Petasites Mill.
Petasites is a genus of ca. 19 species, mainly native in Eurasia. One additional species occurs in North America (Bayer & al. 2006). A single species is originally native in Belgium, Petasites hybridus (L.) P. Gaertn., B. Mey. et Scherb. subsp. hybridus. Several species are cultivated as ornamentals (formerly also as medicinal plants) in Europe, see Akeroyd (2000) and Jäger & al. (2008). Native Petasites hybridus is also cultivated and at least part of the Belgian populations of the latter is alien. In areas where only male plants are present it is possibly not native (Dingwall 1976). The rather similar Petasites paradoxus (Retz.) Baumg. is also cultivated as an ornamental (Jäger & al. 2008) and might have been overlooked. Its inflorescence is more lax (usually with less than 30 flowers) and its basal leaves are broadly triangular with a cordate base, rather than nearly circular.
A useful key to non-flowering European plants is provided by Dingwall (1976). See also Short (1998).
1. Flowers yellowish-white (cream). Involucral bracts green === 2
1. Flowers lilac or pink. Involucral bracts usually tinged purplish === 3
2. Adult leaves 15-30 cm across, persistently white-arachnoid hairy beneath. Scale leaves linear-lanceolate, up to 10 mm wide. Involucral bracts densely glandular hairy === 1. Petasites albus
2. Adult leaves 30-100 cm across, sparsely arachnoid hairy beneath. Scale leaves ovate, 30-60 mm wide. Involucral bracts sparsely glandular to eglandular hairy === 3. P. japonicus
3. Peripheral flowers ligulate. Flowering November-February. Leaves present at anthesis. Flowers vanilla-scented. Leaves regularly toothed === 2. P. fragrans
3. All flowers tubular. Flowering February-April. Leaves appearing at the end of anthesis. Flowers not distinctly scented. Leaves irregularly toothed (native) === P. hybridus subsp. hybridus
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Literature:
Akeroyd J.R. (2000) Petasites. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 636-637.
Bayer R.J., Bogle A.L. & Cherniawsky D.M. (2006) Petasites. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 20. Oxford University Press, New York-Oxford: 635-640.
Dingwall I. (1976) Petasites. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 4. Cambridge University Press, Cambridge: 186-188.
Handeland S. (1992) Forvilla artar av pestrot, Petasites (L.) Mill. Blyttia 50: 163-166.
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.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Leslie A. (1992) Petasites. Garden 117: 186-188.
Sell P. & Murrell G. (2006) Flora of Great Britain and Ireland. Vol. 4 Campanulaceae – Asteraceae. Cambridge University Press, Cambridge: XXVIII + 624 p.
Short M.J. (1998) Petasites Miller/Tussilago L. vegetative material. Plant Crib: 308-309.
Štech M. (2004) Petasites. In: Slavík B. & Štĕpánková (eds.), Kvĕtena České Republiky, vol. 7. Academia, Praha: 285-293.
Toman J. (1972) A taxonomic survey of the genera Petasites and Endocellion. Folia Geobot. Phytotax. 7: 381-406.