12. Cyperus rotundus Linnaeus, Sp. Pl. 1: 45. 1753; Boiss., Fl. Or. 5: 376. 1882; C. B. Clarke in Hook. f., Fl. Brit. Ind. 6. 614. 1893; R.R.Stewart, l.c. 91; Kukkonen in Rech.f., l.c. 99.
Chlorocyperus rotundus (L.) Palla, Allg. Bot. Zeitschr. 6: 61. 1900; Schoenus tuberosus Burm. f., Fl. Ind.: 19. 1768; Holm & al., World's Worst weeds: fig. 1. 1977; Rateeb & Sherif in Fl. Libya 120: fig. 10. 1985.
Perennial, 20-60 cm. Rhizome dark brown, woody; stolons numerous, long-creeping, strong, 1-1.5 mm diam., producing tubers, c. 20 x 9 mm, dark brown. Stem 1-2.5 mm diam., trigonous, green, smooth. Leaves shorter than stem; sheaths to 10 cm, wide, acute, grey-brown or brown, sometimes with reddish tint, mouth margin lingulate; blades to 30 cm, 1-4.5 mm wide, grey-green or green, flat or folded, slightly keeled, margins smooth or towards the apex barbed, apex long-attenuate, acute, trigonous, scabrous. Inflorescence an anthelodium, 40-100 mm; 2-3 lowermost bracts foliose, longer than inflorescence; primary branches 5-8, to more than 50 mm long; secondary anthelodia 25-50 mm; clusters of spikes 15-40 mm, of (2-)5-7(-9) spikes on slightly elongated axis, sometimes nearly digitately; spikes 10-40 x 1.5-2 mm, with 11-51 glumes, glume-like bract c. 2 mm, glume-like prophyll 1.9-2.4 mm, bi-nerved, sterile; spike rachis 4-angled, c. 0.5 mm wide, internodes 0.7-1 mm, winged; glumes (2.3-)2.7-3.7 mm, cymbiform, rather closely imbricate, obtuse, dark reddish brown, or grey-brown, sides with two distinct nerves, nerveless area wide, margin narrowly or sometimes widely scarious, towards apex embracing nut. Nut 1.3-1.8 x 0.7-1 mm, ellipsoid or obovoid, trigonous, brown to blackish brown, clearly or obscurely reticulate papillose.
Fl. Per.: April - October.
Type: "Habitat in India.".
Shores, wet meadows, ditches, rice fields, moist road sides, waste places; Distribution: Tropical and subtropical areas of all continents.
Sometimes called the "Worlds worst weed".
Cyperus rotundus is a variable species readily propagating vegetatively, but frequently producing seeds in S.W. Asia. Three subspecies, subsp. rotundus, subsp. retzii (Nees) Kük. and subsp. tuberosus (Rottb.) Kük. are commonly recognized (Kern, in Fl. Malesiana 1974; Koyama in Fl. of Taiwan, 1979; Haines & Lye, Sedges and Rushes E. Afr. 1983; Bhandari, 1990), and it easier to identify them than in Fl. Iranica material (Kukkonen 1998). The pale coloured subsp. retzii is often mixed with C. esculentus.
Tall plants approaching C. longus in habit occur in central parts of Pakistan (cf. C. rotundus var. centriflora C.B. Clarke in Hook.f., Fl. Brit. Ind. 6: 615. 1893).