seeds are shed and fall directly onto the soil surface. This dispersal is the primary reason crabgrass is a recurring problem, as these seeds are the source of next year’s infestation. The seeds enter dormancy, remaining viable in the soil throughout the winter months.
The volume of seeds produced means that a single season of uncontrolled crabgrass can create a seed bank in the soil that lasts for several years. When soil temperatures warm up the following spring, typically reaching 55 degrees Fahrenheit for several consecutive days, the seeds germinate and begin the cycle again. Identifying the plant before or during the seeding stage is the opportunity to break the cycle and prevent future proliferation.