Baileya 23(3): 139-144 (1991)
xChitalpa tashkentensis (Bignoniaceae), an Intergeneric Hybrid of Ornamental Value
Thomas S. Elias and Walter Wisura
Rancho Santa Ana Botanic Garden, 1500 North College Avenue, Claremont, CA 91711

ABSTRACT. Artificial intergeneric crosses between Catalpa and Chilopsis (Bignoniaceae) have produced sterile F1 hybrids which have considerable potential as ornamental plants in landscaping and in the commercial trade. While the results of the initial crosses were first described in 1964, the hybrid was never named. We relate the history of the introduction into the United States and formally name this hybrid and two flower color forms.

Artificial intergeneric crosses between Catalpa Scop. and Chilopsis D. Don have resulted in sterile, F1 hybrids with intermediate characteristics of the parents. Initially, the likelihood of developing hybrids between the monotypic and xeromorphic Chilopsis linearis (Cav.) Sweet of the southwestern United States and northern Mexico and the eastern broad-leaved mesophytic species seems remote. Chilopsis linearis was recognized as consisting of two subspecies and a new variety in a recent taxonomic treatment (Hendrickson, 1985), whereas Catalpa is a small genus of two sections. Section Catalpa contains broad-leaved deciduous species, two in the eastern United States and four in China, and Section Macrocatalpa with five, small-leaved evergreen species from the West Indies.

Both genera belong to the tribe Tecomeae and share many characters. Hendrickson (1985) pointed out that Chilopsis and Catalpa are nearly identical in fruit, seed, embryo, style, and anther and pollen characteristics. He also noted that they have similar features of the calyx and corolla and that they share a unique pollen type in the family Bignoniaceae. The pollen was described by Gentry and Tomb (1979) as being in tetrads with sculpturing limited to coarsely reticulate areoles. Furthermore, both genera have the same number of chromosomes, 2n = 20 (Goldblatt and Gentry, 1979). Thus, it is not surprising that intergeneric hybrids can be formed.

The hybridization experiments between Catalpa and Chilopsis were carried out at the Uzbek Academy of Sciences Botanical Garden in Tashkent in the Republic of Uzbekistan in the Soviet Union. The breeding program on both herbaceous and woody species was under the direction of the well known plant geneticist Academician Fyodor Nikolaevich Rusanov. One of Academician Rusanov's species was breeding new selections of Hibiscus (1965). His son, Nikolai Fyodorovich Rusanov, worked with his father at the botanical garden and is continuing the hybridization programs his father initiated. Nikolai F. Rusanov conducted hybridization and phenological studies with Catalpa (1964, 1969). The results of other introductions, acclimatizations, and hybridization experiments conducted by this research group are reported regularly in serial publication (Rusanov, 1976).

Rusanov in the 1950s brought several species of Catalpa into the living collections at the botanical garden in Tashkent along with Chilopsis linearis and many other genera. In the early 1960s, Nikolai F. Rusanov began a series of crossing experiments using four species of Catalpa (C. bignonioides Walt., C. speciosa Ward., C. ovata G. Don and one unidentified species) and the monotypic Chilopsis. The results of this crossing study were published by Rusanov in 1964. He reported that all species of Catalpa were pollinated with Chilopsis pollen and that Chilopsis was pollinated with all species of Catalpa pollen. These crossings were repeated over a two-year period. Results showed that Catalpa sp., when pollinated with Chilopsis linearis, yielded only a 2% fruit set. About 20% of the seed sown from this cross germinated and developed into vigorous plants. When Rusanov carried out the reverse cross, he obtained a 90% fruit set and some fertile seed.

Nikolai Rusanov, in his 1964 paper, stated that "we do have plants one or two years of age which have not yet flowered but judging by their morphological features, they are undoubtedly intergeneric hybrids." These would have to be offspring of his Catalpa sp. X Chilopsis linearis and reciprocal crosses. In 1969, Nikolai Rusanov published a paper elucidating the flowering and fruit setting of four species of Catalpa (C. speciosa, C. bignonioides, C. ovata, and C. fargesii Bureau f. duclouxii Dode). It was not clear if the Catalpa sp. in his earlier paper is the same as the Catalpa fargesii of his 1969 work.

This was clarified by N. Rusanov in a detailed account of the intergeneric hybrids developed from reciprocal crossings of Catalpa bignonioides and Chilopsis linearis (1971) and in subsequent papers (1976, 1981, and 1982) in which he described the value and use of these hybrids as new ornamental plants for use in landscaping portions of the Middle Asia region of the U.S.S.R.

Rusanov (1971) reported that the hybrids from reciprocal crossings of Catalpa bignonioides and Chilopsis linearis were brought to flower and that one of the hybrids even bore fruit. He described and compared the basic morphological differences between the hybrids and both parents and presented the results of their investigation as to the reasons for the hybrid sterility. Even though both parents are at the same ploidy level (2n=40), Rusanov found that during megasporogenesis the embryo sac will develop into a tetrad or sometimes a pentad microspore but then in most cases degenerates. Microsporogenesis in the parent species results in uniform normal grains united into tetrads. However, in the hybrids, pollen grains were mostly unites into pentads and hexads and consisted of uninuclear pollen of different ages. These observations indicate a significant degree of incompatibility.

Rusanov (1976) pointed out that there are few shrubs suitable for landscaping in Uzbekistan that flower continuously during the entire summer period. However, the extended summer flowering period of this new intergeneric hybrid fills this gap. He recommended that the hybrids be grown under the same cultural requirements as for Catalpa bignonioides; that is, moist rich soils in sunny locations and with occasional fertilization.

Propagation of the hybrids was accomplished by cuttings obtained in spring and treated with heteroauxin. About 400 hybrid plants had been planted in Uzbekistan by 1976 and he was recommending this hybrid for use in southern Middle Asia as far north as Alma-Ata. In 1982, Rusanov reported that the hybrids were doing well in many landscaping situations in numerous cities in Uzbekistan, thus indicating that the test plantings were successful.

The hybrids were first brought to the United States by Robert Hebb, then Horticulturist at the Cary Arboretum of the New York Botanical Garden. Mr. Hebb and two American botanists had been on a plant collecting expedition in 1977 to the Central Asian region of the Soviet Union under the auspices of the U.S./U.S.S.R. Botanical Exchange Program. The American team toured the botanical garden in Tashkent and had the opportunity to discuss the work of the garden with some of the staff. It was at this time that Mr. Hebb received permission to take cuttings of the hybrid Plants.

Mr. Hebb brought cuttings back to the Cary Arboretum in Millbrook, New York where they were rooted and grown. Two color forms were present, one with deep pink flowers and the other with pale pink flowers. These were grown in containers as they were not winter hardy in the Hudson River Valley of New York. The plants began to flower by the third growing season.

In 1982, fifty rooted cuttings of each of the color forms (Cary Arboretum accession number CA 77-1927) were distributed to members of the American Association of Botanical Gardens and Arboreta (AABGA). This organization held their annual meeting of Vassar College in nearby Poughkeepsie, New York, and toured the arboretum as part of their activities. We believe that all of the living material in the United States of this intergeneric hybrid originated from this single introduction and distribution.

This hybrid was listed and briefly described by Hebb (1982) in a compilation of important Chinese, Soviet and other plants to be distributed at the June, 1982 AABGA meeting. In this paper, he described the hybrid as being a cross between Catalpa bignonioides and Chilopsis linearis. According to his field records and his recollection (pers. comm.), this was the information supplied to him by some of the personnel at the Tashkent Botanical Garden. He noted that the hybrid roots easily from softwood cuttings, is fast growing, and is drought tolerant. Furthermore, he indicated that the plants died back to their root stock in winter in USDA Zone 6A, but that they should be hardy farther south.

The Rancho Santa Ana Botanic Garden received 12 bare-rooted cuttings of Clone no. 1 (deep pink flowers) and 10 of Clone no. 2 (light pink flowers) in November, 1982. In February and March of the following year, this material was planted on the grounds of the garden. The plants have grown rapidly into large shrubs or small trees flowering from May to early September.

This hybrid has adopted the best features of both parents and has now proven to be a successful plant for landscaping in the Southwest. The plant has a low branching pattern, forming a dense, broadly oval-shaped small tree. The leaves are somewhat wider than those of Chilopsis linearis but do not approach the larger coarse leaf size of the eastern catalpas. The inflorescences more closely resemble in size and structure those of the catalpa. However, instead of flowering only in the spring, the flowering time of the hybrid extends from late spring to early fall. This extended flowering time adds considerably to the ornamental value of this plant. Since the plants are sterile hybrids, the mess of the long, narrow pods dropping on the ground beneath the plants is eliminated.

This new hybrid has proven to be drought tolerant, yet flowers at an early age and is vigorous in its growth habit. Test plantings in the southeastern United States have not proven as successful due to the plants' susceptibility to mold (Hebb, personal communication). In the Southwest, however, the plants are of sufficient value that they are now being sold in the nursery trade and have been featured in major newspapers and magazines based in California.

Since this intergeneric hybrid has considerable horticultural merit as an ornamental tree, it is appropriate to describe this nothospecies as well as assign cultivar names to the two flower color forms.

xChitalpa Elias & Wisura, gen. hybr. nov. Catalpa Scop. x Chilopsis D. Don

xChitalpa tashkentensis Elias & Wisura, sp. hybr. nov. Catalpa bignonioides Walt. x Chilopsis linearis (Cav.) Sweet.—Type: U.S.A. California. Los Angeles: cultivated at Rancho Santa Ana Botanic Garden, Claremont, living accession no. 14635, T. S. Elias s.n. (Holotype: RSA; herbarium accession no. 332829).

Hybrida inter parentes (Catalpa bignonioides et Chilopsis linearis) intermedia. Arbuscula vel frutex deciduus; folia pluries alterna, nonnunquam opposita vel ternata, lamina 10-17 cm longa, 2-4.5 cm lata, lanceolata, anguste acuta ad apicem et basim. Inflorescentia terminalis racemosa, erecta, floribus 15-40. Flores pedicellati, magni; calyx campanulatus, bilabiatus; corollae tubus ca. 2.5 cm longus; stamina 4 vel 5, libera, horum 2 longa, 2 brevia, 1 sterile. Capsula et semina non visa.

Intergeneric hybrid forming a deciduous small to medium-sized tree 6-10 m high with a 5-6 m spread, with dense, ascending-spreading branches. Leaves irregularly alternate, occasionally opposite or ternate in dense succession along the young branchlets, estipulate, petiole short, 1-2.5 cm long; lamina 10-17 cm long, 2-4.5 cm wide, dull green, somewhat lighter beneath, attenuate at apex and base, glabrous above, pilose beneath, at least along the veins. Inflorescence a terminal, erect, monopodial raceme of 15 to 40 zygomorphic pink to white flowers. Flowers pedicellate, the pedicels short, pilose, with 1-3 caducous pilose bracts; calyx 2-lobed, 8-12 mm long, pilose without and within, splitting on one side almost to the base, on the opposite side about to the middle, the lobes uncinate tipped; corolla tube about 2.5 cm long, narrow at the base for 5-7 mm, and abruptly curving and widening to a wide, open throat with 5 frilled lobes; corolla tube marked on the inside by purple nectar guides and lines, and with two yellow guide lines leading out to the sinuses between the lower lobes; the two side lobes moderately to strongly reflexed and the lower lobe thrust forward making the corolla about 4.5 cm long; the two upper lobes unmarked, the side lobes and lower lobe marked by vein-like interconnected purple lines leading into the throat. Stamens 4 or 5, two long (18 mm long), two short (12 mm) and one sterile (7 mm), inserted at the base of the upper part of the tube; the longer stamens almost reaching to the mouth of the throat. Ovary linear-terete, ca. 4 mm long, somewhat tuberculate, tapering into a white style 2.2-2.4 cm long; stigma white, bifurcate, stigmatic lobes papillate on the inner surface. Capsule and seed not seen due to the suspected sterility of the hybrid plants.

xChitalpa tashkentensis Elias & Wisura 'Pink Dawn' Elias & Wisura, cv. nov.

This clone is the smaller of the two, with a moderate, somewhat more spreading growth habit and light pink flowers. This is the Clone no. 1 distributed by the Cary Arboretum in 1982 and referred to by Hebb (1982). We suspect that this clone is the result of the original Chilopsis linearis x Catalpa sp. cross since it matches Rusanov's description of being less vigorous in growth than the reciprocal cross.

xChitalpa tashkentensis Elias & Wisura 'Morning Cloud' Elias & Wisura, cv. nov.

This clone displays a more upright, vigorous growth habit and tends to grow larger than Clone no. 1. The flowers are very pale pink to white. This corresponds to Clone no. 2 distributed by the Cary Arboretum. We also suspect that this clone is the result of the Catalpa sp. x Chilopsis linearis cross.

We found that the flower color is not as intense or as deep a pink in Clone no. 1 as was described by Hebb (1982). As a result, the flower colors given above more accurately reflect the materials in cultivation in California and the Southwest.

ACKNOWLEDGMENTS. The introduction of this hybrid and subsequent visits to the Academy of Science Botanical Garden in Tashkent would not have been possible without the help of the U.S./U.S.S.R. Botanical Exchange Program. This activity is part of a program relating to the protection of nature administrated by the U.S. Fish and Wildlife Service and operating under a bilateral agreement on the protection of the environment. The assistance of the International Affairs staff of the U.S. Fish and Wildlife Service is gratefully acknowledged. We also thank Ms. Lydia Newcombe for the translation of pertinent articles from Russian into English and for proofreading this paper.

LITERATURE CITED

xChitalpa taskentensis Hemet, CA (Oct 18, 2018)
xChitalpa taskentensis Hemet, CA (Oct 11, 2018)
 
Chilopsis linearis, southern California (June 3, 2018)
 

CATALPA