Johnsongrass (Sorghum halepense, family Poaceae) is a perennial grassy weed that is native to the Mediterranean region. It was exported world-wide primarily for erosion control and can now be found growing on every continent except Antarctica. Its common name references Alabama plantation owner William Johnson who sowed seeds on his river-bottom land sometime in the 1840s to control water erosion.
This warm season grass was long considered a problem confined to southeastern states. Some online references continue to refer to the 40th northern parallel as an imaginary line of demarcation for this non-native weed. Of course, the 40th parallel splits Ohio (you can see a survey marker on the OSU Columbus campus) meaning that the southern part of the state has long been fertile ground for Johnsongrass. However, this grassy weed has been creeping north no doubt due to changing climate conditions.
Johnsongrass is well-known for becoming established in agricultural fields to reduce yields in corn, soybeans, and leguminous forages. The grass is a prolific seed producer making spread through contaminated crop seed a continual problem. Plants also spread by underground stems called rhizomes making it stubborn recurring weed. Johnsongrass also has a reputation of being capable of killing livestock under certain conditions. Plants stressed by drought, insect damage, or frost will produced hydrocyanic acid (= prussic acid, cyanide). The grass appears on the Ohio Department of Agriculture, Plant Industry Division's list of noxious weeds.
Landscape and nursery managers as well as home gardeners may not be familiar with Johnsongrass particularly since it can be mistaken for other grassy weeds and even some ornamental grasses. However, this stubborn weed is becoming common in Ohio landscapes. Established plants commonly reach 6 - 8' in height and colonies will outcompete preferred landscape annuals and herbaceous perennials.
Control in landscapes and nurseries can be difficult owing to long-term seed viability - over 20 years - and the occurrence of fleshy rhizomes from which new plants can spring in the spring. There is also a developing challenge with herbicide resistance in some southern states. Johnsongrass biotypes have been identified that are resistant to glyphosate (e.g. Roundup), fluazifop-p-butyl (e.g. Fusilade, Ornamec, Grass-B-Gon), sethoxydim (e.g. Poast), and quizalofop (e.g. Assure).
There are no effective pre-emergent herbicide options and post-emergent herbicides including glyphosate (e.g. Roundup) are most effective on plants much younger than the current mature plants that are heavy with seed. Indeed, physical removal of plants right now may miss seed that has already dropped to the ground. However, rouging and destroying plants at this time of the season can be effective in reducing colonies if care is taken to also remove the rhizomes.