This article was originally published on July 30, 2012 and expired on August 6, 2012. It is provided here for archival purposes and may contain dated information.
Grasses are the hardest plants to identify. Why is that? The answer is simple. Unless you know your stuff, most grasses look the same.
So how do weed scientists identify grasses? Correct identification often hinges upon the smallest structures on the grass plant.
The first is the auricle. The leaves of grass plants wrap around the stem. Auricles are arm-like extensions of the leaf blade that wrap around the stem just above this region. They are easily observable, but sometimes a hand lens is required. The most notable example of an auricled grass plant would be quackgrass, Agropyron repens, which has large, clasping auricles.
The second item that taxonomists look at is the collar. The collar region is kind of like the elbow between the blade and the sheath (the region that wraps around the stem). Occasionally, the collar has a projection that emerges vertically from it, termed a ligule. The ligule can be hairy in appearance, as is the case with sandburs, fall panicum, and foxtail, or it may be membranous or skin-like in appearance, as is the case with crabgrass, johnsongrass, and shattercane. The most distinguishing characteristic for a grass plant is the absence of a ligule, or in other words, a naked collar region (a collar without any projections that just bends into the leaf sheath). Grass with these characteristics is distinguished as barnyard grass, the only non-liguled grass plant in our area.
The third item that a taxonomist looks at is the leaf sheath. Once again, the leaf sheath of the plant is the tissue that extends from the grass blade and wraps around the stem. Often this sheath will form a "V" shape. If that "V" shape is very large, in other words, if the sides of the sheath do not meet until a considerable distance down the stem, taxonomists might identify the grass as downy brome grass. Another example of how the leaf sheath can be used for identification is its hairiness.
Finally, taxonomists closely examine the grass blades for hair. Often a taxonomist will bend the leaf and angle it toward a light to see if hairs exist on the plant. Long hairs located near the collar region on the leaf might distinguish a grass as yellow foxtail. Small, stubby pubescence located all over a leaf surface might help the taxonomist to distinguish between the hairy blades of giant foxtail or the relatively hairless blades of green foxtail.
Beyond this, taxonomists use many other plant structures to distinguish one plant from another. Sometimes the appearance of the roots is utilized, sometimes the shape of the first blade to emerge from the soil is utilized, and sometimes taxonomists will even dig up roots to look for the seed from which the plant germinated.
Pull date: August 6, 2012