Johnson Grass

Johnson Grass is native to the Mediterranean region and western Asia, but has since naturalized across every continent except Antarctica.

• Native range spans southern Europe, North Africa, and the Middle East
• Introduced to the Americas in the early 19th century, likely as a contaminant in seed stock or as a prospective forage crop
• First documented in the United States around 1830 in South Carolina; by the 1840s it had spread throughout the southern states
• The common name "Johnson Grass" is believed to derive from Colonel William Johnson, who cultivated it as a forage crop in Alabama around 1840
• Now established in tropical, subtropical, and warm-temperate regions worldwide, including Australia, South America, southern Africa, and southern Asia

Johnson Grass is a tall, robust perennial grass that can form dense, spreading colonies.

Stems & Leaves:
• Culms (stems) are erect, stout, and typically 0.5–2.5 meters tall, occasionally reaching 3 meters
• Leaf blades are flat, linear, 20–50 cm long and 1–2.5 cm wide, with a prominent white midrib
• Ligule is membranous, fringed with hairs (~2–4 mm long)
• Leaves are glabrous (smooth) with rough margins

Rhizomes:
• Extensive, creeping, fleshy rhizomes are a defining feature
• Rhizomes can penetrate soil to depths of over 1 meter and spread laterally several meters per growing season
• Rhizome fragments as small as 2–3 cm can regenerate into new plants, making mechanical control extremely difficult

Inflorescence:
• Open, spreading panicle, 15–50 cm long, with ascending to spreading branches
• Spikelets occur in pairs (one sessile and fertile, one pedicellate and sterile or male)
• Fertile spikelets are lanceolate, ~4.5–5.5 mm long, with awns that are twisted and bent, 10–15 mm long
• Color ranges from purplish to straw-yellow at maturity

Seeds:
• Small, oval caryopses (~3 mm long)
• A single plant can produce 28,000 to 80,000+ seeds per year
• Seeds remain viable in soil for up to 7 years

Johnson Grass thrives in disturbed habitats and is particularly problematic in agricultural settings.

Habitat:
• Roadsides, field margins, ditches, floodplains, and disturbed grasslands
• Prefers fertile, well-drained soils but tolerates a wide range of soil types, including heavy clays and sandy loams
• Grows best in full sun; moderately shade-tolerant
• Tolerates both periodic flooding and extended drought

Climate:
• Optimal growth occurs at temperatures between 25–35°C
• Frost-sensitive; top growth is killed by hard freezes, but rhizomes survive underground and regenerate in spring
• Requires a minimum of ~400 mm annual rainfall but performs best with 600–1,000 mm

Reproduction & Spread:
• Reproduces both sexually (by seed) and vegetatively (by rhizomes)
• Seeds are dispersed by wind, water, contaminated farm machinery, livestock, and contaminated grain or hay
• Rhizome expansion allows a single clone to cover several hundred square meters within a few growing seasons
• Known to hybridize with cultivated sorghum (Sorghum bicolor), potentially transferring herbicide resistance and weedy traits to crop populations

Ecological Interactions:
• Serves as a host for several agricultural pests and pathogens, including the sorghum midge (Stenodiplosis sorghicola), corn earworm (Helicoverpa zea), and various cereal viruses
• Can harbor the bacterium Clavibacter michiganensis subsp. nebraskensis, causal agent of Goss's wilt in maize
• Provides cover and food for some bird species and small mammals

Johnson Grass poses significant toxicity risks, particularly to livestock.

Hydrogen Cyanide (Prussic Acid):
• Contains cyanogenic glycosides (primarily dhurrin), which release hydrogen cyanide (HCN) when plant tissues are damaged
• Young regrowth, drought-stressed plants, and frost-damaged tissues have the highest HCN concentrations
• Poisoning can occur rapidly in ruminants; lethal doses can be reached within hours of grazing
• HCN levels can exceed 200 ppm in young shoots — concentrations above 200 ppm are considered dangerous for cattle

Nitrate Accumulation:
• Under certain conditions (heavy nitrogen fertilization, drought, cloudy weather), Johnson Grass can accumulate toxic levels of nitrates
• Nitrate poisoning in livestock causes methemoglobinemia, leading to oxygen deprivation and potentially death

Other Toxicity:
• Ingestion of large quantities can cause neurological symptoms in horses, including ataxia and urinary incontinence, possibly due to chronic cyanide exposure
• Allergic contact dermatitis has been reported in humans handling the plant

Johnson Grass is not intentionally planted as a crop in most regions due to its status as a noxious weed. However, it has historically been used as a forage grass and erosion control plant in some areas.

Growth Conditions:
• Light: Full sun preferred; tolerates partial shade
• Soil: Adaptable to a wide range of soil types; performs best in fertile, moist, well-drained loams with pH 5.5–7.5
• Temperature: Optimal growth at 25–35°C; killed by hard frost but regenerates from rhizomes
• Water: Drought-tolerant once established; grows vigorously with adequate moisture

Control & Management:
• Mechanical control is difficult due to extensive rhizome systems; plowing can spread rhizome fragments
• Repeated mowing before seed set can reduce vigor over time
• Herbicide options include glyphosate, fluazifop, and quizalofop, though herbicide-resistant biotypes have been documented in multiple countries
• Integrated management combining tillage, herbicides, and competitive crop rotations is most effective
• Biological control agents have been investigated but none are widely deployed

Despite its weedy nature, Johnson Grass has had several historical and limited contemporary uses.

Forage:
• Historically cultivated as a forage and hay crop in the southern United States and parts of Australia
• Nutritious when harvested at the correct growth stage and free of high HCN levels
• Palatable to cattle and horses when properly managed

Erosion Control:
• Extensive rhizome network makes it effective for stabilizing soil on embankments and disturbed sites
• Used in some regions for revegetation of degraded land

Bioenergy:
• High biomass production has led to research interest as a potential bioenergy crop
• Yields of 15–20 tonnes of dry matter per hectare have been recorded under favorable conditions

Phytoremediation:
• Studied for its ability to accumulate heavy metals from contaminated soils

Hybridization:
• Used in sorghum breeding programs as a source of genes for drought tolerance, pest resistance, and perennial growth habit
• Perennial sorghum research aims to transfer rhizomatous growth from S. halepense to cultivated S. bicolor to develop perennial grain sorghum varieties