Crabgrass (Digitaria spp.) is a common and frustrating weed that infests residential lawns. This summer annual completes its entire life cycle within one growing season, dying off when cold weather arrives. It is easily recognizable by its clumpy, coarse texture and light, yellowish-green color, which stands out against desirable turfgrasses. The stems often branch out low to the ground in a prostrate, or “crab-like,” manner, giving the weed its common name.
Competition and Resource Depletion
Crabgrass is detrimental because of its aggressive ability to compete for limited resources. It possesses a C4 photosynthetic pathway, which allows it to thrive during the intense heat and sunlight of summer. Most common lawn grasses, such as Kentucky bluegrass and fescue, are C3 plants that become stressed and slow their growth during summer months.
This physiological difference allows crabgrass to monopolize water and nutrients when turf is weakest. Its rapid, sprawling growth habit enables it to quickly cover bare soil and choke out turfgrass seedlings. Furthermore, the broad leaves of crabgrass shade the soil surface, blocking sunlight from reaching the lower turfgrass below.
This competition starves the established lawn, creating a weakened, thin turf canopy that is susceptible to further weed infestations and environmental stresses. Mowing the lawn too short also encourages this weed by removing the turf’s shading effect, providing ideal light conditions for crabgrass to flourish.
The Persistence of Prolific Seeding
The weed’s annual life cycle ensures its return through an aggressive reproductive strategy. Although a single crabgrass plant lives for only one season, it can produce up to 150,000 seeds before it dies, dropping them directly onto the soil surface.
These seeds replenish the “soil seed bank,” a reservoir of viable seeds lying dormant beneath the lawn. The volume of seeds means that even if the lawn is maintained well for one season, years’ worth of viable seeds remain. Germination begins when the soil temperature at a shallow depth consistently reaches 55°F to 60°F for several days.
This prolonged germination window, which starts in the spring and continues through the late summer, allows crabgrass to emerge in multiple waves throughout the growing season. The long-term viability of the seeds in the soil seed bank makes controlling the weed a multi-year effort.
Structural Damage from Seasonal Die-Off
The infestation manifests in the fall when the plant’s life cycle concludes. As a summer annual, the entire plant is killed by the first hard frost, turning brown almost instantly. This sudden die-off leaves behind large, noticeable patches of dead, straw-colored plant matter.
These dead areas create significant voids in the turf canopy, resulting in a patchy lawn throughout the fall and winter. The bare soil left behind is exposed to the elements, increasing the potential for soil erosion, especially on sloped areas.
These unoccupied areas become ideal niches for other undesirable winter annual weeds to take root. The exposed soil also provides the perfect environment for dormant crabgrass seeds to wait until the following spring.