Why Is It a Noxious Weed?
It is considered a weed in all annual agricultural crops, orchards, vineyards, ditches, roadsides and fence rows. When it is under stress, it produces hydrocyanic acid which is toxic to grazing livestock.
How would I identify it?
Johnsongrass is a tall, upright, perennial grass, reaching 3 to 10 feet tall in height with an extensive system of rhizomes.
Flowers clusters (spikelets) in a branching pyramidal cluster (panicle) up to 16 inches. Spikelets grow in pairs at the lower end of this flowering stalk while spikelets are in threes at the upper end. Male and female spikelets appear on the same plant.
The leaves are alternate, from 12 to 30 inches long and ½ to 1 inch wide. It has a noticeable white mid-vein.
Stems terminate in a reddish inflorescence.
Where does it grow?
Johnsongrass is adapted to a variety of soil types, including fertile moist locations such as riparian areas, and also disturbed areas such as old fields and roadsides. Please click here to see a county level distribution map of Johnsongrass in Washington.
How Does it Reproduce?
Johnsongrass reproduces by seeds and by rhizomes. Each rhizome segment, or node, can produce shoots and roots. Seeds remain dormant in the soil for several years.
How Do I Control It?
General Control Strategy
Since Johnsongrass reproduces by seed and rhizomes, if cultural or mechanical methods are used as control methods, care should be taken to prevent re-sprout of rhizomes. Herbicides will control the upper part of the plant, but dormant buds on the rhizome may still sprout.
Repeated tillage is effective. If tillage is not repeated, preferably monthly, rhizomes can re-sprout and infestation can spread.
Cultural methods on established sites are only effective if rhizome development is controlled.
For More Information
See our Written Findings for more information about Johnsongrass (Sorghum halepense).