Pennisetum setaceum or Pennisetum advena cultivars, what ornamental do we have in our garden

Abstract

Pennisetum Rich. or following recent taxonomic insights Cenchrus L. is a genus with some 120 species worldwide, especially in warm areas. The genus includes some crops, some ornamentals but mostly species that are considered weedy. The name of one of the weedy species Pennisetum setaceum (Forssk.) Chiov. is also found on labels of ornamental grasses as P. setaceum “Rubrum.” It has been debated to belong to a species on its own Pennisetum advena Wipff & Veldkamp or Cenchrus advena (Wipff & Veldkamp) Morrone, only known from cultivation, whereas others still adhere to a broader species concept of P. setaceum. The recent inclusion of P. setaceum on the EU List of Union concern has revitalized the discussion on this issue for commercial reasons. Based on a morphological and molecular comparison (ITS, rbcL, and the trnh-psbA intergenic spacer sequences) of the type specimen of P. advena, five of its “cultivars” in trade and collections of P. setaceum from different regions of the world we conclude that plants currently in trade in Western Europe belong to a separate species P. advena. A drooping inflorescence is consistent as is the difference in width of the leaf blade, the leaf blade being flat or involute, the central vein being swollen or not, and the length of the stipe being 0.3–1.1 mm in P. advena and 1.1–3.1 mm in P. setaceum. On the chloroplast markers rbcL and trnH-psbA, the species consistently differ in 2 and 4 base pairs, respectively. On the nuclear ITS sequence, there is only 90% overlap between the two species. This justifies these ornamentals to be excluded from the List of Union concern of EU regulation 1143/2014.