Plant Profile: Sorghum halepense

Sorghum halepense, commonly referred to as Johnsongrass, is a tall, coarse, herbaceous perennial grass that can reach heights of 2½ to 7 feet. This species is known for its aggressive spreading behavior, making it invasive particularly in the southeastern United States and North Carolina. It often becomes weedy in disturbed environments such as fields and along roadsides. Johnsongrass thrives in a diverse range of habitats, including open forests, abandoned fields, ditches, and wetlands. Its rapid growth can lead to the formation of dense colonies that outcompete native plants and hinder the establishment of tree seedlings. This grass is classified as a noxious weed in 19 states and can sometimes invade gardens, with its seeds and rhizomes being inadvertently transported in contaminated topsoil.

This plant prefers full sunlight, moist to moderately dry conditions, and fertile loamy soil. Nevertheless, it is adaptable and can thrive in various soil types, including those that are drier or contain gravel and clay. Most of its growth occurs during the summer, and it can withstand periods of hot, dry weather. While it is not particularly winter hardy, it can tolerate occasional temperatures as low as 23°F, provided the soil is not overly wet. Johnsongrass is best suited for warm, humid regions with summer rainfall in the subtropics and does not perform well in strictly tropical climates. As a short-day plant, it will not flower if daylight exceeds approximately 13 hours. Its root system is fibrous and long-rhizomatous, often leading to the formation of clonal colonies through its rhizomes. Originally introduced from the eastern Mediterranean or the Middle East as a pasture grass, Johnsongrass has since spread extensively throughout the United States.

In the U.S., Johnsongrass was brought to South Carolina from Turkey around 1830. Named after William Johnson, who established it along the Alabama River in the 1840s as a forage crop, this grass quickly proliferated across the South. Today, it has largely escaped cultivation and is particularly invasive in the Southeast, although it is also prevalent in central California and New Mexico. Johnsongrass can hybridize with other sorghum species (S. bicolor), and in the southern Great Plains and South, some plants identified as Johnsongrass may actually be hybrids with sorghum.

Regarding pests and diseases, there are no significant known issues affecting Johnsongrass. However, its pollen can trigger hay fever. In riparian zones and other areas where it grows abundantly, Johnsongrass may increase the risk of fire by elevating fine fuel loads beyond historical levels. Further research is needed to understand the fire ecology associated with Johnsongrass in North America.