Description: This perennial spike rush forms an unbranched culm about ½–1½' tall. The stiff culm is medium green, hairless, and more or less flattened, spanning about 1 mm. in diameter. A basal sheath surrounds the base of the culm; this sheath becomes light brown to reddish brown with age and it is truncate at the apex. There is no leaf blade. The culm terminates in a single spikelet. This spikelet is lanceoloid to narrowly ovoid, medium to dark brown, and about 4-8 mm. long; it consists of a dense head of minute florets and their scales. The florets are perfect with 3 stamens and 3 styles. The floral scales are 2-3 mm. long and ovate in shape; their margins are membranous. The blooming period occurs during the summer; the florets are wind-pollinated. Fertile florets are replaced by tiny achenes, which can float on water or blow about in the wind. These achenes are 1 mm. in length (or a little less), light brown to dark brown, and obovoid in shape; each achene has a tubercle at its apex and 0-5 bristles that originate from its base (the bristles are usually absent). The tubercle has a flattened conical shape with a tiny point in its middle (apiculate); other tubercular shapes have been reported as well. The root system is fibrous and strongly rhizomatous. This spike rush often forms dense mats of culms from the rhizomes.
Cultivation: The preference is full sun and wet
conditions; shallow standing water is tolerated. The soil can be muddy, sandy,
peaty, or gravelly.
Range &
Habitat: Flat-Stemmed Spike Rush is occasional in central and
northern Illinois, while in the southern section of the state it is uncommon or
absent. Habitats include damp depressions in rocky woodlands and limestone
glades, prairie swales, wet dolomite prairies, sedge meadows, borders of ponds
and streams, and roadside ditches. This species is associated with less
degraded wetlands. It reduces shoreline erosion.
Faunal
Associations: The caterpillars of Oarisma poweshiek
(Poweshiek Skipperling) feed on the culms of Eleocharis spp. (spike
rushes). This oligophagous species is a rare skipper. The caterpillars of
Cisseps fulvicollis (Yellow-Collared Scape Moth) also feed on spike
rush, although this species is more polyphagous and common. Several leafhoppers
suck juices from the culms of spike rush: Deltocephalus gnarus,
Dorydiella kansana, Limotettix bisoni, Limotettix elegans,
Limotettix nigrax, Limotettix urnura, Limotettix
truncatus, and Macrosteles potoria. Semi-aquatic larvae of several
leaf beetles (Plateumaris spp.) derive food and oxygen from the culms.
These include: Plateumaris aurifera, Plateumaris dubia,
Plateumaris flavipes, Plateumaris fulvipes, Plateumaris
germari, Plateumaris nitida, Plateumaris pusilla, and
Plateumaris robusta. Among vertebrate animals, the seedheads of spike
rush are a source of food to ducks, geese, and rails during the summer (see
Bird Table). Mallards and
Canada Geese are typical examples of such birds in Illinois. These birds
probably help to spread spike rush to new wetlands because the tiny seeds can
cling to their damp feathers and muddy feet. The dense vegetation of spike rush
provides cover for semi-aquatic insects, frogs, and other small
animals.
Photographic Location: A
roadside ditch along the Red Bison Railroad Prairie in Savoy, Illinois. The
spikelets of the photographed plants are beginning to disarticulate their
floral scales and achenes.
Comments:
This medium-sized spike rush is fairly easy to identify because of its flat
culms and the appearance of its achenes (which usually lack bristles). The
small seedheads are often a darker shade of brown than those of other
Eleocharis spp. (spike rushes), and they develop a chaffy appearance
because of the transparent margins of the scales. Because there is some
variability across local populations, some specimens may be more difficult to
identify; hybridization across different species of spike rush may be
responsible for some of these anomalies. Flat-Stemmed Spike Rush is regarded as
a variety of Slender Spike Rush by Drepalik & Mohlenbrock (1960), or
Eleocharis elliptica compressa. Under this taxonomic treatment, the
typical variety has less flattened stems than var. compressa, otherwise
they are very similar to each other.