Virginia Tech Weed Identification Guide

Johnsongrass: Sorghum halepense

Weed Description:   A perennial from rhizomes that may reach 6 1/2 feet in height.  Johnsongrass is capable of rapidly colonizing a variety of different environments due to the large amounts of seed and rhizomes produced by this plant.  Originally introduced as a forage crop, this weed is now one of the most common and troublesome weeds of most agronomic and horticultural crops, as well as roadsides, pastures, and hay fields.  Found in the United States from Massachusetts to Iowa, south to Florida and Texas, and also in southern California.
Seedling:  Leaves are rolled in the shoot, auricles are absent, and the ligule is membranous and may be toothed at the top.  Leaf blades are without hairs (glabrous) on both surfaces and develop a prominent white midvein with maturity. 
Leaves:   Rolled in the shoot, without auricles, 6 to 20 inches long by 10 to 30 mm wide, with a prominent white midvein.  Leaf blades are usually without hairs (glabrous) on both surfaces, however some hairs may be present at the base of the leaf blade.  The ligules are 3 to 4 mm long, membranous, and often toothed at the top.  With maturity, some ligules may develop a fringe of hairs in the upper portion of the ligule, and remain membranous towards the base.
Stems:   Round to somewhat flattened, usually without hairs but sometimes hairs may be present along the margins.  Sheaths may be green to maroon, especially near the base of the plant. 
Roots:  A fibrous root system and thick rhizomes.
Flowers:  Seedhead a large, open panicle, often with a purplish tint.  Seed are oval, 3 to 5 mm in length, and dark red to black at maturity.
Identifying Characteristics:   Johnsongrass is often mistaken with Barnyardgrass (Echinochloa crus-galli) and/or Fall Panicum (Panicum dichotomiflorum) prior to seedhead formation.  However, johnsongrass has a membranous ligule unlike that of fall panicum or barnyardgrass and johnsongrass seedlings do not have hairs on the lower leaf surface like those of fall panicum.  Johnsongrass seedlings and mature plants also resemble shattercane (Sorghum bicolor), but shattercane does not have rhizomes like johnsongrass.