The term “oat grass” most commonly refers to Tall Oat Grass, scientifically known as Arrhenatherum elatius, a perennial grass species often mistaken for cultivated oats (Avena sativa). This cool-season grass, native to Eurasia, is now widely naturalized and is a common sight in fields and along roadsides across North America. To accurately identify this plant, one must look closely at its overall size, the specific characteristics of its leaves and stems, and the highly distinctive structure of its seed head.
Overall Plant Structure and Height
Tall Oat Grass is a loosely-tufted, perennial bunchgrass that forms dense clumps. When viewed from a distance, the plant appears substantial and coarse, a result of its height and relatively broad leaf blades. The overall hue is typically a bright, slightly glaucous, or grayish-green.
The culms, or stems, of the plant grow erect and can reach 3 to 5 feet tall, though some robust specimens may reach nearly 6 feet. This height makes it one of the taller grasses found in meadows and along fence lines. One subspecies, Arrhenatherum elatius subsp. bulbosum, is distinguished by a swollen, bulb-like structure at the base of the stem, visible when the plant is pulled from the ground.
Identifying Features of the Leaves and Stems
The leaf blades are notably flat and coarse in texture, often feeling rough to the touch. These leaves are relatively broad, measuring between 1/8 to 3/4 inches wide, and they can be up to a foot long.
The color of the foliage is typically a distinct bright green, sometimes possessing a grayish cast. The stems themselves are smooth and jointed, with nodes that may occasionally display a slight hairiness. A critical feature for confirming the identity of any grass is the ligule, the small membrane where the leaf blade meets the sheath wrapped around the stem.
In Tall Oat Grass, the ligule is short, membranous, and square-shaped, usually measuring only 1 to 3 millimeters long. This ligule is often described as being finely ciliate, or hairy-edged. Unlike some other grasses, the base of the leaf blade lacks auricles, which are small, ear-like projections that clasp the stem.
The Distinctive Appearance of the Seed Head
The most definitive characteristic for identifying Tall Oat Grass is its seed head, or inflorescence, which is a narrow, open panicle. This panicle is typically long and somewhat loose, measuring between 3 to 12 inches in length and up to 3 inches wide when fully developed. When the grass is flowering, the seed head is a shiny green, which develops a noticeable purple tinge as it matures.
The panicle is composed of numerous spikelets, which are small clusters containing two florets. The defining feature of these spikelets is the prominent awn, a stiff bristle that projects from the lower floret. This awn is stout and long, often measuring up to 3/4 of an inch, and it is characteristically twisted and bent, or geniculate, near its base.
The twisted, angled nature of the single exposed awn on each spikelet gives the entire seed head a ragged, bristly appearance. As the seed head dries and the seeds mature, the entire structure often turns a straw-yellow or purplish-brown color, with the bent awns remaining clearly visible.