9. Setaria viridis (L.) Beauv. (warm-temp. areas of the Old World, probably Asia; see Layton & Kellogg 2014) – A common alien – casual as well as naturalised – and maybe even an archaeophyte. Already documented from Belgium in the 18th century. At present chiefly introduced with cereals (including birdseeds). Formerly also a frequent wool alien. Now mostly found in port- or urban areas, by motorways, on dumps, etc. In such habitats usually rather ephemeral. However, in the past decades also increasingly found as a naturalised and spreading weed of maize fields (Hoste 2006). Apparently more frequent in Flanders than in Wallonia.
Setaria viridis is often poorly delimited from S. italica (the latter probably is derived from it). In some cases ripe caryopses are required for an accurate determination. Setaria viridis was furthermore long confused with S. faberi (see under that species).
Setaria viridis itself is a very variable species with numerous infraspecific taxa (see for instance Conert 1998) but most probably are of little taxonomic value. Apart from typical plants with moderate dimensions (hight up to 80 cm, leaves up to 12 mm wide, inflorescence up to 12 mm wide = var. viridis), a taller plant with wider leaves and inflorescences is increasingly found nowadays [= var. major (Gaudin) Pospichal, syn.: subsp. pycnocoma (Steud.) Tsvelev] but both varieties are linked through intermediates. Another remarkable variety, also regularly found, is characterised by purplish bristles. Such plants are ascribable to subsp. weinmannii (Roem. et Schult.) Tsvelev (syn.: S. weinmannii Roem. et Schult.). The two latter varieties are probably ephemeral aliens in Belgium.
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Selected literature:
Conert H.J. (ed.) (1998) Gustav Hegi Illustrierte Flora von Mitteleuropa. Band I, Teil 3 Poaceae (3.Auflage). Parey Buchverlag, Berlin: XXVII + 898 p.
Hoste I. (2006) Setaria viridis. In: Van Landuyt W., Hoste I., Vanhecke L., Van den Bremt P., Vercruysse W. & De Beer D., Atlas van de flora van Vlaanderen en het Brussels gewest. Instituut voor Natuur- en Bosonderzoek, Nationale Plantentuin van België en Flo.Wer: 828.
Layton D.J. & Kellogg E.A. (2014) Morphological, phylogenetic, and ecological diversity of the new model species Setaria viridis (Poaceae: Paniceae) and its close relatives. Am. J. Bot. 101(3): 539-557.