Chilopsis linearis (Cav.) Sweet

The sole member of its genus, the Desert Willow Chilopsis linearis is an attractive multi-stemmed shrub with pale pink flowers and willow-like leaves, native to the southwestern deserts of North America and widely cultivated within its natural range. It is commonly planted as a shelterbelt species or for erosion control, its rapid growth and ability to thrive in loose, poor soils recommending it for use during the dustbowl era of the 1930s when vast numbers were planted in the Texas plains (United States Forest Service 1935; Simpson, Hipp & McWilliams 1989). Having naturalised in such areas, it is increasingly grown as a landscape specimen further afield, flourishing best in areas with consistently high summer temperatures, though in its native habitat it is able to withstand significant cold. In the context of a warming climate Chilopsis represents a viable option for planting in locations that might have been regarded as marginal only a few decades ago (cf. Dirr 1998), in Europe as well as the United States.

Chilopsis linearis is typically a plant of dry river courses in the deserts of the southwestern United States and northern Mexico, adapted to hot conditions with low rainfall. While it shows some characteristic xeric traits such as stomata on both sides of the leaves (amphistomaty) (Henrickson 1985), it is really a riparian species that has partially adjusted to seasonal aridity without developing a comprehensive suite of xeromorphic adaptations. Instead, plants have evolved to avoid drought stress rather than tolerate it, shedding their leaves in hot, dry conditions, a tactic that enables them to wait out extended periods of high evapotranspiration and long daylengths. In the Sonoran desert plants typically shed their leaves in June, producing a late flush of new growth in August (Nilsen, Sharifi & Rundel 1984). This strategy of total defoliation may appear counterintuitive (plants growing in resource-poor environments tend to have evolved to conserve hard-won assimilates), but many drought-deciduous species are able to resorb nutrients back into the stems before the leaves are shed (Marchin, Zeng & Hoffmann 2010). A penetrating taproot allows Chilopsis to access groundwater beneath the riverbeds and washes where it grows: rooting depths of 160 cm have been recorded (DePree & Ludwig 1978; misread as 16 m in Turner, Bowers & Burgess 2005). The ability of Chilopsis to produce two flushes of leaves in some years reflects this ability to access moisture at depth, as well as a high photosynthetic capacity and growth rate at moderate temperatures (Strain & Chase 1966; DePree & Ludwig 1978). In cultivation, these characteristics translate into larger, more vigorous trees when given sufficient moisture and less heat.

Three geographically distinct infraspecific taxa are recognised. The type subspecies linearis, native to the Chihuahuan desert of central Mexico, Texas and eastern New Mexico, has straight, moderately short leaves, strongly coloured corollas, and a generally erect-ascending habit, while subsp. arcuata (Fosberg) Henrickson, from the more westerly part of the range – the Sonoran, Mojavean and extreme northwestern Chihuahuan deserts – is characterised by long, distinctively curved, delicately drooping leaves, flowers with lighter corollas, and a more spreading ‘willowy’ habit. Within the type subspecies, the northeastern Mexican populations with densely white-tomentose young stems and larger, more strongly coloured corollas are recognised as var. tomenticaulis Henrickson (Henrickson 1985).

A handful of knowledgeable and enterprising nurserymen from Arizona and New Mexico have done much to explore the natural variation within the species, between them reponsible for the introduction of many of the most successful named varieties. Populations in eastern New Mexico, the most northerly part of the range, which is subject to exceptionally low temperatures (–29°C) in rare ‘polar vortex’ conditions, are the source of some of the particularly hardy cultivars (David Salman, quoted in Aal 2021). At the other end of the hardiness scale, Jones and Sacamano (2000) reported a ‘nearly evergreen,’ more densely leaved and fewer-fruited form within the Chihuahuan subsp. linearis that drops its leaves only after a hard frost (see the cultivar ‘Warren Jones’).

The winter-deciduous state (and dormancy) in Chilopsis is triggered at around 5°C, all leaves being lost in response to the first hard frost (DePree & Ludwig 1978). Given sufficient summer heat and adequate drainage, some selections may exhibit greater cold-tolerance than has often been assumed. The hardiest selections are rated as low as USDA Zone 5 (see the cultivars ‘Conchas Dam Pink’, ‘Hope’ and ‘Lucretia Hamilton’), though Zone 6 (–23°C) is more normal. In central European conditions the cultivar ‘Burgundy’ has withstood –20°C without injury (Havlis 2025). Trees are able to regenerate from the root crown, and in some conditions, at least, are reported to regrow if frozen to the ground (High Country Gardens 2025). Nevertheless, as with many species from arid environments, there is a significant difference between surviving and thriving: position, aspect, shelter and drainage are all factors that need to be carefully considered if wood is to be ripened sufficiently to maximise flowering. Newly planted specimens benefit from protection of the crown by a dry mulch during the first few winters (High Country Gardens 2025). In ideal conditions the species can make a reasonably tall tree: the North American champion in Gila City, Arizona, was 13.7 m tall 1.49 m dbh when measured in 2024 (NCTP 2024).

Chilopsis is bee-pollinated (chiefly by Bombus sonorus, other visiting species being ineffectual pollinators) (Richardson 2004), and has the foliar nectaries found in most species of Bignoniaceae, which provide food for ant species (Pemberton 1988) in exchange for protection from other herbivorous insects (for an insight into the complexity of these mutualistic relationships elsewhere in the family see the discussion of extra-floral nectaries under Catalpa speciosa). Curiously, in view of the bee-pollination syndrome, Sargent (1894) describes the flowers of Chilopsis as ‘exhaling at night the odor of violets’, a characteristic more commonly associated with pollination by night-flying moths (one of the closely related American catalpas has evolved to cater to multiple – diurnal and nocturnal – pollinator guilds, a possibility that appears not to have been investigated in Chilopsis).

There is some question as to the extent of the species’ natural range, which is sometimes held to extend into Kansas and Oklahoma on the basis of the county-based survey data in the NAPA and PLANTS databases (Kartesz & Biota of North America Program 2015; Natural Resources Conservation Service 2025), though this incursion into the marches of the Midwest is likely attributable to naturalisation resulting from the 1930s shelterbelt project: Chilopsis is generally held to be unsuited for true Midwestern conditions (High Country Gardens 2025). On the other hand, the PLANTS record of native Chilopsis in Georgia is clearly spurious, evidently reflecting escapes from cultivation.

Like many riparian species, young Chilopsis plants are fast-growing in their native environments, putting on 1 m of growth annually, considerably more given sufficient moisture (Kraus 1998). The rather gangly habit and weak wood means that pruning is advisable to form a well-structured tree. Flowers are produced on new wood, so the timing of pruning is not critical. Trees tend to retain dead twiggy growth, and may benefit from periodic deadwooding (Creech 2016).

Chilopsis is self-incompatible and an obligate outcrosser (Richardson 2004). Propagation is by seed, or by semi-hardwood (late summer) or hardwood (winter) cuttings (Row 2014). As might be predicted for a species of river banks and mobile soils, mound-layering is also a practical technique (Dreesen & Harrington 1997). While seed-grown trees are often sold in the southwestern United States, for the purposes of horticulture (rather than conservation) numerous named cultivars have been selected for flower colour, habit and hardiness. Some of the most recent selections have been bred to minimise or eliminate fruiting, since the overwintering fruits tend to produce an unkempt, untidy effect, and volunteer seedlings and fruit debris may represent a nuisance. The importance of flower colour in distinguishing between cultivars causes problems, since colour descriptions often vary markedly between observers (the category of ‘pink’ is particularly troublesome).