Johnson Grass

Biological Name:

Johnson-Grass: Sorghum halepense

Natural Habitat:

Johnson-Grass: The natural habitat for Johnson-Grass is in fields, pastures, and waste areas in North and South America.


Johnson-grass is a type of grass that is commonly found in fields and other grassy areas. It is a member of the Poaceae family which also includes plants such as wheat and corn. Johnson-grass is an annual or perennial plant that produces small green or brown flowers and clusters of seeds. The plant is often used as a cover crop to improve soil health and suppress weeds. It is also known for its ability to tolerate a wide range of growing conditions including wet or dry soils. In some areas Johnson-grass is considered a weed because of its ability to invade cultivated areas and cause allergies and other health problems.

Frequently Asked Questions (FAQs)

Q: Is Johnson grass invasive?
A: Ecological threat: This species is considered one of the 10 worst invasive weeds in the world. Fifty-three countries, ranging in latitude from 55 N to 45 S report johnsongrass as invasive. Invades wetlands, floodplains, irrigation ditches, undisturbed grasslands, prairies, savannas and riparian zones.

Q: Why is Johnson grass a problem?
A: Under certain conditions, the leaves of johnsongrass (and sorghum) can produce toxic amounts of hydrocyanic acid, which can poison livestock when ingested. It is a highly variable species with some regional biotypes.

Q: Is Johnson grass toxic to animals?
A: Johnsongrass, which can be found in pastures, can produce toxic levels of prussic acid, especially when stressed during cold temperatures and can then poison cattle. Prussic acid is one of the most potent toxins in nature.

Q: Does Johnson grass come back every year?
A: Johnsongrass (Sorghum halepense) is a warm-season perennial weed in pastures and roadsides throughout central and northern Georgia. It germinates from seed in spring after overwintering in the soil, but it primarily emerges from dormant rhizomes in areas with a long-term history of infestations.

Q: What is another name for Johnson grass?
A: Sorghum halepense (Johnson grass)

Q: Can you graze Johnson grass?
A: Johnsongrass is a species that doesn’t tolerate frequent and low grazing because its growing point is 4 to 8 inches above the soil surface. If johnsongrass is to be used as a forage resource in times of short feed supplies such as during drought conditions, it needs to be grazed between 6 and 18 inches.

Q: Is Johnson grass edible for humans?
A: Edible parts of Johnson Grass: Seed – raw or cooked. It can be used whole in a similar manner to rice or millet, or it can be ground into a flour and used as a cereal in making bread, cakes etc.

Q: Is Johnson grass the same as crabgrass?
A: Crab grass tends to populate lawns, vegetable gardens and ornamental yard areas, as well as orchards, vineyards and agricultural zones. Johnson grass specializes in areas where the soil has been disturbed in preparation for planting, often of agricultural crops or vegetable gardens.

Q: Is Johnsongrass good for anything?
A: Believe it or not Johnsongrass can be just as high in crude protein and energy (TDN) than coastal bermudagrass. Its an excellent grazing and haying forage, you just need to pay attention and manage it correctly. When I was growing up we depended on our Johnsongrass meadows for hay.

Q: How does Johnsongrass spread?
A: Biology. Johnson grass flowers from May to October and reproduces by seeds, which stay viable for up to 20 years. Rhizomes also help it to reproduce quickly and one plant can spread across areas up to 200 feet. New stands readily establish from small pieces of rhizome.

Q: Why is Johnsongrass invasive?
A: Invasive. The dense clumps of this very aggressive, perennial grass spread by seed and rhizomes to form nearly pure stands. A single plant may produce more than 80,000 seeds per year. Rhizome cuttings commonly form new plants, making it very difficult to eradicate.

Q: What Round Up Kills Johnsongrass?
A: To keep johnsongrass from wreaking havoc in your landscape, apply Roundup® Landscape Weed Preventer in the spring. It forms a weed barrier to prevent johnsongrass weed seeds—not to mention other listed grass and broadleaf weed seeds—from sprouting and growing for up to 6 months.

Q: What kills Johnsongrass and not Bermuda?
A: Outrider (sulfosulfuron) is an effective herbicide on Johnsongrass found in bermudagrass or bahiagrass pastures and hay meadows. For successful control, Outrider must be applied during active growth that is at least 18 to 24 inches tall and up to the heading stage.

Q: Does Johnsongrass grow fast?
A: Johnsongrass is a fast-growing perennial weed species that can grow 7 to 9 feet tall. It has become a weed menace due to its ability to readily reproduce from seed and vigorous branched rhizomes (horizontal underground stems).

Q: How do you get rid of Johnson grass?
A: For sites with established infestations, a fall application of Roundup or Touchdown will kill emerged tissue and often developing rhizomes. Growing early-maturing crops, plowing immediately after harvest, and tilling as needed are common methods to break up rhizomes and weaken johnsongrass stands in cultivated areas.

Q: Is Johnson grass toxic after cutting?
A: Prussic acid poisoning potential is very high for johnsongrass forage that is shorter than 18 inches tall, wilted forage or for a new flush of growth soon after a rainfall or after hay cutting.

Q: Does Roundup work on Johnson grass?
A: For long-term Johnsongrass control, glyphosate (Roundup, others) is another systemic herbicide that works more effectively when applied in the fall compared to spring treatments. Glyphosate is nonselective and should be limited to spot treatments at rates required to control Johnsongrass.

Q: What will choke out Johnson grass?
A: Dense patches can be controlled by spraying the foliage with 2 percent Roundup (a formulation of glyphosate). Best results are obtained when glyphosate is applied to plants that are 18 inches tall to early flowering stage. During this period the herbicide will be most effectively translocated to the roots and rhizomes.

Q: What time of year is Johnson grass poisonous?
A: Johnsongrass is toxic when under stress, including for about 72 hours after a “killing” frost. After a “burn back” frost, it can be toxic for at least 10 days and possibly longer. When the plant is under stress, it produces a chemical called prussic or hydrocyanic acid (HCN), or more commonly called cyanide.

About the author

Samuel is a gardening professional and enthusiast who has spent over 20 years advising homeowners and farm owners on weed identification, prevention and removal. He has an undergraduate degree in plant and soil science from Michigan State University.