3. Eleocharis compressa Sull. (flat-stemmed spike rush)

Pl. 74 a–c; Map 276

E. elliptica Kunth var. compressa (Sull.) Drapalik & Mohlenbr.

Plants perennial, with stout rhizomes 2–5 mm in diameter. Aerial stems 5–40 cm long, 1–2 mm in diameter, strongly flattened and finely ridged, often appearing somewhat twisted, lacking cross-lines. Basal sheaths tinged reddish purple to straw-colored, the tip firm and somewhat thickened, truncate, 1 side of the margin sometimes with a short, raised tooth. Spikelets 5–12 mm long, oblong-elliptic to ovate in outline, pointed or rounded at the tip, with 1 sterile, basal scale. Scales 2–3(–3.5) mm long, ovate, pointed and usually tapered at the tip, many of the scales usually becoming bifid at maturity, brown to purplish brown with white-membranous tips and margins. Perianth bristles usually lacking or rarely 1–5, shorter than the fruit, and often becoming detached during fruit development, usually finely retrorsely barbed. Stigmas 3. Fruits 1.0–1.5 mm long, the main body obovate in outline, unequally 3-angled in cross-section, the surface finely roughened or honeycombed with a fine network of ridges and pits, often appearing somewhat wrinkled, golden yellow to brown. Tubercles depressed-conical. 2n=18, 20, 24, 36. May–July.

Relatively common in the Ozark and Ozark Border Divisions, less common in the Unglaciated Plains, Mississippi Lowlands, and north of the Missouri River (eastern U.S. and adjacent Canada west to Texas). Most commonly on limestone and dolomite glades, less commonly in moist depressions of upland prairies and openings of mesic upland forests; also in roadside ditches.

Steyermark (1963) noted that some populations in northwestern Missouri possess more slender aerial stems and are in some ways intermediate in morphology between E. compressa and E. verrucosa. He hypothesized that these may represent hybrids between the two species, but this has not been investigated further. Aside from differences in shape and width of the stems, E. compressa also tends to have a slightly more rounded fruit than does E. verrucosa and the spikelet scales have a greater tendency to split at the tip at maturity.

Catling (1994) characterized a hybrid between E. compressa and E. erythropoda from Ontario, which eventually may be found in Missouri. This hybrid is especially interesting because the parents are trigonous-fruited and lenticular-fruited species that are usually classified in different sections of the genus. It is variable for most of the characters separating the parental taxa but tends to resemble E. erythropoda more closely in overall aspect.

Harms (1972) cited a specimen of the closely related E. elliptica (sensu stricto) from Missouri as a voucher (without further data) for a chromosome count of 2n=38 for that taxon. Subsequent attempts to locate this specimen have failed and it is presumed to have been cited incorrectly in the publication. Eleocharis elliptica occurs in the northeastern United States and adjacent Canada west to Illinois and Minnesota, and it differs from E. compressa in its 6–8-angled stems and often finely warty fruits.

Eleocharis compressa is a characteristic species of calcareous glades in the Ozarks and tends to grow in drier habitats than any of the other Missouri spike rushes. Apparently, it is unpalatable to cattle and is sometimes referred to as “kill-cow” in Canada (Catling, 1994).