Johnsongrass, a rhizomatous, perennial member of the Poaceae, was introduced from Europe into Alabama in the 1830's for use as pasture forage. It now is distributed widely throughout the United States and has become a serious weed of agronomic field crops, particularly in the Midwest eastward, southward to the Gulf Coast, and across Texas to Arizona. It hybridizes with grain sorghum under field conditions to produce a fertile plant without a loss of aggressiveness. When Johnsongrass is stressed by drought, frost, freezing, herbicides, and cutting, prussic acid may form in the leaves and cause cyanide poisoning in cattle, which consume them. Johnsongrass also is a strong competitor in disturbed environments, including roadside rights-of-way, ditches, waste sites, and fallow fields. Johnsongrass is found abundantly throughout North Carolina.

Johnsongrass grows to a height of up to 10 ft. and at elevations up to 6,000 feet. The "stems" are smooth, but have hairy joints. Johnsongrass spreads by seeds and by an extensive system of thick, scale-like rhizomes, which may grow 2 1/2 feet deep. Fragmentation of the rhizomes by cultivation or with earth-moving equipment results in rapid spread to adjacent or distant sites. The leaves may be up to 12 inches in length and more than an inch wide, often with a pale midvein. Johnsongrass, as in case of all grasses, is wind pollinated. Flowers are open panicles, blooming from spring until late fall and producing many reddish-brown seeds, which may remain dormant for many years, making eradication nearly impossible.