Johnson grass (Sorghum halepense) is a highly invasive and problematic perennial grass globally recognized as a noxious weed. Native to the Mediterranean region, this warm-season grass spreads aggressively, rapidly forming dense colonies that outcompete native vegetation and significantly reduce crop yields. Given its widespread distribution, accurately identifying Johnson grass is necessary for implementing effective management and control strategies. Identification relies on recognizing distinct physical markers on the plant’s above-ground structure, its subterranean system, and its reproductive organs.
Identifying Features of the Leaf and Stem
The most recognizable feature of Johnson grass is the prominent, wide, whitish midrib running down the center of the leaf blade. The leaf blades are flat, lance-shaped, and broad, typically reaching up to one inch wide and 8 to 24 inches long on mature plants.
The stout, erect stems, known as culms, are a major visual cue, often growing between 3 to 8 feet tall. These stems are generally smooth, round, and unbranched. At the junction where the leaf blade meets the sheath, a short, translucent, membranous ligule is present, measuring about 1/8 to 1/4 inch long. This ligule may appear fringed or toothed, which helps distinguish it from other similar grasses.
The Role of Rhizomes in Identification
The subterranean structure provides key identification proof and explains the plant’s persistence. Johnson grass spreads via an extensive system of stout, creeping, fleshy rhizomes, which are horizontal underground stems. These rhizomes are segmented and scaly, and a single system can grow to exceed six feet in length, forming a dense network.
The rhizomes are typically yellowish-white to brownish, often marked with purple or reddish splotches. The presence of these thick structures is the primary factor distinguishing Johnson grass from annual grasses, such as shattercane. The rhizomes act as storage organs, allowing the plant to overwinter and produce new shoots rapidly in the spring.
Recognizing the Seed Head
When the plant is mature in mid-to-late summer, the reproductive structure, or seed head, becomes a key identification feature. This structure is a large, open, and loose panicle that often takes on a pyramidal or triangular outline. The panicle is highly branched and can measure between 6 and 20 inches long, giving the top of the plant a bushy or feathery appearance.
The color of the seed head changes as it matures, starting green but eventually developing a purplish, reddish, or dark brown hue. The panicle contains numerous spikelets, which are often arranged in pairs or threes and may have a conspicuous awn, a bristle-like appendage. The mature seeds are oval, reddish-brown to black, measuring approximately 3 to 5 millimeters long.