×Chitalpa T.S.Elias & W.Wisura

Selections of ×Chitalpa are widely cultivated in the United States for their attractive and plentiful flowers, resembling those of Catalpa, but produced over a far longer season. They are ideally suited to use as urban street trees, the broader leaves providing more useful shade in hot climates than those of the willowy Chilopsis; in more temperate contexts, the protection offered buildings and radiant and reflected heat from hardscapes assists with the ripening of wood that is essential for good flowering. The absence of fruit debris and the quality of leaf – smaller and less coarse than Catalpa – are additional advantages in metropolitan situations where litter is undesirable.

Only relatively recently has molecular data confirmed that Catalpa and Chilopsis together constitute a well supported clade (Tribe Catalpeae) within Bignoniaceae (Olmstead et al. 2009), but their close kinship – long recognised on the basis of gross morphology, as well as biogeography and shared ploidy (2n = 40 in both genera) – was ample justification for early experiments in hybridisation. The two genera are distinguished from the rest of Bignoniaceae by their simple leaves and unique pollen architecture, and by their common flower and fruit structure: other than leaf shape, in taxonomic terms the most significant difference between Chilopsis and the more derived Catalpa clade is the number of fertile stamens (reduced from four to two in the latter). Even where their divergence might seem to be greatest – in the question of habitat – the desert-dwelling Chilopsis linearis (Desert Willow) is less distant from Catalpa than might at first appear: Chilopsis is a deep-rooting plant that depends on the availability of moisture beneath the arroyos and dry riverbeds where it grows, adapted to cope with the periodic flash-floods that scour these channels. Like its humidity-loving, river-dwelling North American Catalpa cousins, Chilopsis is an uppish riparian plant at heart.

The successful cross between Chilopsis and Catalpa was first brought about successfully in the early 1960s at the Uzbek Academy of Sciences Botanical Garden in Tashkent, Uzbekistan by the geneticist Nikolai Fyodorovich Rusanov, son of the garden’s founder Fyodor Nikolaevich Rusanov (1895–1979). Nikolai Fyodorovich’s experiments with Catalpa and Chilopsis emulated his father’s earlier work on Hibiscus in the 1940s and 50s, which had used North American germplasm to develop cold-hardy hybrids capable of tolerating harsh Central Asian conditions (Elias & Wisura 1991). The younger Rusanov attempted the Chilopsis / Catalpa cross in both directions, obtaining fertile seed in both cases, though crosses with Catalpa as pollen parent resulted in much higher fruit set than the converse (Elias & Wisura 1991). Robert Hebb, of the Cary Arboretum at New York Botanical Garden, brought cuttings of two of Rusanov’s selections to the United States in 1977, this material being subsequently propagated and distributed to botanic gardens and commercially, under the names ‘Pink Dawn’ and ‘Morning Cloud’ (Elias & Wisura 1991; Barnes 2000).

These early hybrid clones were initially referred to the nothospecies ×Chitalpa tashkentensis T.S.Elias & W.Wisura, which was originally described as the product of a cross between Catalpa bignonioides Walter and Chilopsis linearis (Cav.) Sweet (Elias & Wisura 1991). However, subsequent research into the parentage of these cultivars by Li, Shoup and Elias (2006) indicated not only that the female parent was Chilopsis and Catalpa the male, but also – more problematically – that the two clones have different catalpa fathers, rendering a single nothospecific epithet inaccurate: the pollen parent of ‘Pink Dawn’ appears to have been Catalpa × galleana (C. ovata × C. speciosa), and C. speciosa that of ‘Morning Cloud’. The type of ×Chitalpa tashkentensis (Chilopsis linearis × Catalpa × galleana) is ‘Pink Dawn’, but ‘Morning Cloud’ (Chilopsis linearis × Catalpa speciosa) has not been assigned a nothospecies: foreseeing the nomenclatural complexity that might accrete as new cultivars of complex parentage are developed, it was recommended that further nothospecies not be published for ×Chitalpa crosses (Grimshaw & Bayton 2009).

Early experience with ×Chitalpa in the eastern United States showed it to be prone to powdery mildew and leaf spots, particularly in areas of high humidity (Dirr 1998; Olsen, Ranney & Hodges 2006). Dirr (2011) dismissed the species as ‘a mildew farm’, but great strides have been taken in the breeding programme at the US National Arboretum, building on more than two decades of research and experimentation by Richard Olsen and colleagues, who – in the quest for a disease-resistant, long-flowering small tree with broad enviromental tolerances – have brought to bear on this ornamental plant a degree of scientific firepower normally reserved for lucrative crop breeding. Techniques included manipulation of ploidy levels using the chemical oryzalin (rather than the notoriously toxic colchicine) to restore fertility in normally sterile ×Chitalpa by inducing polyploidy in flowering shoots, as well as embryo rescue to faciliate germination of triploid crosses (Olsen 2006).

×Chitalpa tolerates a broad range of conditions, though it will only flower well if the summer is hot enough to encourage flowering on the new growth and ripen the wood. Growth can be very rapid in favourable conditions, but if the new wood is unable to ripen sufficiently, some dieback is likely to occur, and in Zone 6 conditions plants will be killed to the base; they are hardy from Zone 7 upwards. At least one cultivar has demonstrated notable drought resistance, a trait inherited from the Chilopsis parent (see ‘NCXC1’).

In Europe ×Chitalpa is ideally suited to Mediterranean areas, or the western Atlantic seaboard, but even in Britain it shows potential to thrive in more northerly latitudes as the climate warms (a sheltered position in full sun is indicated). In recent hot summers in England, especially during 2025, there have been widespread reports of profuse flowering, doing much to redeem the plant’s earlier reputation for producing little more than ‘a few spindly metres with a thin tuft of growth on top’ (Grimshaw & Bayton 2009).

×Chitalpa does comes with a health warning, being susceptible to a bacterial leaf scorch disease caused by the xylem-living pathogen Xylella fastidiosa, first identified in the species in New Mexico in 2006 and subsequently found to be widespread among cultivated ×Chitalpa in Arizona, New Mexico and California, in both landscape trees and nursery stock (Randall et al. 2006, 2007). The primary symptoms are spotting, chlorosis and scorching of leaves, leading to thinning of the canopy and branch dieback (New Mexico State University 2025), weakening affected trees and shortening their life. X. fastidiosa is one of only three highly specialised pathogens known to exist exclusively in xylem sap – an uncongenial environment for bacteria, being nutrient-poor, subject to constant flow and turbulence, and affording no opportunity for interaction with living plant tissues (xylem vessel elements are dead cells) (Fuente, Navas-Cortés & Landa 2024). The xylem-limited, endophytic character of the pathogen makes treatment with antimicrobials difficult and impractical. The dimensions of the tree pose practical obstacles to potential antibiotic therapies, given the potential dispersion of Xylella infection to all vascular tissue, and there is currently no treatment for the disease beyond supportive cultural interventions to avoid further stressing affected trees. The presence of X. fastidiosa in the xylem sap also means that any infection will be passed on to all vegetative progeny, the disease potentially spreading widely if stock plants are contaminated, as has happened in the southwestern United States. In landscape settings the disease is spread between plants by xylem-feeding bugs (Order Hemiptera). In the US, this is the glassy-winged sharpshooter (Homalodisca vitripennis Germar), but it should be assumed that all sap-feeders are potential vectors (Fuente, Navas-Cortés & Landa 2024).

Genetic analysis of Xylella found in southwestern ×Chitalpa showed it to be distinct from known strains, and it was given the name subsp. tashke, the epithet reflecting the ×Chitalpa host species (Randall et al. 2009). Not yet formally described, it appears closely related to subsp. pauca (the South American strain causing olive decline in Southern Italy) and subsp. multiplex (North American, infecting a broad range of ornamentals in France). Although different Xylella subspecies tend to be associated with particular plant species, or groups of species, information on host range is wanting, as well as an understanding of the molecular basis of host specificity. As Fuente, Navas-Cortés and Landa (2024) put it: ‘the assumption that certain strains are host specific is based on very minimal information that may not reflect their biology’. More to the point is Xylella subspecies’ propensity for hybridisation-like interactions (‘homologous recombination’) involving mutual gene transfer. Such recombination facilitates the emergence of new strains, increasing the likelihood of jumps to new host species – the shift to mulberry and blueberry by X. fastidiosa subsp. morus was a consequence of this phenomenon (Nunney et al. 2014). This problem is compounded by the possibility of silent infections in some hosts, and extended incubation periods during which infected plants remain asymptomatic (Fuente, Navas-Cortés & Landa 2024).

Xylella fastidiosa subsp. tashke has not been reported outside the United States, nor has infection (by any Xylella subspecies) been reported in ×Chitalpa outside North America. Nevertheless, given the continuing phytosanitary emergency precipitated by the outbreak of Xylella fastidiosa in Mediterranean Europe from 2013, it would be wise to be mindful of the genus’s evident susceptibility to infection. Prudent, too, to be aware of the tendency of changes in nomenclature to create ambiguities and loopholes: in the United Kingdom, for example, guidance and legislation framed to apply only to ×Chitalpa tashkentensis (DEFRA 2023) does not cover ×Chitalpa hybrids outside this nothospecies, such as ‘Morning Cloud’.

The susceptibility of the parent species has not been systematically examined, though casual investigation by Randall et al. (2009) found no evidence of Xylella fastidiosa in Chilopsis, and one instance in Catalpa. There are sporadic reports of infection in Catalpa speciosa (the species whose range comes closest to the current focus of ×Chitalpa infection in the United States), but broadly speaking the pathogen does not appear to cause serious disease in that genus.

Cuttings of only two clones were brought back from Tahkent by Hebb in 1977, but given the extent of Rusanov’s breeding programme, it seems possible or likely that other instances of the Chilopsis / Catalpa cross could survive in cultivation in Uzbekistan. Experimental work there involved C. bignonioides, C. speciosa, C. ovata and another unidentified catalpa (perhaps C. bungei (as C. fargesii f. duclouxii); one of Rusov’s crosses using C. bignonioides is reported to have borne fruit – whether fertile or (presumably) not was not specified (Elias & Wisura 1991). There certainly appears to be potential to explore other combinations, say, C. ovata × white Chilopsis, or crosses using forms of C. bungei, C. × erubescens – even, conceivably, evergreen species from sect. Macrocatalpa.