Coarse grasses with reddish to purplish-black panicles, to 2 m tall . Johnsongrass and shattercane grow rapidly, are highly competitive with crops, and can be difficult to control. Infestations in crops can reduce harvest yields significantly. Both species hybridize with each other and with cultivated sorghum varieties, diminishing the quality and value of grain harvested for seed. Plants are highly variable and many regional biotypes exist.
Healthy plants can provide good forage for livestock.
However, foliage of johnsongrass and other sorghums can produce toxic amounts of hydrocyanic acid when exposed to frost, stressed by drought, or damaged by trampling or herbicides and may be poisonous to livestock when ingested. Some biotypes are potentially more cyanogenic. Young shoots and second growth are typically more dangerous than uncut mature plants. Dried plant material does not lose its toxicity. Well-cured hay from healthy mature plants is usually safe.