Johnson Grass



Johnson grass is a common weed that occurs throughout Kansas, and one of the most expensive weeds that farmers have to deal with.

Johnson Grass

Johnson Grass

Johnson grass (Sorghum halepense) grows sturdy upright stems ranging from two to eight feet in height. Some stems are branched and others are not, but one thing can be counted on with this species—there will be quite a few of them packed into each clump! Look for a pink or red color near the base of the stem, if you can comb through the thick clumps well enough to find any bases.

The leaves grow in an alternating pattern up the stem and tend to droop. They can vary from 8 to 36 inches in length, and may be up to 1-1/2 inches wide. The midrib is prominent in this species.

The inflorescence displays an open, branching structure known as a panicle. The panicle may vary from 6 to 20 inches in length and is typically purplish and hairy. The spikelets grow in pairs or threes. Note the awns and the tiny reddish-brown seeds, no more than 1/8 of an inch long.

Johnson grass has a fibrous root system, but of more interest are its thick rhizomes. These tend to start out white and develop a creamy color and orange scales as they grow. In most soils, the rhizomes tend to stay close to the surface, extending about four to six inches downward. Where cracks in the ground permit more ambitious growth, the rhizomes may stretch down as far as 20 inches!


Johnson grass is a common weed that occurs throughout Kansas. It prefers open, disturbed ground with plenty of moisture and nutrients. Thus, it regularly appears in fields and pastures, along roads and streams, and anywhere the soil has been turned up and abandoned.

Life Cycle

Johnson Grass

This perennial grass gets its start in the spring by growing from the rhizomes, the leaves developing by late spring. The plant flowers beginning in July and continues until frost hits, sometimes producing over 80,000 seeds in one season. The seeds can remain dormant in the soil for up to 25 years, but they may germinate and develop a rhizome system from which to grow the following spring—it takes no more than six weeks for a Johnson grass seedling to establish a viable network of rhizomes.


Johnson grass is native to the Mediterranean region, but it has been introduced to all continents except Antarctica. It was first brought to the United States around 1830. It takes its name from Colonel William Johnson of Alabama, who established it on his plantation about ten years later. Johnson grass quickly spread throughout a good portion of the United States, reaching Kansas by 1880.

This aggressive weed can be a nuisance in poorly managed natural areas, crowding out the native plants. However, it typically does not become a major problem in healthy ecosystems with the exception of stream banks, where it tends to take over quickly.


Johnson Grass

Johnson grass is not a particularly useful plant. Even though it is sometimes used for forage and hay, it is not very palatable and can be downright deadly. When wilted due to frost or drought stress, Johnson grass contains high levels of cyanide. Also, this weed can accumulate nitrates in large enough quantities to cause bloat in livestock.

Due to its aggressive tendencies, Johnson grass is highly effective when planted to prevent soil erosion. However, the drawbacks to planting this invasive species tend to outweigh the benefits.

Kiowa children made whistles from the stems and leaves of this plant.


Many agricultural experts believe that Johnson grass is one of the most expensive weeds farmers have to deal with. It can compete with most crops and win, it has strong rhizomes to keep it going when cut down, and it is resistant to glyphosate. Eradicating Johnson grass from a field will require a great deal of persistence. Make it easier on yourself by adding some small grains to your rotation, as the dense stands these crops form can keep the Johnson grass at bay to some degree. When working in fields infested with Johnson grass, always clean all machinery thoroughly to avoid spreading plant parts from one field to the next. Cultivate infested fields three to five inches deep every other week to weaken the plants over time.

Johnson grass can be high in bloat-causing nitrates, but the most deadly threat to livestock from this plant is cyanide. New shoots and wilted grass are particularly high in cyanide, a toxin that can causes progressive neurological damage and death. Ruminants are particularly susceptible, but horses are not entirely immune. Never allow livestock to graze Johnson grass during droughts or after a frost, and avoid feeding any hay which might contain Johnson grass. Prevention is the best cure when it comes to avoiding this species in pastures—keep your grass stand healthy, and do not introduce any grass seed or animal feed that might contain weed seed. If you need to eradicate Johnson grass from an infested pasture, mow it down closely every two to three weeks throughout the entire growing season. After fall frosts begin, plow up the pasture to expose the roots and rhizomes to the elements over the winter.

This weed is also a pain to get rid of in lawns and gardens. Begin the eradication process by tilling up the yard or garden and letting it lie exposed over the winter. The following growing season, tackle the weeds at least every other week, pulling and digging them them out of the ground by hand whenever the soil is soft enough and hoeing them down when it is not. Whenever weeding, be sure to remove all plant parts and dispose of them in a place where they cannot sprout. Once you have the situation back under control, avoid future problems by taking care not to introduce seeds from outside sources, such as hay used for mulch or stands of Johnson grass growing in nearby ditches. Also, keep your lawn healthy, and it should be able to fight off invasion on its own.

Finally, Johnson grass pollen is a major allergen in some areas.

Similar Species

Johnson Grass

Eastern Gamagrass
This species may resemble Johnson grass in its early stages of growth due to its clumping tendencies. A quick peek at the base of the stem should clear up the dilemma immediately—Eastern gamagrass stems are flat near the base, while Johnson grass stems are round. Once the plants flower, there should be no further confusion.

Helpful Resource

Parts of a Grass Plant: A Glossary
Definitions for technical terms used in this post.

Complete Series

Grasses of Kansas

Grasses of Kansas