Johnsongrass
(Sorghum halepense)
Erect, perennial, rhizomatous, warm-season grass.
Ranked among the top ten world's worst weeds
Distinguished from other grasses by:
*Ribbed leaf sheath
*Leaves have conspicuous midrib
*Large, purplish, panicled seed head
*Extensive large, fleshy, purple rhizomes
Habitat:
*Fertile, porous soils, pH 5.0-7.5
*Disturbed, flooded lands
*Ditches and waterways
*Cultivated fields
Flowers:
*Monoecious
*Flowering occurs approximately 8 weeks after emergence in spring
*Exact time of flowering depends on temperature, plant vigor and photoperiod (8-16 hours)
Rhizomes:
*Primary spring growth is from apical and axillary nodes
*Secondary growth is annual above and below ground structures
*Rhizome spurs and tillers initiate growth 30 days after onset of primary growth, usually at six leaf
stage
*Majority of rhizome growth occurs after flowering
*Minimum temperature of rhizome growth is 59 degrees
*Rhizome depth depends on soil type, shallow in clay-deeper in loam, average is 8
Seeds:
*Dispersal mechanisms: contaminated feed and forage, livestock, machinery, water and wind.
Growth and reproduction:
*Plants can regenerate from small rhizome fragments (2")
*Most seeds germinate during second year
*Carbohydrate reserves are lowest in early spring and early fall, and are at the absolute lowest
10-30 days after flowering
Management:
*Constant cultivation to chop, desiccate and/or freeze rhizomes
*Frequent mowing to deplete carbohydrate reserves
*Herbicides: glyphosate, imazapic
Threats:
*Alleopathic
*Rapid growth and height shades smaller plants
*Aggressively competes for light, nutrients and water
*Stress can produce hydrocyanic acid, cured hay ok
*Johnsongrass serves as host for crop diseases
*Highly allergenic due to the amount of pollen produced