HPIPM:Johnsongrass

Taxonomy

Domain	Eukarya
Kingdom	Plantae
Phylum	Magnoliophyta
Class	Magnoliopsida
Superorder	Lilianae
Order	Poales
Family	Poaceae
Subfamily	Panicoideae
Tribe	Andropogoneae
Genus	Sorghum

Scientific Name

Sorghum halepense

Scientific Name Synonyms

Sorghum miliaceum

Holcus halepensis

Common Name

johnsongrass

Johnsongrass (Sorghum halepense)

Author: Tim Prather, University of Idaho, Department of Plant, Soil, and Environmental Sciences

Adapted from: Idaho Weed Resource webpage on johnsongrass.

Identification and Life Cycle

Johnsongrass (Sorghum halpense) is an introduced perennial weed in the grass family (Poaceae). Johnsongrass grows from 3 to 10 feet tall and the erect stems are generally solid (pithy) with prominent swollen nodes. Johnsongrass produces freely branching fibrous roots and long, scaly rhizomes. The characteristic rhizomes are white with purple or red areas, 3/4 inch in diameter and up to several feet in length. Leaves are 12 to 30 inches long with flat blades 1/2 to 1 inch wide. The short membranous appendage where the leaf blade meets the sheath (ligule) is fringed with fine hairs. The seed head is a foot or more long, with whorled spreading branches. The flowers (spikelets) occur in threes at tips of small branches (one stalkless seed-producing spikelet and two nonseed-producing spikelets on short stalks). Below the tips, the spikelets occur in pairs of one sessile (fertile) and one stalked (infertile) spikelet. Fertile spikelets have bent bristles that fall off. Some have no bristles. Seeds are 1/8 to 3/16 inch long, purplish or a glossy red mahogany.

Habitats

Johnsongrass grows in cultivated fields, pastures, banks of ditches, irrigated canals, moist sites, roadsides, and waste areas.

Impacts

Although johnsongrass is a useful forage crop in pastures, its undesirable properties have earned it the ranking of one of the 10 worst weeds in the world. Highly competitive, it severely reduces yields of many crops. It is an alternate host for the sorghum midge, corn stunt disease and the sugar cane mosaic virus, which infects sorghum and corn. Its pollen contaminates sorghum grown for seed. Because its height restricts visibility, it is a safety hazard on roadsides. When stressed by frost or drought, johnsongrass produces hydrocyanic acid, which is toxic to livestock.

Biology and Ecology

Johnsongrass reproduces by seeds, creeping rhizomes and rooting of old aboveground stems when they are plowed under into moist soil. Freezing soil temperatures kill rhizomes. In cold northern climates, johnsongrass may behave as an annual, producing seed the first year. Johnsongrass flowers from late July to frost. An individual plant may produce 28,000 seeds. The hard seeds remain dormant in the soil and germinate over a period of years. Johnsongrass seedlings usually begin to develop rhizomes within 3 to 4 weeks after emergence. Individual plants may produce more than 500 rhizome buds in a season and 200 to 300 feet of rhizomes in one month after flowering. Rhizomes produced early in the growing season may die in the fall or winter. Unless killed by cold, rhizomes produced late in the growing season usually survive the winter to produce plants the following spring. A rhizome fragment with a single bud can produce a new plant.

Management Approaches

Chemical and mechanical controls are available for johnsongrass.

Biological Control

There are currently no biological controls for johnsongrass.

Mechanical and Cultural Control

Disking to chop rhizomes into small pieces, followed by use of a springtooth harrow to bring rhizomes to the surface can be effective if fields are cultivated every 4 to 5 weeks.

Chemical Control

Consult herbicide labels for additional rate, application, and safety information. Additional herbicide information can be found at http://www.greenbook.net.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.

References

For more information and images please visit IPM Bugwood. http://www.ipmimages.org/search/action.cfm?q=johnsongrass