What Is Johnson Grass and Why Is It a Problem?

Johnsongrass (Sorghum halepense) is a tall, fast-growing perennial grass originally from the Mediterranean and central Asia that has become one of the most damaging weeds on Earth, capable of slashing crop yields, poisoning livestock, crowding out native plants, and shrugging off the herbicides farmers throw at it. It was brought to the United States as a promising forage crop in the early 1800s, and within decades it had escaped cultivation and colonized farmland, roadsides, and prairies across nearly every state. What makes johnsongrass so stubbornly difficult to deal with is its dual reproductive strategy: it spreads both by prolific seed production and by a dense underground network of rhizomes that can regenerate a full plant from a small fragment.

How Johnsongrass Got Here

The common story credits a farmer named William Johnson of Marion Junction, Alabama, with introducing the grass to his state, which is how it got its name. But the historical record shows johnsongrass was already being grown across the southeastern United States by 1830, possibly earlier, before any single introduction event could explain its spread.¹ Farmers at the time saw it as a high-yield forage and hay crop, and they shared it enthusiastically. By the late nineteenth century its presence was almost nationwide, and reports of the damage it was causing were already widespread.¹ In other words, the weed problem is nearly as old as the introduction itself. A grass that seemed like a gift to livestock operations turned into an ecological and agricultural headache within a single generation.

What the Plant Actually Looks Like

If you have walked past a weedy ditch or a neglected field edge in the southern or central United States, you have probably seen johnsongrass without recognizing it. It is a coarse, upright grass that can reach two to three meters tall in a single growing season, with broad leaves and a large, open seed head that can produce thousands of seeds per plant. It looks somewhat like cultivated grain sorghum, which is a close relative, but it grows in thick, tangled stands rather than neat rows.

Below the soil surface is where the real trouble lives. Johnsongrass produces fleshy, segmented rhizomes that spread horizontally, store carbohydrates, and sprout new shoots from buds at each node.² In clay soils, about 80% of rhizomes sit in the top 7.5 centimeters, while in sandy loam they spread a bit deeper, with 80% in the top 12.5 centimeters.³ Cutting a rhizome into pieces does not kill it; in fact, smaller fragments actually germinate at a higher rate, which means tillage that chops up rhizomes can make the problem worse rather than better.³ This is the central frustration for anyone trying to manage it: the plant has a built-in backup system underground that conventional weed control can inadvertently activate.

Chemical Warfare Below the Soil

Johnsongrass does not just outgrow its neighbors. It actively poisons them. The roots exude a compound called sorgoleone, which inhibits photosynthesis in surrounding plants. All sorghums produce some sorgoleone, but johnsongrass produces far more than its cultivated relatives, yielding roughly 14.8 milligrams of root exudate per gram of fresh root weight compared to about 2 milligrams for other sorghum varieties.⁴ Sorgoleone makes up more than 85% of the exudate mixture, and it works by blocking the photosynthetic machinery in the leaves of competing plants that absorb it through their roots.

Research on native tallgrass prairie species confirmed this effect directly. When little bluestem, a native grass, was treated with johnsongrass leachate in both field and greenhouse settings, the treated plants grew significantly less biomass and produced fewer seed heads than control plants that were not exposed.⁵ So even in areas where johnsongrass rhizomes have not yet physically arrived, the chemical front can move ahead, weakening native grasses and softening up the ground for the invasion that follows.

Damage to Crops and Farm Economics

For row-crop farmers, johnsongrass is a constant battle. It competes aggressively for light, water, and nutrients, and because of its height it can shade out shorter crops like soybeans and young corn. Beyond direct competition, the grass physically contaminates harvested grain. In soybean fields, uncontrolled johnsongrass raised the foreign-material content in harvested seed samples from about 0.8% with full control to nearly 6% without any control, and it increased seed moisture enough to trigger price deductions.⁶ Soybean yields climbed roughly 5 to 6% for every 10% improvement in johnsongrass control, and net returns from fields with complete control were nearly double those of fields with no control at all.⁶ Those numbers give you a sense of scale: this is not a minor nuisance weed but a direct hit on farm income.

The trouble extends beyond yield loss in the field. Johnsongrass serves as a refuge for agricultural pests and plant viral diseases, essentially acting as a year-round host that keeps pest populations alive between cropping seasons.⁷ One of the best-documented examples is Maize dwarf mosaic virus, which persists in johnsongrass stands and then spreads to corn fields during the growing season. Studies of the virus across major corn-growing areas in Spain found that johnsongrass acts as a native reservoir for genetically diverse strains of the virus, which then differentiate as they move into corn.⁸ Eliminating the weed from field margins does not just remove a competitor; it reduces disease pressure on the crop.

Why Herbicides Keep Losing Ground

The standard answer to a weed problem is herbicides, and for decades glyphosate and grass-specific herbicides called ACCase inhibitors were the primary tools against johnsongrass. Both are losing effectiveness. In Argentina, where johnsongrass infestations in soybean and corn fields are severe, testing of field populations found that 70% showed resistance to glyphosate.⁹ About 15% survived haloxyfop-methyl (an ACCase-inhibiting herbicide), and roughly a fifth showed multiple resistance, meaning they could tolerate both glyphosate and ACCase inhibitors simultaneously.⁹

The mechanisms behind this resistance are diverse and evolving. In an Arkansas population tested against fluazifop and pinoxaden, resistance factors were 181-fold and 133-fold respectively, meaning you would need over a hundred times the normal dose to achieve the same kill rate. Researchers traced the resistance to specific mutations in one of the plant’s two copies of the ACCase gene, but not all resistant plants carried those mutations, suggesting additional resistance mechanisms are at work that have yet to be fully characterized.¹⁰ Evidence pointed toward metabolic resistance as well, where the plant breaks down the herbicide before it can do damage, on top of the target-site mutations.¹⁰ In practical terms, this means farmers are running out of effective chemical options, and the weed is getting ahead faster than new solutions can be developed.

A Poison in the Pasture

Ironically, the same plant that was originally brought in as livestock feed can kill the animals it is meant to nourish. Johnsongrass contains cyanogenic glycosides, compounds that release hydrocyanic acid (hydrogen cyanide) in the stomachs of cattle, sheep, and other ruminants.¹¹ The concentration varies with the plant’s growth stage, and young, rapidly growing tissue or regrowth after a frost or drought stress tends to have the highest levels. The toxic threshold is generally considered to be about 20 milligrams of HCN per 100 grams of dry matter.¹¹

The practical danger for ranchers is highest in spring when new shoots emerge and after weather events that stress the plants and trigger a flush of new growth. Mature johnsongrass that has been cut and properly dried as hay is generally much safer, because the drying process allows the cyanide gas to dissipate. But animals grazing freely on young, stressed stands can consume a lethal dose before any symptoms become obvious. Cyanide poisoning in cattle progresses rapidly from labored breathing to collapse, and animals can die within minutes if the dose is high enough. This makes johnsongrass one of the more dangerous pasture weeds from a livestock safety standpoint.

Displacement of Native Grasslands

Outside of agriculture, johnsongrass poses a serious threat to native plant communities, particularly the remnant tallgrass prairies of the central United States that are already reduced to a small fraction of their historical range. Research in Oklahoma documented johnsongrass advancing into undisturbed native prairie at an average rate of about 0.45 meters per year via rhizome growth.⁵ That may sound slow, but it is relentless and cumulative. The invaded areas showed distinct plant communities: a native zone, a heavily invaded zone dominated by johnsongrass, and a transitional zone between them where native grass cover was already declining.⁵

Competitive exclusion experiments showed just how lopsided the matchup is. When johnsongrass seedlings were grown alongside native prairie grasses like big bluestem, little bluestem, and switchgrass, the native species were suppressed to a fraction of their normal size. By the end of the study, big bluestem had achieved only about 5% of its normal biomass, and little bluestem and switchgrass were down to roughly 2%.¹² Johnsongrass simply grows faster early in the season, capturing light and space before the native grasses can get established. That developmental head start, combined with the chemical effects described earlier, creates a one-two punch that native species struggle to survive.

Gene Flow into Cultivated Sorghum

One of the more quietly alarming dimensions of the johnsongrass problem is its ability to hybridize with grain sorghum, a major food and feed crop grown worldwide. Because the two are related species, cross-pollination can occur when they grow near each other. Research found that the frequency of hybridization varied significantly depending on the sorghum variety and the environment, but pollen competition from fertile sorghum plants reduced the rate of cross-pollination by up to two orders of magnitude compared to male-sterile sorghum lines.¹³

Why does this matter? If herbicide-resistance genes from cultivated sorghum cross into johnsongrass populations, the weed could acquire resistance to additional herbicide classes through a shortcut. And the flow can go the other way too: weedy traits from johnsongrass, like aggressive rhizome production or seed shattering, could introgress into cultivated sorghum and degrade the crop. Managing this gene flow is a real concern for sorghum breeders, particularly as biotech traits become more common in commercial sorghum varieties. Choosing sorghum parent lines that are fully fertile and highly competitive pollinators is one strategy researchers have identified for reducing the rate of unwanted hybridization.¹³

Johnsongrass and Seasonal Allergies

Most conversations about johnsongrass focus on farms and prairies, but the plant affects human health directly through airborne pollen. Johnsongrass pollen is a recognized respiratory allergen, and in subtropical and warm-temperate regions where the grass is abundant, it contributes to seasonal allergic rhinitis and asthma symptoms. Research into the allergen profile of johnsongrass pollen identified two major allergen components, Sor h 1 and Sor h 13, which triggered immune responses in about 76% and 44% of grass-pollen-allergic patients tested, respectively.¹⁴ The pollen also contains allergen groups that are restricted to subtropical grasses, meaning standard allergy panels focused on temperate grass species like ryegrass may not capture johnsongrass sensitivity.

In a study of patients in a subtropical region, about 16% tested positive only to subtropical grass pollens like johnsongrass and Bahia grass, while just 6% reacted solely to ryegrass.¹⁵ This means people living in the southern United States, parts of South America, Australia, or the Mediterranean who have grass-pollen allergies might be reacting to johnsongrass without knowing it, especially if their allergy testing used only temperate-grass extracts. For allergists working in regions where johnsongrass is common, including subtropical grass pollens in testing panels gives a more complete picture of what is triggering a patient’s symptoms.

Carbon Balance and an Expanding Range

Climate change is likely to make the johnsongrass problem worse. The plant uses the C4 photosynthetic pathway, which is highly efficient in warm, sunny conditions, and it thrives as growing seasons lengthen and temperatures rise. Three years of flux measurements in the Southern Great Plains found that a johnsongrass field acted as a net carbon sink, absorbing carbon from May through September, with a cumulative annual net carbon exchange of about −434 grams of carbon per square meter.¹⁶ That same field was producing hay biomass of around 7.5 tonnes per hectare annually.

Those numbers might sound like a silver lining, and in isolated terms, any plant that sequesters carbon is doing something useful. But from an ecological perspective, a carbon-efficient invasive species is not good news. It means johnsongrass is very good at converting atmospheric carbon into the underground rhizome reserves and aboveground biomass that allow it to dominate landscapes. As its range pushes northward into regions where cold winters previously kept it in check, the areas vulnerable to invasion will grow. Rhizome buds die when exposed to temperatures around −3 to −5 degrees Celsius for more than a few hours, so milder winters open the door for establishment in places that were once too cold.³ Seeds, which tolerate cold better than rhizomes, can arrive years before a population establishes through vegetative spread, seeding future invasions that only take hold once the climate crosses a threshold.

Why Eradication Is Essentially Impossible

Putting all of this together, you start to see why johnsongrass is considered one of the world’s worst weeds rather than merely an annoying one. It reproduces both sexually through seed and asexually through rhizomes. It chemically suppresses competitors. It serves as a disease reservoir. It hybridizes with its crop relative. It is evolving resistance to the herbicides used against it. And it can poison livestock that eat it at the wrong growth stage.

Managing it is possible; eliminating it from a landscape essentially is not. Rhizome fragments as short as a few centimeters can regenerate, and seeds remain viable in the soil for years. Intensive cultivation can reduce rhizome biomass but, as noted earlier, can also fragment rhizomes and spread them. Heat treatment works in principle since rhizome buds die at 50 to 60 degrees Celsius within one to three days, but achieving those temperatures across an entire field is impractical outside of solarization in very hot climates.³ Most farmers rely on integrated approaches: rotating herbicide modes of action, combining tillage timing with herbicide applications, and using competitive cover crops to shade out new seedlings. None of these provide complete control on their own, and the economic math of fighting johnsongrass year after year weighs heavily on farm budgets, particularly in soybean and corn systems where the weed is most competitive.

In natural areas, the management picture is even bleaker. Conservation managers in tallgrass prairie preserves face an opponent that advances slowly but steadily, shrugs off most mechanical control, and chemically degrades the habitat ahead of its physical front. Prescribed burning, a standard tool for prairie management, can reduce johnsongrass aboveground growth but does not reliably kill the rhizomes below the surface. The result is that many remnant prairies are in a slow-motion losing battle against an invasion front that moves less than half a meter a year but never stops.

1
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2
Weeds. Characteristics of Johnsongrass Rhizomes
3
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4
Weed Technology. Mode of Action, Localization of Production, Chemical Nature, and Activity of Sorgoleone: A Potent PSII Inhibitor in Sorghum spp. Root Exudates
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6
Cambridge University Press. The Technical and Economic Effects of Johnsongrass (Sorghum halepense) Control in Soybeans (Glycine max)
7
Biologia. Invasive Johnsongrass, a threat to native grasslands and agriculture
8
Plant Pathology. Maize dwarf mosaic virus diversity in the Johnsongrass native reservoir and in maize: evidence of geographical, host and temporal differentiation
9
Adv Weed Sci. Accessions to glyphosate and ACCase-inhibiting herbicides in Argentina. Sorghum halepense response to herbicides
10
PubMed Central. Target-site mutations Ile1781Leu and Ile2041Asn in the ACCase2 gene confer resistance to fluazifop-p-butyl and pinoxaden herbicides in a johnsongrass accession from Arkansas, USA
11
Agricultural Reviews. Assessment of Hydrocyanic Acid (HCN) Content during Different Stages of Growth in Johnson Grass (Sorghum helepensis)
12
PLOS ONE. Accelerated development in Johnsongrass seedlings (Sorghum halepense) suppresses the growth of native grasses through size-asymmetric competition
13
PubMed Central. Rate of crop‐weed hybridization in Sorghum bicolor × Sorghum halepense is influenced by genetic background, pollen load, and the environment
14
Journal of Allergy and Clinical Immunology. Total transcriptome, proteome, and allergome of Johnson grass pollen, which is important for allergic rhinitis in subtropical regions
15
PubMed Central. Subtropical grass pollen allergens are important for allergic respiratory diseases in subtropical regions
16
Science of The Total Environment. Dynamics of CO2 and H2O fluxes in Johnson grass in the U.S. Southern Great Plains