Echinochloa is a genus of ca. 40-50 species, confined to tropical and warm-temperate regions of the world. None is native in Belgium (but Echinochloa crus-galli is sometimes accepted as an archaeophyte). Several species are economically important. Some are noxious agricultural weeds, for instance in maize or rice (a few species are even obligate weeds of rice). Echinochloa esculenta and possibly also E. frumentacea are more or less widely cultivated as crop plants (cereals) or for birdseed.
Specific boundaries in the genus Echinochloa are unclear. Very many names have been published but most of these taxa are now reduced to synonymy. The taxonomic rank of the remaining species is often questionable. Many characters are exceedingly variable: presence or absence of awns on spikelets, spikelet length, presence or absence of hairy leaf sheaths, etc. A thorough molecular study will probably be required for a better understanding of the relationships between some of the species (groups).
Even relatively well-separated species are often poorly known. Echinochloa muricata, for instance, is now more or less widely naturalised in Belgium but long remained unnoticed.
1. Inflorescence very compact, contracted at maturity with axes hardly visible. Spikelets always awnless, fertile florets not disarticulating at maturity === 2
1. Inflorescence not contracted at maturity, axes showing through. Spikelets awned or awnless, fertile florets disarticulating at maturity === 3
2. Inflorescence pale at maturity (green or yellowish) === Echinochloa frumentacea
2. Inflorescence dark at maturity (blackish-brown to purplish) === E. esculenta
3. Spikelets 2-3 mm long, remarkably regularly and neatly arranged (often in four distinct rows). Inflorescence without secondary branches === 4
3. Spikelets usually longer, very irregularly and untidily arranged (never in distinct rows). Inflorescence usually with secondary branches present === 5
4. Leaves narrow, ca. 3-6 mm wide. Inflorescence branches without bristles. Lower glume ca. ½ as long as spikelet. Caryopsis whitish === E. colona
4. Leaves wider, the widest commonly ca. 10 mm wide. Inflorescence branches with sparse to numerous bristles. Lower glume ca. 1/3 as long as spikelet. Caryopsis brownish === E. crus-galli var. praticola
5. Inflorescence pyramidal. Inflorescence axes with sparse to numerous bristles. Lower floret neuter === 6
5. Inflorescence spike-like. Inflorescence axes without or with few bristles. Lower floret often, at least partially, staminate === E. inundata
6. Spikelets ca. 4-5 mm long. Caryopsis 2-3 mm long. Inflorescence pure green, erect. Always associated with rice cultivation === E. oryzicola
6. Spikelets less than 4 mm long. Caryopsis less than 2 mm long. Inflorescence often tinged purple and/or nodding. Weed from various sources === 7
7. Blade of the uppermost leaf strongly decurrent along the culm. Upper lemma with a stiff acute tip, not clearly differentiated from the coriaceous body of the lemma. Bristles of upper glume and lower lemma always more or less papillose-based, strongly spreading or not === 8
7. Blade of the uppermost leaf at most moderately decurrent along the culm. Upper lemma with a membranous tip, clearly differentiated from the coriaceous body of the lemma. Bristles of upper glume and lower lemma papillose-based or not, not conspicuously spreading === 9
8. Spikelets usually awnless with short tip. Bristles strong and spreading, with distinct papillose base === E. muricata var. muricata (incl. var. microstachya)
8. Spikelets slender with acuminate upper lemma and palea, always awned. Bristles more appressed with less prominent papillose base === E. muricata var. wiegandii
9. Spikelets ovate-elliptic, ca. 3,5-3,8 mm long. Caryopsis ovoid to oblong, 2-2,2 x 1,5-1,8 mm. Inflorescence rarely pyramidal, nearly always greenish. Weed of rice fields === E. hispidula
9. Spikelets ovate, 2,8-3,4 mm long. Caryopsis ovate, 1,4-1,9 x 1,3-1,6 mm. Inflorescence often pyramidal, often tinged purplish. Weed in a wide range of habitats === E. crus-galli var. crus-galli
Additional alien: Echinochloa jubata Stapf (Afr., wool alien). This record was initially erroneously ascribed to the Australian E. elliptica Michael et Vickery (Verloove 2006). Its single collection was revised and identified by Peter Michael in 2009.
Literature:
Carretero J.L. (1981) El género Echinochloa Beauv. en el suroeste de Europa. An. Jard. Bot. Madrid 38(1): 91-108. [available online at: https://dialnet.unirioja.es/servlet/articulo?codigo=2987498]
Conert H.J. (ed.) (1998) Gustav Hegi Illustrierte Flora von Mitteleuropa. Band I, Teil 3 Poaceae (3.Auflage). Parey Buchverlag, Berlin: XXVII + 898 p.
Costea M. & Tardif F.J. (2002) Taxonomy of the most common weedy European Echinochloa species (Poaceae: Panicoideae) with special emphasis on characters of the lemma and caryopsis. Sida 20: 525-548.
Gould F.W., Ali M.A. & Fairbrothers D.E. (1972) A revision of Echinochloa in the United States. Amer. Midl. Natur. 87(1): 36-59.
Hitchcock A.S. (1920) The North American species of Echinochloa. Contr. U.S. Nat. Herb. 22: 133-153.
Jauzein P. (1993) Le genre Echinochloa en Camargue. Monde Pl. 446: 1-5.
McKenzie P.M., Michael P.W., Urbatsch L.E., Noble R.E. & Proctor G.R. (1993) First record of Echinochloa stagnina (Poaceae) for Puerto Rico and key to the Echinochloa in the West Indies. Sida 15(3): 527-532.
Michael P.W. (1983) Taxonomy and distribution of Echinochloa species with a special reference to their occurrence as weeds of rice. Proc. Conf. Weed Control in Rice (31 Aug. – 4 Sept. 1981): 291-306.
Michael P.W. (1994) Distribution and taxonomy of Echinochloa – a world view with a key to the species occurring in China. Proc. 5th Weed Sci. Conf. China: 161-166.
Michael P.W. (2001) The taxonomy and distribution of Echinochloa species (barnyard grasses) in the Asian-Pacific region, with a review of pertinent biological studies. Proc. 18th Asian-Pacific Weed Sci. Soc. Conf. 1: 57-66.
Michael P.W. (2003) Echinochloa. In: Barkworth M.E. & al. (eds.), Flora of North America north of Mexico, vol. 25. Oxford University Press, New York-Oxford: 390-403.
Shouliang C. & Phillips S.M. (2006) Echinochloa. In: Zhengyi W. & Raven, P.H. (eds.), Flora of China, vol. 22: 515-518. Science Press, Beijing & Missouri Botanical Garden Press, St. Louis.
Verloove F. (2006) Catalogue of neophytes in Belgium. Scripta Botanica Belgica 39: 89 p.
Wiegand K.M. (1921) The genus Echinochloa in North America. Rhodora 23: 49-65.
Yabuno T. (1966) Biosystematic study of the genus Echinochloa. Jap. J. Bot. 19: 227-323.