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The progeny of artificial crosses between Chilopsis and Catalpa, plants of the nothogenu × Chitalpa are large shrubs or small trees to 10 m, usually single-stemmed, and forming a broad, dense crown. The leaves are deciduous, 10–17 × 2–4.5 cm, alternate or rarely opposite or ternate, the upper surface dull green, the lower lighter green and pilose, the margins entire, the apex attenuate; the petiole is distinct, 1–2.5 cm long. The inflorescences are terminal, erect and racemose or paniculate, with 15–40 flowers. The flowers are hermaphrodite, zygomorphic, the corolla tubular with five lobes, to 2.5 cm long, white to pink; the corolla lobes are frilled, the two upper lobes unmarked, the two lateral lobes and one lower lobe with interconnected purple lines entering the corolla. Fruit is not produced, due to hybrid sterility. In the United States the flowers appear from May to September (Elias & Wisura 1991, Barnes 2000).
Until very recently all clones of × Chitalpa were known as the hybrid ×Chitalpa tashkentensis T.S. Elias & W. Wisura, which was described as the product of a cross between Catalpa bignonioides Walter and Chilopsis linearis Sweet, first produced by Nikolai Rusanov at the Uzbek Academy of Sciences Botanical Garden in Tashkent, Uzbekistan in 1964, and reaching the United States in 1977 (Elias & Wisura 1991, Barnes 2000). Recently published research (Li et al. 2006) has revealed, however, that Chilopsis was the female parent and Catalpa the male – and, more problematically, that the two original clones named have different Catalpa fathers, rendering a single nothospecific epithet inaccurate. Catalpa ×galleana (C. ovata × C. speciosa) was responsible for ‘Pink Dawn’, while Catalpa speciosa fathered ‘Morning Cloud’. Richard Olsen has confirmed (pers. comm. 2007) that the type specimen is ‘Pink Dawn’, which should now be known as × Chitalpa tashkentensis ‘Pink Dawn’ (Chilopsis linearis × Catalpa ×galleana). To avoid excessive proliferation of nothospecific epithets it is recommended that no new hybrid name is published for ‘Morning Cloud’, as it is likely to be joined by numerous other hybrids with diverse Catalpa parentages.
The results of crosses between Chilopsis and Catalpa have become widely cultivated for their attractive flowers – and perhaps for their novelty value. The Desert Willow Chilopsis linearis is typically a plant of the (normally) dry river courses of the southwestern United States, where it produces long whippy growths of narrow leaves and terminal inflorescences. It is adapted to hot conditions with low rainfall, although the riverbeds and washes where it is most frequently found may hold some moisture through the year. It is not hardy enough to be growable in most of our area. Crossing it with catalpas from the southeastern United States, adapted to hot humid areas with summer rainfall, was a bold move, but the resulting seedlings have proved their worth in a range of conditions, so long as the summer is warm and preferably hot – to encourage flowering on the new growth and also to enable the wood to ripen. If this does not occur some dieback will result, and in Zone 6 conditions plants will be killed to the base; they are hardy from Zone 7 upwards. Growth can be very rapid in good conditions. In the eastern United States it is prone to powdery mildew and leaf spots and is not recommended there for areas of high humidity (Dirr 1998, Olsen et al. 2006). In Europe it is probably ideal for Mediterranean areas, or western France, but in the United Kingdom it is not very impressive, seldom achieving more than a few spindly metres with a thin tuft of growth on top. Of the two original clones named, ‘Pink Dawn’ is smaller and has light pink flowers while ‘Morning Cloud’ is larger and more vigorous, with a slightly pendulous habit and pale pink or white flowers (Elias & Wisura 1991), with prominent purple striations in the corolla. ‘Minsum’, sold under the name Summer Bells, is a more recent introduction and has slightly richer flower colour. In favourable conditions the hybrid flowers profusely and over a long season. As Dirr (1998) says, it does not ‘measure up to the better deciduous shrubs’ – but Richard Olsen is conducting a breeding programme at the US National Arboretum, in the hope of changing this.