Eleocharis obtusa (Willd.) Schult.

Extremely uncommon plants of Eleocharis obtusa without perianth bristles may be called E. obtusa var. peasei (type from New Hampshire). Robust plants with distinct caudices, floral scales 2.5 mm, and achenes 1.2–1.3 mm (Eleocharis obtusa var. gigantea Fernald) are rare (specimens seen from the Washington-British Columbia border [type], Arkansas, and the Hawaiian Islands). Dwarf plants (E. obtusa var. jejuna Fernald, type from Maine), with unusually small achenes and floral scales, and tubercles often less than 0.5 mm wide, are occasional in the East and are easily confused with E. ovata and E. aestuum. A few specimens are intermediate with E. engelmannii. Eleocharis obtusa is sometimes treated as conspecific with E. ovata, which consistently differs in its mostly 2-fid styles, mostly two stamens, and especially its narrower tubercles (B. M. H. Larson and P. M. Catling 1996).

Eleocharis macounii Fernald has been treated as a synonym of E. obutsa (H. K. Svenson 1957) but is more probably a hybrid between E. intermedia and E. obtusa (P. M. Catling and S. G. Hay 1993; see 34. E. intermedia).

Culms 3–50(–90) cm × 0.2–2 mm. Leaves: apex of distal leaf sheath obtuse to acute, tooth to 0.3 mm. Spikelets broadly ovoid (to ellipsoid or lanceoloid), apex rounded (to acute), (2–)5–13 × (2–)3–4 mm; floral scales 15–150+, 8–20 per mm of rachilla, orange-brown (to stramineous), elliptic, 1.5–2.5 × 1–1.5 mm, midribs seldom keeled, apex broadly rounded. Flowers: perianth bristles (5–)6–7, rarely 0, brown, stout, slightly to usually greatly exceeding tubercle; stamens usually 3; anthers brown to yellow, 0.3–0.6 mm; styles usually 3-fid and 2-fid in same spikelet. Achenes 0.9–1.2(–1.3) × 0.7–0.9 mm. Tubercles deltoid 0.35–0.5 × (0.4–)0.5–0.8 mm, 1/3–2/3 as high as wide, 1/3–1/2 as high and 2/3–9/10 as wide as achene. 2n = 10.

B.C., N.S., Ont., P.E.I., Que.; Ala., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., Ohio, Okla., Oreg., Pa., R.I., S.C., Tenn., Tex., Vt., Va., Wash., W.Va., Wis., Wyo.; Pacific Islands (Hawaii).

Fruiting summer–fall.

Fresh shores, marshes, disturbed places; 10–1600m.

Scirpus obtusus Willdenow, Enum. Pl. 1: 76. 1809; Eleocharis obtusa var. ellipsoidales Fernald; E. obtusa var. gigantea Fernald; E. obtusa var. jejuna Fernald; E. obtusa var. peasei Svenson

Eleocharis obtusa (Wilid.) Schultes in R. & S Syst. Veg. Mant. 2 : 89. 1824.
Scirpus obtusus Willd. Enum. 76. 1809. (Pennsylvania.)
Scirpus elegantuhis Steud. Syn. Cvp. 317. 1855. (New Orleans.)
Eleocharis diandra C. Wright, Bull. Torrey Club 10: 101. 1883. (Connecticut.)
Eleocharis obtusa var. jejuna Fernald, Proc. Am. Acad. 34: 492. 1899. (Maine.)
Eleocharis ovata var. gigantea Clarke; Britten, Jour. N. Y. Micr. Soc. 5: 103. 1889. Nomen
nudum. (Oregon.) Eleocharis obtusa var. gigantea Fernald, Proc. Am. Acad. 34: 493. 1899. Eleocharis Macounii Fernald, Proc. Am. Acad. 34 : 497. 1899. (Quebec.) Heleocharis ovata var. obtusa Kiikenth. ; Skottsb. Acta Hort. Gothob. 2: 212. 1926. Eleocharis obtusa var. Peasei Svenson, Rhodora 31 : 217. 1929. (New Hampshire.) Eleocharis obtusa var. ellipsoidalis Fernald; Svenson, Rhodora 31: 218. 1929. (Massachusetts.)
Annual (rarely perennial, as in var. ellipsoidalis), usually erect; culms numerous, 0.3-5 dm. long, yellowish-green, capillary to l.S mm. in diameter ; sheaths purplish at base, at the apex firm and somewhat oblique ; spikelets globose-ovoid to ovoid-cylindric, obtuse, manyflowered, 2-13 mm. long, closely or loosely flowered ; scales ovate-oblong to suborbicular, brown, with a narrow scarious margin and usually a greenish midrib; style bifid or trifid; achene 1-1.5 mm. long, turbinate-obovoid, narrowed at the base, pale to deep brown, smooth and shining; style-base strongly flattened, deltoid, acute, nearly as wide as the achene; bristles 6 or 7, dark brown, coarse, exceeding the achene, retrorsely toothed.
Type locality : Pennsylvania.
Distribution: Nova Scotia, west to Minnesota, south to northern Florida and Texas : Colorado and New Mexico ; British Columbia and Idaho to northern California ; Hawaiian Islands.

Eleocharis obtusa is a species of spikesedge known by the common name blunt spikerush. This plant is widely distributed across Canada and the United States, where it grows in wet areas such as riverbanks and moist forest floors. It is also a weed of rice paddies, especially when the rice plants are young sprouts. This is an annual spikesedge approaching half a meter in maximum height. It has many green cylindrical stems which are erect but weak and spongy. There may be thin, grasslike leaves toward the base of the plant, which are generally straw-colored. Atop each stem is a rounded or oblong spikelet containing at least ten flowers, each covered by an oval-shaped brown bract.

Eleocharis obtusa là loài thực vật có hoa trong họ Cói. Loài này được (Willd.) Schult. mô tả khoa học đầu tiên năm 1824.