Common names: Johnson grass, Johnsongrass
Family: Poaceae
Life form: Perennial grass
History & Introduction
Johnson grass was originally a Mediterranean grass, native to parts of Europe, Africa, and Turkey which reside on the coastline. In the 1830’s and 40’s, it was introduced as a forage crop to the United States in South Carolina and, more notably, in Alabama by William Johnson, from whom the plant’s common name was formed. It has since spread widely across the states due to the agricultural trade and the plant’s aggressive nature, with the most successful populations persisting in the Southeast.
Identification & Biology
Johnson grass thrives in open-canopy areas that receive plenty of sunlight. In particular, disturbed habitats can be quickly overcome by this species, as the plants take advantage of remaining resources by propagating by seed or rhizomes. Like many other invasive species, Johnsongrass is a generalist and able to adapt to a wide variety of soil types and moisture levels.
Form: This is a tall grass that can form dense patches if allowed. Plants have been known to range between 3-12 ft tall, though will most commonly stop growing at about 8 ft. Stems are hollow and round. There can be multiple flowering stalks per individual, with upward arching leaves forming the rest of the vegetative silhouette. Underground, a dense and extensive rhizomatous network can be found, allowing the plant to produce neighboring clonal individuals.
Leaves: Long, narrow, and with entire margins, mature Johnson grass leaves are often warm-green in coloration and extend up to 20 in. long. Before tapering at the tip, the average width of these leaves is about 1 in. At the center of each leaf is a notable white midrib that parallels the rest of the leaf’s lateral venation. The underside of the leaf is a paler green than the upper surface, although both sides are hairless. Coloration of the leaves may tinge purple if the plant itself is under stress.
Leaves will primarily stay in the lower half of any stem that they are growing along, and are arranged alternately. The leaf sheath which adjoins the leaf to the stem is usually round, hairless, with overlapping margins. The ligule, or where the base of the leaf blade meets the sheath, is pronounced and membranous in texture. Ranging between ⅛-¼ in. wide, unlike the rest of the leaf, the ligule typically exhibits fine, white hairs.
Flowers: Johnsongrass flowers come in mass on purple-to-brown branched structures called panicle inflorescences. Inflorescences are found atop flowering stems, and when fully open they can take on a diamond to pyramid-like silhouette that is 18-24 in. long by 9 in. wide. Individual flowers are copper and are only 4-5 mm long. Flowering season typically occurs in midsummer, May-August.
If looking at the bracts enclosing the flower, note that the outermost-bract (the glumes) are awn-less, while the inner bract (the lemma) can vary between having no awns to occasionally exhibiting an awn that is twisted and up to 15 mm long.
Seeds: Medium to dark brown, football-shaped, and 2-5 mm long. Bracts surrounding the seeds are pubescent.
Look-alikes: to avoid misidentification with native look-alikes, the leaves of the plant are a good place to look. Mature switchgrass (Panicum virgatum) leaves are half the width (0.6 in) of Johnson grass leaves (1.0 in), while leaves of yellow prairie grass (Sorghastrum nutans) will never exhibit the prominent white midrib that Johnson grass leaves do. An invasive look-alike, Chinese silvergrass (Miscanthus sinensis), is another grass with a white midrib on its leaves. Although the most obvious difference between these two species is the flowering structure—with Johnson grass exhibiting branched panicles and Chinese silvergrass with fan-like, almost feathery panicles—vegetative differences also exist. Chinese silvergrass leaves are notably serrate, and can be much longer (up to 3 ft.) and wider (up to 4 in.).
Damage & Concern
Crops and Livestock: Although originally introduced as a crop for livestock feed and birdseed, it has since been learned that Johnson grass poses significant threats to the agricultural business. It is a notorious weed in corn and soybean fields, and is not easy to remove either as there are various herbicide resistant strains present throughout the United States. Due to its tall stature and allelopathic abilities (Johnson grass can release chemicals into the soil which inhibits the growth of neighboring plants), this invasive plant actively limits resource access to crop plants and has been known to reduce productivity by 30-40%. For corn fields in particular, Johnson grass poses a unique threat as it can host various viruses and pests that impact productivity, including Maize Dwarf Mosaic Virus, Sugarcane Mosaic Virus, and Sorghum Midge.
Like a few other members of the Sorghum genus, Johnson grass has the capacity to induce cyanide poisoning when grown in certain conditions. Containing the cyanogenic glucoside, dhurrin, in all of its vegetative parts, Johnson grass is not inherently toxic to livestock. However, if grown in stressful conditions such as drought, dhurrin can release hydrogen cyanide into the plant, serving to defend the plant against predation. Unfortunately, this has led to death in grazing livestock such as cows and horses.
Native Ecosystems: Due to its allelopathic tendencies, toxicity to grazing animals, and ability to form rhizomatous colonies, Johnson grass poses a significant threat to native ecosystems. Not only is this plant able to outcompete native biodiversity for space and nutrients, but it has high reproductive success and many pathways of spread. In one year, a singular individual of Johnson grass may produce anywhere between 30,000-80,000 seeds per year. In turn, these seeds can be dispersed through nearby waterways, the wind, and bird and mammal droppings. Although less likely to occur in more natural environments, Johnson grass seeds and rhizome fragments have also been discovered to spread via contaminated livestock feed and gardening equipment. In providing less ecological services to their surrounding environment than native plants might, Johnson grass colonization has a net negative impact in environments outside of its native range.
Management
It is always best to manage Johnson grass before plants have gone to seed. If managed by hand, take extra care to remove all parts of the rhizome structure underneath, and expect to continue treating the area for multiple years. An alternative mechanical treatment could include a mixture of mowing the area regularly to prevent flowering stalks, and if viable, tilling. Tilling the soil every 2 weeks for a total of six times throughout the growing season will help to expose and kill off root systems. A multi-year management timeline, however, should still be expected with this option, and even in chemical treatment with herbicides.
If working with herbicides, you may apply systemic herbicides to the foliage in the spring or fall. Refer to your herbicide label and state regulations surrounding herbicide application to Johnson grass as there are numerous herbicide resistant strains that exist.
Monitoring or Reporting
As a Tier 2 in LIISMA’s Early Detection and Rapid Response program, we urge you to please report any sightings of Johnson grass to iMap or iNaturalist. Your observations are key to our mission, as they allow us to respond to emerging populations before becoming a larger threat to New York. To gain a better understanding of how we assign tiers to species, see this article.
iMap is a network for invasive species reporting, allowing concerned parties like LIISMA to discern where invasive species may be popping up, or their geographic extent. Similarly, iNaturalist functions as a biodiversity reporting platform for all kinds of living things—native or non-native—and can help anyone identify and share what species are inhabiting a given area.
If you are interested in playing a bigger part in protecting our local flora and fauna, consider joining LIISMA’s Invasive Species Spotter Program! This program teaches you how to spot emerging invasive species as they arise, giving you valuable detection skills against occurrences of invasive species that were not previously on LIISMA’s radar.
References:
Center for Invasive Species and Ecosystem Health. (2018) Johnsongrass. University of Georgia. https://www.invasive.org/browse/subinfo.cfm?sub=3075
Cowan, M., Møller, B. L., Norton, S., Knudsen, C., Crocoll, C., Furtado, A., Henry, R., Blomstedt, C., & Gleadow, R. M. (2022) Cyanogenesis in the Sorghum Genus: From Genotype to Phenotype. Genes, 13(1), 140. https://doi.org/10.3390/genes13010140
Go Botany. (2025) Sorghum halepense – Johnson grass. Native Plant Trust. https://gobotany.nativeplanttrust.org/species/sorghum/halepense/
Minnesota Department of Natural Resources. (2025) Johnsongrass (Sorghum halepense). Minnesota DNR. https://www.dnr.state.mn.us/invasives/terrestrialplants/johnsongrass.html
Missouri Department of Conservation. (2025) Johnson Grass. Conservation Commission of Missouri. https://mdc.mo.gov/discover-nature/field-guide/johnson-grass
Mitich, L. (1987) Intriguing World of Weeds: Colonel Johnson’s Grass: Johnsongrass. https://my.ucanr.edu/repository/fileaccess.cfm?article=167414&p=DHWRWB
North Carolina Extension Gardener Plant Toolbox. (2025) Sorghum halepense. NC Cooperative Extension. https://plants.ces.ncsu.edu/plants/sorghum-halepense/
Ohio Perennial and Biennial Weed Guide. (2025) Johnsongrass (Sorghum halepense). The Ohio State University College of Food, Agricultural, and Environmental Sciences. https://weedguide.cfaes.osu.edu/singlerecord.asp?id=62
Waggy, M. (2011) Miscanthus sinensis. In: Fire Effects Information System. U.S. Department of Agriculture, Forest Service: Rocky Mountain Research Station, Fire Sciences Laboratory. https://www.fs.usda.gov/database/feis/plants/graminoid/missin/all.html
