The desert willow (Chilopsis linearis) from the desert washes of the American Southwest, and the southern catalpa (Catalpa bignonioides), from the southeastern United States, were brought together in the central Asian Republic of Uzbekistan in the Soviet Union in the early 1960s. From this unlikely event came one of the most exciting drought-tolerant woody plants for arid and semi-arid regions of the world.
Chitalpas are strikingly attractive small to medium-sized ornamental trees that have inherited the best features of both parents — the desert willow (Chilopsis linearis) and the catalpa (Catalpa bignonioides) — and are ideally suited for most of the soils and climates of the American Southwest. Among the better features of these plants are abundant trusses of fifteen to forty large flowers ranging from white to pink. Each flower is about an inch long, with a descending funnel-shaped throat, spreading petal lobes, and conspicuous purplish nectar guides on the inside of the flower. In southern California they begin to flower in mid to late May, after many of our native trees and shrubs have finished flowering, and they continue into October or November. Plants sometimes flower as early as three or four years old.
Although these hybrids were made in 1964 and first introduced to the United States in 1977, they were not given a name under the International Rules for Botanical Nomenclature until 1991. Recently, Walter Wisura, curator of living collections at Rancho Santa Ana Botanic Garden, and I named them x Chitalpa tashkentensis. The generic name is a combination of the scientific names of both parents, while the specific epithet identifies the city where the crosses were made. We also named two cultivars. A tree with light pink flowers and more spreading growth was named ‘Pink Dawn’. A more upright and vigorous tree with white flowers was named ‘Morning Cloud’.
These rapidly growing trees branch readily near the base and have ascending branches, forming dense, broadly oval crowns. They probably will reach nineteen to twenty-five feet in height, an estimate based on the current age and height of specimens in the botanic garden and assuming they will be intermediate between the parents. Our two native catalpas of the eastern United States mature into large trees from forty-five to nearly one hundred feet tall. The desert willow is a shrub or small tree, often with a short, twisting trunk, slender upright branches, and an irregular shape.
Chitalpas have other features that make them good plants for urban and suburban landscapes. They have inherited much of the drought tolerance of the desert willow, a highly desirable trait for new landscape plants in southern California and the Southwest, where fresh water supplies are limited and population continues to increase rapidly. The four- to seven-inch-long, narrow, pointed leaves of the chitalpa are more attractive and create less litter than the coarse, much larger and broader leaves of the catalpa. The leaves of the chitalpa are, however, larger and more conspicuous than those of the desert willow. Leaves fall in October or November, depending on location and temperature.
Chitalpas are sterile and do not produce the long, narrow pods characteristic of both parents. This also reduces the litter problem and makes them more suitable for urban environments. The flowers wither and dry while still on the tree, which lessens the risk of accidents to pedestrians, who have been known to slip on the decaying flowers of trees that release them in fresher condition.
Our experience at the Rancho Santa Ana Botanic Garden has shown that large chitalpas can be successfully transplanted. Plants obtained in 1982 and subsequently planted out in experimental plots were boxed and moved to our new California cultivar garden in winter 1989. These plants, ranging from nine to twelve feet in height and spreading up to fifteen feet, survived the move in good condition. Leaves and flowers were fewer the first season after transplanting, but production of both had returned to normal by the second season. This and other experiences with chitalpas suggest that mature specimens can be transplanted as well as young ones.
Even the most promising new cultivated plants have deficiencies, and chitalpas are no exception. These trees are being grown commercially as unbranched standards three or four feet in height. Single-trunked plants often produce sprouts at the base, indicating a tendency toward low branching or multiple stems. Further evaluation of the growth of these plants will show whether they can be maintained as small, single-trunked trees. A second potential problem is its rapid growth. In many trees rapid growth produces weak or brittle branches that are subject to breakage in adverse conditions. However, the larger chitalpas at the botanic garden have withstood several strong wind storms without signs of damage.
Chitalpas have been planted experimentally in the southeastern United States, where they are subject to mold in humid conditions. They will tolerate frost and some freezing, but may die back to the ground in Zone 6A of the USDA hardiness zone map.
The history of this new intergeneric cross can be traced to the early 1960s, when Soviet geneticist Nikolia F. Rusanov conducted an experiment with four species of catalpa and the desert willow. Only two percent of his crossings set fruit, but he did obtain offspring from crosses of Catalpa bignonioides and Chilopsis linearis. After testing and evaluating these hybrids in various parts of the Middle Asian region of the U.S.S.R., Rusanov recommended them for use in their landscape. Few other plants suitable for landscaping in Uzbekistan flower throughout the summer months.
In 1977 cuttings of the two hybrids were brought to the United States by Robert Hebb, horticulturist at the Cary Arboretum of the New York Botanical Garden. Rooted cuttings from these introductions were distributed in 1982 to members of the American Association of Botanical Gardens and Arboreta, and these became the source of the plants in cultivation today.