Description: This introduced perennial grass is about 3-7' tall and more or less erect. The culms are light green, terete, and glabrous. The alternate leaves occur primarily along the lower half of each plant. The green to dark green leaf blades are up to 2' long and 1" across. On the upper surface, each blade is glabrous and it usually has a prominent central vein that is pale-colored. In this respect, the leaf blades of Johnson Grass resemble those of Zea mays (Corn), except the latter are larger in size. The leaf sheaths are light green to green and often glaucous. Each ligule at the junction of the leaf blade and sheath has a strip of white hairs.
The culm terminates in a panicle of
floral spikelets. Each panicle is up to 1½' long and less than half as
much across; it is broader toward the bottom than the top and its branchlets
are ascending to spreading. These branchlets are light green, slender,
glabrous, and slightly wiry; each branchlet is naked toward its base. The
spikelets usually occur in pairs or triples: a single sessile spikelet and 1-2
spikelets with pedicels. The sessile spikelets are up to ¼" (6 mm.) long
and perfect, while the pedicellate spikelets are about the same length and
either staminate or sterile. Both kinds of spikelets have a pair of glumes that
enclose the lemmas; these glumes are the same length as the spikelets. While
the florets are in bloom, the glumes of the sessile spikelets are lanceolate,
shiny, softly hairy, and light green with reddish or yellowish tints. At the
same time, the glumes of pedicellate spikelets are narrowly lanceolate, shiny,
softly hairy, and reddish; their pedicels are up to ¼" long and softly
hairy. The fertile lemmas of the sessile spikelets can produce awns about
½" long, but these are early deciduous. Each floret has an ovary, a pair
of plumose stigmas that are pale purple to white, and 3 anthers that are yellow
to light brown. The blooming period occurs during mid-summer to fall. Each
fertilized sessile spikelet produces a single grain. The root system is fibrous
and it has spreading rhizomes. This grass often forms vegetative
colonies.
Cultivation: Typical
growing conditions are full to partial sun and slightly dry to moist fertile
soil. This robust grass adapts to soil containing loam, clay loam, or gravel.
It can spread aggressively.
Range
& Habitat: Johnson Grass is occasional to locally common in the
southern half of Illinois, while in the northern half of the state it is
uncommon or absent (see
Distribution
Map). This species was introduced from the Middle East as a possible
pasture grass in the SE states, and it has since spread across a large area of
the United States. In some states, Johnson Grass is considered a noxious weed.
Habitats include thickets, moist meadows, fields, areas along railroads and
roadsides, and miscellaneous waste areas. Sometimes this grass invades gardens
and prairie restorations.
Faunal
Associations: The caterpillars of Lerema accius (Clouded
Skipper), Lerodea eufala (Eufala Skipper), and Nastra lherminier
(Swarthy Skipper) feed on the foliage. Canada Geese have been observed eating
the seeds. Foliage that becomes wilted from frost or hot dry weather can
contain sufficient amounts of prussic acid to kill cattle and horses if it is
eaten in quantity. Also, the foliage can cause 'bloat' in such herbivores from
the accumulation of excessive nitrates; otherwise, it is
edible.
Photographic Location: A
gravelly area along a railroad in Savoy, Illinois.
Comments: In Illinois, Johnson Grass can become
quite large in size and it has a tendency to stand out from the surrounding
vegetation. In spite of its weedy character, this grass is rather attractive
while it is in bloom. The foliage of Johnson Grass has a similar appearance to
the foliage of Tripsacum dactyloides (Gama Grass). For example, their
leaf blades are quite long and broad, shiny green, and pale-striped from their
prominent central veins. However, the inflorescence of Johnson Grass is quite
different from the inflorescence of Gama Grass. The latter species has a
cluster of stout floral spikes, while Johnson Grass has a spreading panicle of
florets with slender branchlets. Another species, Sorghum bicolor
(Sorghum), is occasionally cultivated as an agricultural crop. Sorghum is an
annual and its inflorescence is more dense and upright in appearance than the
inflorescence of Johnson Grass. Another annual species, Sorghum almum
(Columbus Grass), has been introduced into the United States from Argentina.
This species resembles Johnson Grass, but it is even larger in size with leaf
blades typically 1-2" across. Columbus Grass is restricted to southern
Illinois, where it is rare.