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There are seven species of Paulownia, all occurring in China and Taiwan, though P. fortunei extends into Laos and Vietnam. They are deciduous trees (evergreen in tropical regions) with smooth bark on the trunk and conspicuous lenticels on the immature stems. The leaves are opposite (rarely in whorls of three) and entire or with three to five shallow lobes; when coppiced or growing vigorously, Paulownia species produce extremely large leaves. The thyrsoid inflorescences are large and numerous; they may be pyramidal or cylindrical and contain (one to) three to five (to eight) flowers. The flowers are purple or white; the calyx is campanulate and tomentose; the corolla campanulate or tubular with five lobes and a slightly curved and constricted base; there are four stamens. The fruit is a woody capsule with two to four valves, containing numerous seeds with membranous wings (Hong et al. 1998). Paulownia species are widely cultivated in China, and assessing their natural distributions is difficult. A key to the genus is provided by Hong et al. (1998).
Despite the beauty and popularity of Paulownia tomentosa, other members of the genus are seldom seen, though most have been in cultivation for many years. Paulownia tomentosa is an aggressive weed species in some parts of the United States, and in areas where its weediness is a problem any other Paulownia species should be planted only after due care and consideration. The other side of the coin is that Paulownia are very fast-growing trees, producing timber that is highly valued in the Far East. While environmentalists wring their hands over its invasiveness, others actively promote its culture as a plantation tree! It is also used in reclamation of degraded land – after strip-mining, for example. Dirr (1998) wisely points out that it is a short-lived pioneer that will be out-competed by later successional species. The species principally used as a plantation tree in warm areas is P. elongata, on account of its particularly rapid growth (World Paulownia Europe 2007–2008). As a whole the genus appreciates hot summers and good living in terms of soil and moisture conditions. Propagation is by seed or from root cuttings – both playing a part in the weediness problem.
A small genus of deciduous trees, natives of China, Korea, Indochina, and Formosa, in which six species are recognised by Dr S.-Y. Hu in her monograph (Qtly Journ. Taiwan Mus., Vol. 12 (1959), pp. 1-54). The leading characters are: Bark smooth, branchlets pithy; leaves opposite, entire or lobed; flowers in three- or five-flowered stalked or almost sessile cymes in the axils of the fallen leaves, produced in spring from buds developed in the previous autumn; calyx fleshy, five-lobed; corolla between tubular and funnel-shaped, obscurely two-lipped, the upper lip two-lobed, the lower three-lobed; stamens four; style one, with a small stigma; capsules ovoid to ellipsoid, with numerous winged seeds.
Dr Hu follows Pennel in retaining Paulownia in Scrophulariaceae as a monotypic tribe. Other botanists have placed it in the Bignoniaceae and indeed it bears a very close resemblance to Catalpa, which certainly belongs to the latter family. However, the Bignoniaceae and Scrophulariaceae are closely allied, and it has been suggested that Catalpa and Paulownia are near to the ancestral stock from which the two families have branched. The most obvious differential character of the catalpas is that their trunks are rugged and that the flowers are borne in summer at the ends of the seasonal growths; they also differ markedly from any paulownia in their long, cylindrical fruits.
The genus was named by Siebold in honour of Anna Paulowna, Hereditary Princess of the Netherlands.
For propagation and cultivation, see under P. tomentosa.