Paulownia Siebold et Zucc.
As currently understood, Paulownia is a genus of seven species (Deyuan & al. 1998, Mabberley 2008, Freeman, in prep.), although many more have been described in the past. All species occur in China but as a result of cultivation the wild range of some has been obscured. In western Europe Paulownia is mostly cultivated as an ornamental tree but elsewhere (for instance in South America; Mabberley 2008) also for timber. According to Mill (2000) four species are cultivated in Europe but only one, Paulownia tomentosa, is common. However, in general appearance all species are much alike and more than one species might occur as an escape in Belgium. In the British Isles a fifth species, Paulownia kawakamii Ito was observed as an escape (Gibson 2003). Hybridization between several species has also been demonstrated (Wang & al. 1994, Kumar & al. 1999) and might further blur species boundaries.
Saplings of Paulownia have long been confused with Catalpa. The latter is far rarer as an escape and their separation is straightforward: leaves are always opposite, much larger (often ca. 50 cm across in young trees) and more hairy in Paulownia, while mostly in whorls of three, subglabrous (but often glandular-sticky!) in Catalpa. Moreover, in Paulownia twigs are chambered (or even hollow) while Catalpa has twigs with continuous pith (see also Nowack 1987 for differences between both genera).
Additional information on the genus Paulownia and its propagation is provided by the “American Paulownia Association” at http://www.paulowniatrees.org/.
Literature:
Campbell D.H. (1930) The relationships of Paulownia. Bull. Torrey Bot. Club 57: 47-50.
Carolus H. (1979) Paulownia - der Blauglockenbaum. Palmengarten 43(1): 36-37.
Deyuan H., Hanbi Y., Cunli J. & Holmgren N.H. (1998) Scrophulariaceae. In Flora of China, vol. 18. Science Press, Beijing and Missouri Botanical Garden Press, St Louis: 449 p.
Freeman C.C. (in prep.) Paulownia. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 17. Oxford University Press, New York-Oxford. [available online at: http://floranorthamerica.org/files/Paulowniaceae04a%20SI.CH%20for%20Web.pdf]
Gibson C. (2003) Paulownia kawakamii Ito, in Essex: a mystery solved. BSBI News 94: 33. [available online at: http://archive.bsbi.org.uk/BSBINews94.pdf]
Hu S.Y. (1959) A monograph of the genus Paulownia. Quart. J. Taiwan Mus. 12: 1-54.
Hu S.Y. (1961) The economic botany of the paulownias. Econ. Bot. 15: 11-27.
Kumar P.P., Rao C.D., Rajaseger G. & Rao A.N. (1999) Seed surface architecture and random amplified polymorphic DNA profiles of Paulownia fortunei, P. tomentosa and their hybrid. Ann. Bot. (UK) 83(2): 103-107.
Liang Z.Y. & Chen Z.Y. (1997) Studies of the cytological taxonomy of the genus Paulownia. J. Huazhong Agric. Univ. 16: 609-613.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Mill R.R. (2000) Paulownia. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 344-345.
Nowack R. (1987) Verwilderungen des Blauglockenbaums (Paulownia tomentosa [Thunb.] Steud.) im Rhein-Neckar-Gebiet. Floristische Rundbriefe 21(1): 23-32.
Vujičić R., Grubišić D. & Konjević R. (1993) Scanning electron microscopy of the seed coat in the genus Paulownia (Scrophulariaceae). Bot. J. Linn. Soc. 111(4): 505-511.
Wang W.Y., Pai R.C., Lai C.C. & Lin T.P. (1994) Molecular evidence for the hybrid origin of Paulownia taiwaniana based on RAPD markers and RFLP of chloroplast DNA. Theoretical and applied genetics 89: 271-275.
Wharton A.P. (1981) The foxglove trees. Davidsonia 12(3): 57-61.
Zhao-Hua Z., Ching-Ju C., Xin-Yu L. & Yao X. (1986) Paulownia in China. Cultivation and utilization. Chinese Acad. Sci., Beijing.