Crabgrass

Crabgrass is a coarse, light-green summer-annual grass that grows in a low, spreading, star-like clump, with stems that radiate outward and root at the lower nodes where they touch the soil. The leaf blades are relatively wide and coarse, with leaves rolled in the bud; the ligule is tall, membranous, and jagged-edged, and auricles are absent. Two species predominate: large (hairy) crabgrass has noticeably hairy leaf blades and sheaths, while smooth crabgrass is mostly hairless except for a few hairs near the collar. The finger-like seed heads, with several spike-like branches radiating from the top of the stem, are diagnostic, as is the prostrate, node-rooting, mat-forming habit.

Crabgrass disrupts turf with coarse, light-green, spreading clumps that contrast in color and texture with finer desirable grasses and form low mats that smother and crowd the stand through summer. As the annual matures it produces conspicuous seed heads, and when it is killed by frost the patches collapse and die back, leaving large thin or bare areas that expose soil and invite further weed encroachment and a renewed flush the following spring.

Crabgrass is a warm-season summer annual that reproduces by seed and by rooting at the stem nodes. It is an extraordinarily prolific producer, with a single plant capable of generating many hundreds of tillers and on the order of 150,000 seeds. Seed germinates when soil temperatures at the surface reach a sustained 55 degrees Fahrenheit in spring (a benchmark often coinciding with full forsythia bloom), and germination continues as soils warm; plants grow vigorously through summer, set seed, and are killed by the first hard frost. The large seed bank ensures recurring infestation year after year.

Crabgrass exploits thin, stressed, and open turf and is favored by close mowing, compaction, drought stress, and excessive moisture from frequent shallow irrigation, all of which weaken the desirable stand and expose soil to the warming, sunlight, and warmth crabgrass seeds need to germinate. It is most aggressive along edges, in compacted high-traffic areas, and in lawns mowed too short, where the canopy is open and soil temperatures climb. Hot summer conditions that thin cool-season turf give crabgrass the openings it needs to dominate.