Paspalum dilatatum, commonly known as Dallisgrass, is a resilient and pervasive perennial grass that often invades lawns and turfgrass areas. As a warm-season weed, it thrives during the summer months, presenting a coarse, unwanted texture within fine turf. Accurately identifying this plant is the first step toward effective management. This guide provides a detailed look at the specific visual markers that distinguish Dallisgrass from other common grassy weeds.
Overall Growth Pattern and Appearance
Dallisgrass has a characteristic growth habit, forming dense, circular clumps or loose bunches that spread outward from a central base. Unlike turfgrasses that spread uniformly, this clumping pattern makes the weed highly noticeable in a manicured lawn. The foliage generally appears a lighter or grayish-green color, contrasting sharply with the darker, finer texture of desirable lawn species.
If left uncut, Dallisgrass can grow quite tall, with stems sometimes reaching up to five feet in height. Homeowners often first identify it by the coarse, upright growth that quickly overtakes the surrounding, shorter grass after mowing. A close inspection of the plant’s base will often reveal a reddish or purplish coloration on the lower leaf sheaths. This color distinction at the base is a helpful, early visual cue for identification.
Specific Features of the Leaf and Stem
The leaf blades of Dallisgrass are relatively wide, typically measuring between 6 to 15 millimeters (0.25 to 0.5 inches) across, and are notably flat. A prominent, raised mid-vein runs down the center of the blade, which is another useful identification feature when observed up close. Leaves emerge rolled in the bud and taper to a sharp point at the tip.
The leaf sheath, which wraps around the stem, is compressed and can be slightly hairy, especially along its margins and near the collar. At the junction where the leaf blade meets the sheath, a noticeable, tall membranous ligule is present, generally measuring between 1 to 5 millimeters. Dallisgrass lacks the ear-like appendages called auricles.
Beneath the soil, the plant possesses short, thick, and knotty rhizomes (underground stems) that help it survive as a perennial. These rhizomes often exhibit distinctive rings, providing confirmation of the plant’s identity. The presence of these rhizomes explains why the plant returns persistently each year.
The Defining Characteristic: The Seed Head
The most distinct and reliable feature for identifying Dallisgrass is its unique seed head, which emerges on a tall, slender stalk that rises above the foliage. This inflorescence is a raceme structure, featuring between three and seven spike-like branches, or rachises, arranged alternately along the central stem. These branches often droop slightly and may give the seed head a recognizable “V” or “Y” shape.
Along the underside of these spike-like branches, the spikelets—which contain the seeds—are arranged in two distinct rows. Each spikelet is ovate, slightly flattened, and covered in fine, silky hairs. The seeds themselves are elliptic, measuring approximately 2 millimeters long, and may appear green, yellow, or purplish as they mature.
Mature seed heads are susceptible to infection from the ergot fungus, Claviceps paspali. This fungus causes the spikelets to become dark or black, sticky, and hardened, confirming identification. The fungus-infected seeds can be toxic to livestock if ingested. The prolific seed production is a major contributor to the weed’s spread.
How to Distinguish Dallisgrass from Other Common Weeds
Dallisgrass is frequently mistaken for other common grassy weeds, such as Crabgrass, but their life cycles are a key difference. Dallisgrass is a perennial plant that returns year after year from its root structure, while Crabgrass is an annual that dies completely in the winter. Visually, Crabgrass grows in a low, sprawling, “crab-like” pattern, whereas Dallisgrass maintains a more upright, clumping habit.
The seed heads are also distinct; Crabgrass typically has smaller, finer seed heads, while Dallisgrass produces a larger, darker seed head with the notable three-to-seven alternating branches. It is also sometimes confused with Bahiagrass, a related species. However, Bahiagrass typically has fewer seed head branches, often just two forming a distinct Y-shape, and its leaves have a different texture.
Another distinguishing factor is found underground: Bahiagrass rhizomes lack the dark, ringed appearance found on Dallisgrass rhizomes. Recognizing these differences in growth habit, leaf structure, and seed head morphology is necessary for correct identification.