Crabgrass is arguably the most common offender among unwanted grasses in manicured lawns. This aggressive annual weed, which includes large crabgrass and smooth crabgrass species, is highly competitive with desirable turfgrasses. Accurate identification is the necessary first step, as misidentification often leads to ineffective control methods. Crabgrass can be distinguished by examining its anatomical features, overall growth habit, and the specific timing of its annual life cycle.
Key Anatomical Markers
Identifying crabgrass requires a close inspection of the plant’s structure, focusing on the leaf and the junction where the blade meets the stem. Crabgrass leaves are generally wider than those of typical lawn grasses, often measuring between one-quarter and one-half inch across, and they have a coarse texture. Large crabgrass leaves are notably covered with fine, stiff hairs on both the blade surface and the sheath. Smooth crabgrass, the other common variety, is less hairy, sometimes only having a few hairs near the base.
A definitive feature lies in the collar region, where the blade separates from the sheath. Crabgrass lacks auricles, the small, ear-like appendages found on some other grasses. Instead, it features a prominent, translucent membrane called a ligule, which is often jagged or cut off straight across the top. This membranous ligule is a reliable marker for positive identification. Furthermore, the leaf blades emerge from the stem in a rolled-up fashion, known as rolled vernation, which distinguishes it from other grassy weeds.
Identifying the Growth Pattern
Crabgrass exhibits a distinct growth pattern within the lawn that contributes to its name and appearance. It is a bunch-type grass, meaning it does not spread via underground runners (rhizomes) or above-ground runners (stolons) like many desirable turf species. Instead, the plant grows from a central point, sending out numerous tillers that root at the nodes as they spread outward. This results in a dense, spreading clump that often creates a noticeable star-like or crab-leg pattern low to the ground.
The stems of the mature plant typically lie prostrate, or flat, against the soil surface, which allows the plant to avoid being cut by a lawnmower set at a normal height. This low-growing habit enables it to form thick, circular mats that effectively smother the surrounding desirable turfgrass. As the season progresses, the base of the stems may develop a purplish or reddish tint, and the overall plant color is usually a lighter, yellowish-green hue compared to the darker green of most cool-season lawn grasses.
Comparing Crabgrass to Common Look-Alikes
Comparing crabgrass to common look-alikes is helpful, as many wide-bladed grasses are mistakenly identified. Goosegrass, a warm-season annual, is a frequent source of confusion because it also grows in clumps and forms a low mat. However, goosegrass typically germinates two to eight weeks later than crabgrass, and its leaves have a distinct silvery-white center at the base of the plant. Furthermore, goosegrass leaves are folded in the bud, a structural difference from the rolled vernation of crabgrass.
Tall fescue and quackgrass are two other weeds that are often misidentified as crabgrass, but they are both perennial grasses, meaning they survive the winter. Tall fescue grows in large, tough clumps and tends to grow more upright than the sprawling crabgrass, making it easier to spot in a mowed lawn. Quackgrass, a cool-season perennial, also grows upright and can be definitively identified by the presence of a clasping auricle, the small claw-like structure that is absent on crabgrass. Observing whether the weed survives a hard frost or has a deep, spreading root system can quickly rule out the annual crabgrass.
Seasonal Appearance and Annual Cycle
The predictable life cycle of crabgrass is one of the most reliable ways to confirm its identity, dictating when the plant will be visible. Crabgrass is a summer annual, meaning it completes its life cycle within a single warm growing season before dying off. Seeds begin to germinate in the late spring or early summer, specifically when the soil temperature at a one-inch depth consistently reaches 55°F to 60°F for several consecutive days.
The plant then grows rapidly throughout the hottest months, spreading and maturing until the days begin to shorten in late summer. At this time, the plant shifts its energy to reproduction, producing the characteristic finger-like seed heads that can generate up to 150,000 seeds per plant. The entire plant is highly sensitive to cold temperatures and will be killed completely by the first hard frost, leaving behind bare patches in the lawn and seeds ready to sprout the following spring.