Johnsongrass (also called grass sorghum, egyptian millet) is a perennial species over most of its range. Leaves are grasslike, up to 1" wide, with a prominent whitish midvein.
The ligule is short and membranous with a hairy fringe; auricles are lacking. Stems can grow up to 8 feet in height, but our annual specimens will be closer to 3 or 4' tall. Large, open panicles are up to 1' long and emerge in midsummer. Spikelets are reddish in color and most are tipped by bent awns. Scaly, finger-thick rhizomes are produced from the crown.
Johnsongrass is adapted to a large range of conditions. An ideal environment for Johnson grass is the subtropics: warm and humid with summer rainfall. Most of the ecotypes are frost sensitive; the top dies back during winter freezes.
Hundreds of seeds are produced on each panicle throughout the summer flowering period . Seedlings emerge later and grow slower than rhizome sprouts, but the pattern of development is similar between the two structures. Seeds that land in a water source may be carried, possibly quite far, to new sites. Other modes of dispersal include wind, livestock and contaminated machinery, grain or hay. Seeds pass unharmed through the digestive systems of birds and cattle. Most seeds do not germinate the year they are produced, but germinate readily the
Complete eradication of Johnson grass is extremely difficult. The immense number of seeds produced on each panicle and which remain dormant until favorable conditions exist provide a countless number of potential plants. Rhizomes which over-winter deep in the soil make the subterranean portion of the plant difficult to control. The majority of the buds on the rhizomes remain dormant and thus herbicide or cultivation techniques will not harm these inactive regions. However, fragmenting the rhizomes will relieve axillary buds from apical dominance, allowing for their growth and thus increasing their susceptibility to control techniques. In addition, seed viability is drastically reduced from 50% to 2% after 5 - 6 years in the soil. The seed-supply in the soil can be removed if seed development is prevented. The most likely mode of preventing the spread of Johnson grass is by maintaining undisturbed land next to invaded areas.
Mowing the plants on a regular basis from spring till winter result in a severe decrease in growth.