(incl. Beckeropsis Fig. et De Not.)
Pennisetum, as traditionally circumscribed (Mabberley 2008), is a genus of ca. 80 species, possibly more (up to 130 according to Wipff 2003). It is mostly confined to the tropics and subtropics. Its generic boundaries have long been unclear. The essentially African genus Beckeropsis, distinguished by a compound, panicle-like inflorescence and spikelets with a single bristle, is quite characteristic but merges into Pennisetum on a global scale. Therefore, it is currently included in the latter. Pennisetum is furthermore particularly closely related to Cenchrus (see also under that genus). Recent molecular phylogenetic studies shed a new light on the relationships in Cenchrus and Pennisetum. Donadio & al. (2009) first demonstrated that Cenchrus and Pennisetum are very closely related and that most species of Cenchrus are in fact nested in Pennisetum. At most a core group of a few American species (mainly with a basic chromosome number of x=17) could be retained in Cenchrus s.str. Chemisquy & al. (2010) confirmed these results and strongly suggested to merge both genera. Cenchrus having priority, all species of Pennisetum should be transferred to Cenchrus. Pending further studies Pennisetum is here accepted in its traditional sense but synonyms in Cenchrus are, if available, added.
A useful account of the genus (incl. cultivars) as present in the horticultural trade in Western Europe is provided by Costerus & van Valkenburg (2018). See also here for an online alternative: https://keys.lucidcentral.org/keys/v3/pennisetum/.
1. Plant annual. Flowerheads (fascicles) with only 1 bristle and 1 spikelet. Inflorescence an axillary panicle, exserted from the upper leaf sheaths, borne on long capillary peduncles. Leaves flat, up to 25 mm wide, abruptly cordate basally, pseudopetiolate. Bird seed alien === 3. Pennisetum petiolare
1. Plant perennial. Flowerheads (fascicles) with at least 6 bristles and 1-12 spikelets. Inflorescence a dense spicate terminal panicle. Leaves flat or folded, up to 8 mm wide. Garden escapes === 2
2. Spikelets solitary. Fascicles distinctly pedicellate (pedicel up to 3 mm long). Rachis scabrous-pilose, with scattered hairs. All bristles scabrous, none plumose; the primary bristle not noticeably longer than the others. Longest bristles up to 30 mm long. Tufted, rhizomes absent === 1. P. alopecuroides
2. Spikelets usually 3-4(-9), rarely solitary. Fascicles subsessile. Rachis scabrous, without hairs. Primary bristle sometimes slightly plumose, noticeably longer than the others. Bristles (except the primary bristle) up to ca. 15 mm long. Strongly rhizomatous === 2. P. flaccidum
Chemisquy M.A., Giussani L.M., Scataglini M.A., Kellogg E.A. & Morrone O. (2010) Phylogenetic studies favour the unification of Pennisetum, Cenchrus and Odontelytrum (Poaceae): a combined nuclear, plastid and morphological analysis, and nomenclatural combinations in Cenchrus. Ann. Bot. 106: 107-130.
Costerus M.E. & van Valkenburg J.L.C.H. (2018) Pennisetum. Het handelssortiment ontrafeld. Dendroflora 54: 44-67.
Donadio S., Giussani L.M., Kellogg E.A., Zuloaga F.O. & Morrone O. (2009) A preliminary molecular phylogeny of Pennisetum and Cenchrus (Poaceae-Paniceae) based on the trnL-F, rpl16 chloroplast markers. Taxon 58(2): 392-404.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Phillips S.M. (1995) Poaceae. In: Hedberg I. & Edwards S. (eds.), Flora of Ethiopia and Eritrea. Vol. 7. The National Herbarium, Addis Ababa University, Addis Ababa (Ethiopia) and Department of Systematic Botany, Uppsala University, Uppsala (Sweden): 420 p.
Shouliang C. & Phillips S.M. (2006) Pennisetum. In: Zhengyi W. & Raven, P.H. (eds.), Flora of China, vol. 22: 548-552. Science Press, Beijing & Missouri Botanical Garden Press, St. Louis.
Tsvelev N.N. (1984) Grasses of the Soviet Union, part 1 and 2. A.A. Balkema, Rotterdam: XVI + 1196 p.
Veldkamp J.F. (2014) A revision of Cenchrus incl. Pennisetum (Gramineae) in Malesia with some general nomenclatural notes. Blumea 59(1): 59-75. [available online at: http://www.ingentaconnect.com/content/nhn/blumea/2014/00000059/00000001/art00012]
Wipff J.K. (2003) Pennisetum. In: Barkworth M.E. & al. (eds.), Flora of North America north of Mexico, vol. 25. Oxford University Press, New York-Oxford: 515-529.