Sorghum halepense aka Johnsongrass

Sorghum halepense, or Johnsongrass, is a perennial in the grass family. Its native range runs from Macaronesia to Central Asia and Indo-China. It now grows widely in warm and temperate regions. People brought it from Turkey to South Carolina around 1830. They later grew it along the Alabama River for forage. It has since become a serious weed in many places.

This coarse grass grows upright, with fast summer growth. Plants reach about 2.5–7 feet [0.76–2.13 metres] tall. Some accounts report heights near 9.5 feet [2.9 metres]. Thick rhizomes spread below ground and build dense stands. These stands crowd out native plants and hinder young trees. The smooth, round stems are pale green. Most leaves grow on the lower half of each stem. Their blades reach 2 feet [0.61 metres] long. The upper surface is darker than the underside. Large leaves have a pale midrib and smooth edges. Fine white hairs mark the join between blade and sheath.

The stems carry loose panicles of small brown to copper flowers. These heads reach 1.5 feet [0.46 metres] long. Their lower branches spread wider than the upper ones. Flowers appear in summer and fall, and wind carries pollen. Long days can prevent flowering when daylight exceeds about 13 hours. Seeds ripen in September and October. Each plant can produce hundreds of seeds. Both seeds and rhizomes can travel in contaminated topsoil.

Johnsongrass grows best in full sun and fertile, moist loam. It also grows in clay, gravelly ground, and drier sites. It tolerates drought and soils from acid to alkaline. Sources place it in USDA zones 6–10. It grows in fields, open woods, ditches, and wetlands. Its forage use does not remove its risks in the landscape. It needs much upkeep, and control can take several years. Dense growth can add fuel and help fires spread. NC State reports no known insect or disease problems.

The seeds have food uses, either whole or ground into flour. NC State limits its food guidance to small amounts. Sources also record seed uses in medicine and potential biomass use. Leaves can cause low toxicity in people who eat them. The evidence reports no known cases of human poisoning. Wilted foliage can contain enough hydrogen cyanide to harm livestock. Frost, heat, and dry weather can cause this hazard. Young plants pose a greater risk than mature ones. The pollen can also trigger hay fever.