Fouquieria macdougalii

Fouquieria macdougalii is a xerophytic plant endemic to northwestern Mexico, particularly common in the deserts and tropical deciduous forests of the state of Sonora. This member of the genus Fouquieria is known by several vernacular names: Mexican tree ocotillo in English, ocotillo macho, torote verde, or jaboncillo in Spanish. The specific epithet honors the American botanist Daniel Trembly MacDougal (1865–1958); a living specimen cultivated at the New York Botanical Garden provided George Valentine Nash with the decisive material for his 1903 revision of the family Fouquieriaceae.

Fouquieria macdougalii is one of the most easily recognizable species of the genus, owing to a combination of features that set it apart from its congeners. Its bright yellow-green, photosynthetic bark, exfoliating in characteristic papery sheets, is unique within the family and gives the species its distinctive appearance even at a distance. Its clearly arborescent habit, with a true main trunk surmounted by a crown of branching stems, contrasts with the multi-stemmed candelabra of Fouquieria splendens and even with the more compact small-tree habit of Fouquieria diguetii. Its tubular red flowers, blooming mainly in spring, attract a diverse cohort of hummingbirds, making Fouquieria macdougalii a key resource within the Foothills Corridor, one of the three major nectar corridors used by migratory hummingbirds along the western flank of the Sierra Madre Occidental.

The species is also remarkable for its long history of ethnobotanical use, particularly its bark, which produces a saponaceous lather when crushed in water and has been used as a natural soap by the rural communities of Sonora. This local use directly inspired the Spanish vernacular name jaboncillo (literally “little soap”) and the synonym Fouquieria jaboncillo coined by Theodor Loesener in 1911. Fouquieria macdougalii is, in this regard, one of the few Fouquieria species whose practical uses by local human communities are still widely documented and partially practiced today.

Recognizing Fouquieria macdougalii

Fouquieria macdougalii is a deciduous tree, typically reaching 5 to 8 m in height, exceptionally up to 10 m at the most favorable sites. Its silhouette is the most distinctly arborescent of the genus along with Fouquieria formosa: the plant develops a clearly individualized main trunk, 50 cm to 1.5 m tall and up to 30 cm in basal diameter, surmounted by a spreading crown of well-branched stems. This habit gives Fouquieria macdougalii the appearance of a small spiny tree rather than a shrub, distinguishing it immediately from the multi-stemmed candelabra of Fouquieria splendens and even from the more compact small-tree habit of Fouquieria diguetii.

The most distinctive feature of the species is its bark. The trunk and main branches are clothed in a bright yellow-green to greenish-bronze bark, smooth, strongly photosynthetic, and exfoliating in papery sheets ranging from beige to coppery in color. This characteristic exfoliation, which proceeds gradually with age, gives the trunk a particularly graphic appearance, with successive layers contrasting in texture and tint. The yellow-green color is most intense on younger parts and fades slightly with age, but the photosynthetic activity of the bark remains substantial throughout the life of the specimen. This combination of features is unique within the family Fouquieriaceae and constitutes the most reliable identification character at any age.

The branches of the crown are armed with conical, sharp spines of typical Fouquieria type, formed by the woody persistence of leaf petioles after the leaf blades have fallen. The spines are 1 to 3 cm long, somewhat shorter and less rigid than those of Fouquieria splendens.

The species produces two types of leaves, as in all Fouquieria. Primary leaves are simple, alternate, oblanceolate to spatulate, 2 to 5 cm long and 6 to 15 mm wide, bright green and slightly fleshy. Secondary leaves are smaller and emerge in fascicles at the base of each spine after rain events. The leaf size and shape are intermediate between those of Fouquieria splendens (narrower) and Fouquieria formosa (broader and more glaucous).

The flowers are arranged in terminal panicles at the apex of the crown branches, of moderate size compared with the long inflorescences of Fouquieria splendens. Individual flowers are tubular, about 2 cm long, slightly zygomorphic, scarlet to bright red, with stamens slightly exserted from the corolla. The flowering season extends mainly from March to June, with a peak in April and May, and corresponds well to the spring flowering of Fouquieria splendens — a temporal coincidence that has profound ecological implications for the migratory hummingbirds that use the Foothills Corridor (see Ecology and interactions below).

The fruit is an elongate loculicidal capsule containing flat, winged, wind-dispersed seeds.

The chromosome number is 2n = 48, corresponding to the diploid state typical of the genus. No infraspecific taxon is currently recognized by Plants of the World Online. The morphological variability observed across the range is interpreted as ecotypic, in response to climatic gradients between the Sonoran Desert, the tropical deciduous forests of the Sierra Madre Occidental, and the thorn scrub of Sinaloa.

Possible confusion with other species

Fouquieria macdougalii is in general one of the easiest Fouquieria species to identify, thanks to its distinctive yellow-green exfoliating bark. Confusion may nonetheless arise with several arborescent species of the genus, particularly outside the species’ native range or on juvenile specimens that have not yet developed the typical bark.

Distinguishing from Fouquieria diguetii

Fouquieria diguetii, native to the Baja California peninsula and the coasts of Sonora and Sinaloa, shares with Fouquieria macdougalii the arborescent habit with a short trunk and partial caudiciform character. The two species locally meet on the Mexican mainland. Several criteria nonetheless distinguish them without ambiguity. The bark of Fouquieria macdougalii is bright yellow-green, strongly photosynthetic, and exfoliates in characteristic papery sheets. That of Fouquieria diguetii is more brown, sometimes coppery, but never displays the papery exfoliation typical of the Mexican tree ocotillo. The general habit of Fouquieria macdougalii is also more strictly arborescent, with a taller trunk and more branched secondary stems than in Fouquieria diguetii. The flowering period differs as well: spring (March–June) for Fouquieria macdougalii, summer monsoon (July–September) for Fouquieria diguetii.

Distinguishing from Fouquieria splendens

Fouquieria splendens is occasionally confused with Fouquieria macdougalii, especially on juvenile specimens before the latter has developed its true trunk. The distinction is, however, immediate at maturity: Fouquieria splendens produces a strictly multi-stemmed candelabra silhouette of 6 to 100 unbranched primary canes radiating from a compact woody base, with a grey, longitudinally furrowed bark. Fouquieria macdougalii, by contrast, displays a clearly individualized main trunk surmounted by a branched crown, with a bright yellow-green exfoliating bark. The geographic ranges also differ: Fouquieria splendens covers the deserts of the southwestern United States and northern Mexico, while Fouquieria macdougalii is strictly endemic to northwestern Mexico (Sonora, Sinaloa, western Chihuahua).

Distinguishing from Fouquieria formosa

Fouquieria formosa shares with Fouquieria macdougalii the strictly arborescent habit with a true main trunk and a developed crown. The two species are nonetheless allopatric, Fouquieria formosa being centered on central and southern Mexico (Jalisco to Oaxaca) while Fouquieria macdougalii is restricted to northwestern Mexico. Several morphological criteria distinguish them: Fouquieria formosa develops the most massive trunks of the family (up to 40 cm in basal diameter) with grey, longitudinally fissured bark, whereas Fouquieria macdougalii has a more slender trunk and the diagnostic bright yellow-green exfoliating bark. The leaves of Fouquieria formosa are also broader and more glaucous, and the flowering period shifts from spring (Fouquieria macdougalii) to winter (Fouquieria formosa, October to February).

Taxonomy and systematic position

Fouquieria macdougalii was described by George Valentine Nash in 1903, in his foundational paper A Revision of the Family Fouquieriaceae, published in the Bulletin of the Torrey Botanical Club (vol. 30, p. 449–459). The species was named in honor of Daniel Trembly MacDougal (1865–1958), an American botanist affiliated with the New York Botanical Garden. A living specimen of the species, cultivated at that institution, came into flower and provided Nash with the decisive material for the taxonomic revision of the family.

Daniel Trembly MacDougal was one of the leading American botanists of the early twentieth century. After his time at the New York Botanical Garden, he founded and directed the Desert Laboratory of the Carnegie Institution in Tucson, Arizona, from 1906 to 1928, which became one of the world’s leading centers for the study of arid-zone plants. His pioneering ecological and physiological work on desert plants of the American Southwest and northwestern Mexico laid the foundations for the modern understanding of xerophytic adaptations in the family Fouquieriaceae and beyond. The dedication of Fouquieria macdougalii by Nash recognized both his institutional role and his sustained scientific contribution to the study of the family.

According to Plants of the World Online (POWO), the principal synonym of Fouquieria macdougalii is:

  • Fouquieria jaboncillo Loes. (1911)

The name Fouquieria jaboncillo, given in 1911 by Theodor Loesener, refers directly to the local use of the bark as a soap substitute (jaboncillo meaning “little soap” in Mexican Spanish), a use still documented in some rural areas of Sonora. This synonym also reflects the deep ethnobotanical roots of the species in local culture.

The species belongs to the family Fouquieriaceae, monogeneric in the strict sense, placed in the order Ericales (Magnoliopsida). The genus Fouquieria, comprising eleven accepted species, was named in honor of the French physician Pierre Éloi Fouquier (1776–1850).

No subspecies is currently recognized by POWO or by the major international nomenclatural databases (Tropicos, GBIF). The morphological variability observed across the range is interpreted as a simple ecotypic expression, in response to climatic gradients between the Sonoran Desert, the tropical deciduous forests of the Sierra Madre Occidental, and the thorn scrub of Sinaloa.

Phylogenetically, Fouquieria macdougalii belongs to the diploid arborescent group of the genus (2n = 48), alongside Fouquieria diguetii, Fouquieria formosa, Fouquieria ochoterenae, and Fouquieria leonilae. Its precise position within this group remains partially unresolved, but the studies of De Nova et al. (2018) suggest a relatively basal position consistent with its broad ecological niche and its distribution in northwestern Mexico, an area regarded as one of the cradles of the Neogene radiation of the genus.

A xerophyte with distinctive ecological behavior

Fouquieria macdougalii combines the classic xerophilous adaptations of the genus with one of the most original anatomical features of the family: a photosynthetic exfoliating bark that confers a particularly efficient stem photosynthesis and gives the plant its ornamental signature.

Like all Fouquieria, the species displays an opportunistic leafing strategy, with the rapid emergence of leaves a few days after a significant rainfall, followed by their abscission once the soil dries again. The species can refoliate several times per year, in close coordination with the bimodal rainfall regime of its native range (winter rains and summer monsoon). This phenology is, however, slightly more conservative than in Fouquieria splendens, with leaves often persisting longer when conditions remain favorable, particularly in the tropical deciduous forests of the Sierra Madre Occidental.

The most distinctive ecophysiological feature of Fouquieria macdougalii is its bright yellow-green photosynthetic bark, much more developed than in other Fouquieria species. The intense chlorophyll concentration of the bark is visible to the naked eye and reflects an unusually well-developed cortical photosynthesis. This stem photosynthesis allows the plant to maintain an active carbon balance during the long defoliated periods of the dry season. The papery exfoliation of the bark, which proceeds gradually, ensures regular renewal of the photosynthetic surface and prevents the accumulation of corky tissues that would compromise light penetration. This combination of bright pigmentation, regular exfoliation, and substantial photosynthetic activity probably represents the most extreme example of stem photosynthesis within the family Fouquieriaceae.

Like other Fouquieria, Fouquieria macdougalii relies on a shallow but extensive lateral root system, capable of rapidly capturing surface water after a rainfall, combined with a thick cuticle that limits transpirational losses.

Fouquieria macdougalii in the wild

Distribution of Fouquieria macdougalii

Fouquieria macdougalii is strictly endemic to northwestern Mexico. Its range covers mainly the state of Sonora, where it is common, and extends southward to northern Sinaloa and westward to western Chihuahua, following the foothills of the Sierra Madre Occidental. This continental distribution distinguishes it from Fouquieria diguetii, which is more strictly tied to the Baja California peninsula and the immediate coast of the Gulf of California.

The altitudinal range extends from sea level to about 1500 m, with the species reaching its highest abundance between 200 and 1000 m on the western foothills of the Sierra Madre Occidental. Fouquieria macdougalii shows a marked preference for rocky, well-drained substrates: volcanic outcrops, granitic slopes, alluvial fans, and stony tropical-deciduous-forest soils. It tolerates a wide range of substrate textures provided that drainage is excellent, and accepts both moderate slopes and exposed mesa edges.

The species is common across much of its range and integrates into several characteristic plant communities: the Sonoran xerophytic scrubland, the coastal thorn forests of Sinaloa, and most notably the tropical deciduous forest (bosque tropical caducifolio) of the Sierra Madre Occidental, where Fouquieria macdougalii is often associated with Bursera species, Pachycereus pecten-aboriginum, Lysiloma divaricatum, and various arborescent legumes.

The climate of its native range is characterized by very hot, dry summers, mild winters with rare and brief frosts at altitude, and a bimodal rainfall regime combining summer monsoonal precipitation (July–September) and Pacific-derived winter rains (December–February). Annual rainfall is moderate by desert standards, generally between 300 and 700 mm depending on the locality and elevation, allowing the species to develop a more substantial size than Fouquieria diguetii in more arid coastal sites.

Conservation status

Fouquieria macdougalii is not currently considered globally threatened by the IUCN Red List, and its populations remain extensive and locally abundant across much of its range. The species is not listed in any CITES appendix.

In Mexico, Fouquieria macdougalii benefits from the general protection extended to native plant species under NOM-059-SEMARNAT, without specific listing in any risk category. The principal local pressures are agricultural expansion (notably extensive grazing and irrigated crops), coastal urbanization, and tourism development along the Sonora coast. The development of intensive agriculture on the alluvial plains of Sonora and Sinaloa has historically led to the conversion of significant areas of original habitat, but the species’ affinity for rocky, hard-to-cultivate substrates has provided a degree of natural protection.

The collection of adult specimens for the regional horticultural market remains a localized concern, particularly because of the slow growth of the species and the high failure rate of bare-root transplantation, characteristic of the entire genus.

Ecology and interactions

Fouquieria macdougalii plays a major ecological role in the desert and tropical deciduous forest ecosystems of northwestern Mexico, especially as a key resource for migratory hummingbirds.

The species participates actively in the nectar corridors used by migratory hummingbirds between the Sierra Madre Occidental and the southwestern United States. It is notably integrated into the Foothills Corridor, one of the three major nectar corridors used by the rufous hummingbird (Selasphorus rufus) during its northward migration. This corridor, which runs along the foothills of the Sierra Madre Occidental between the tropical deciduous forest and the transition zone to the Sonoran Desert, depends on a small set of key species including Fouquieria macdougalii, Ipomoea arborescens (the morning glory tree), Anisacanthus andersonii, and Justicia candicans. Other hummingbirds also visit its flowers, particularly the black-chinned hummingbird (Archilochus alexandri) and Costa’s hummingbird (Calypte costae).

The synchronization of the spring flowering of Fouquieria macdougalii with the northward migration of hummingbirds illustrates a remarkable case of co-adapted phenology between a desert plant and its main pollinators. The plant offers an abundant nectar source at a critical time of the migratory journey, while the hummingbirds ensure efficient pollination over considerable distances, with pollen transfer between geographically distant populations.

Carpenter bees (Xylocopa spp.) and various solitary bees also contribute to pollination, particularly during the warmer hours of the day and on populations remote from the main migratory corridors. The trunk and stems of Fouquieria macdougalii offer microhabitats for many invertebrates and small vertebrates, particularly in the tropical deciduous forest where the structure of the crown provides nesting sites for several small bird species. The papery exfoliating bark also provides shelter for various invertebrates that take advantage of the spaces between the successive layers.

Cultivation of Fouquieria macdougalii

Fouquieria macdougalii is a moderately rare species in horticulture in Europe but is more widely cultivated in the southwestern United States and in northwestern Mexico, where it is appreciated for its rapid growth, its true tree habit, and the ornamental value of its photosynthetic exfoliating bark. It is one of the Fouquieria species best suited to cultivation in dry Mediterranean climates, on condition that the basic requirements of the genus are respected.

Light and exposure

The plant requires full sun, with no prolonged shading. It tolerates intense Mediterranean and continental summer heat without difficulty but quickly suffers under partial shade, where it grows weakly, flowers sparsely, and develops a less brightly colored bark. Plantings against a heat-reflecting wall, with full southern exposure, give the best results in cultivation in southern Europe.

Substrate

Drainage is the absolute criterion for this species. The substrate must be mineral, low in organic matter, predominantly sandy or gravelly, with a neutral to slightly alkaline pH (6.5 to 8.0). Fouquieria macdougalii tolerates a relatively wide range of mineral substrates, including coarse calcareous soils and volcanic substrates, provided drainage is irreproachable. In the ground, on heavy or water-retentive soils, planting on a raised, well-drained mound dressed with pumice, coarse gravel, or crushed rock is essential. In containers, a mix combining 50% pumice or scoria and 50% standard mineral substrate for cacti and succulents gives excellent results.

Watering

Fouquieria macdougalii tolerates regular watering poorly. During the warm season, a generous watering every six to eight weeks is largely sufficient for an established adult specimen. In winter, watering should be substantially reduced or suspended altogether, particularly in cool, humid climates. The species responds especially well to a moderate summer watering simulating its native bimodal regime, which can be useful for inducing both growth and flowering in cultivation.

In-ground versus container cultivation

In dry Mediterranean climates with mild winters, Fouquieria macdougalii may be grown in the ground in sites well sheltered from frost. Its hardiness is intermediate between that of Fouquieria splendens (the most cold-hardy species) and that of Fouquieria diguetii (one of the least), allowing in-ground cultivation in the warmest microclimates of the European Mediterranean coastline (Ligurian Riviera, Côte d’Azur, Costa del Sol, Algarve). Container cultivation, with overwintering in a cold greenhouse or unheated veranda, remains the safer option in regions with prolonged cool winters. The container should be deep enough to accommodate the root system and equipped with a substantial drainage layer.

Transplanting and acclimation

As with all Fouquieria species, Fouquieria macdougalii tolerates transplantation poorly, especially from adult specimens collected or purchased bare-root. Specimens propagated from seed and grown in containers from the seedling stage establish far more easily than imported plants. The success rate of bare-root adult transplants is comparable to that of Fouquieria splendens, generally not exceeding 40 to 50% even in the species’ native range.

In Mediterranean climates, the principal limiting factor remains the combination of cool winter conditions and atmospheric humidity. Cultivation under a bright shelter, away from winter rainfall and with adequate ventilation, gives much better results than permanent outdoor cultivation in regions with humid winters.

Behavior in Mediterranean climates

In the dry Mediterranean climate of southern Europe, Fouquieria macdougalii shows good performance, on condition that drainage is impeccable, atmospheric humidity is controlled, and frost protection is provided in the coldest winters. The bimodal rainfall pattern of its native range fits naturally with the Mediterranean climate, which favors a relative climatic acclimation. The critical season remains winter, where the combination of prolonged rainfall, cool nights, and atmospheric humidity weakens exposed specimens. Container-grown specimens overwintered under a dry, bright shelter remain the most reliable option for the cooler Mediterranean situations.

Propagation

Seed propagation

Seed propagation is the most reliable method for obtaining vigorous, well-rooted specimens, and the path of choice for the responsible production of this species. The flat, winged seeds do not display marked dormancy. A light scarification or a few-hour soak in lukewarm water improves germination uniformity. Sowing is best performed in spring or early summer, at a temperature of 22 to 28 °C, in a highly drained mineral substrate. Germination generally occurs within two to four weeks when fresh seeds are used.

Seedling growth is moderately rapid for a Fouquieria — among the fastest of the genus along with Fouquieria splendens — and a specimen with a clearly developed trunk and crown can generally be obtained in five to seven years of cultivation. The rapid emergence of the photosynthetic exfoliating bark, often visible by the third or fourth year, gives this species particular appeal for relatively impatient collectors.

Stem cuttings

Stem cutting of lignified branches is theoretically possible in Fouquieria macdougalii but remains irregular and rarely practiced for serial production. Mature fragments must be allowed to callus for several days in a dry atmosphere before being planted in a highly mineral, lightly moistened substrate. Specimens obtained from cuttings tend to develop a less harmonious silhouette than seed-grown plants, particularly with respect to the trunk-and-crown architecture that gives the species its ornamental value. For collectors’ cultivation, seed propagation almost always gives better long-term results.

Diseases, pests, and common problems

Fouquieria macdougalii is overall little prone to parasitic diseases when its fundamental requirements are respected. Almost all problems encountered in cultivation derive from cultural errors — excess water, insufficient warmth, poor ventilation — rather than from specific pathogens.

Root and basal collar rot is the leading documented cause of mortality in cultivation, particularly in cool, humid climates. It manifests as a progressive softening of the basal collar, browning of the internal tissues, and the gradual decline of the crown. Once established, root rot is almost always fatal. Prevention by means of mineral drainage, bright shelter for overwintering, and strict suspension of winter watering remains the most effective strategy.

The papery exfoliating bark, while ornamental and ecologically functional, can occasionally provide refuge to mealybugs and other small phytophagous insects. Regular inspection of the bark crevices and prompt treatment with diluted alcohol or specific products is recommended in greenhouse cultivation. Weakened specimens may also be attacked by various opportunistic molds (Fusarium, Phytophthora).

Cold hardiness of Fouquieria macdougalii

Documented USDA zones

The cold hardiness of Fouquieria macdougalii is intermediate within the genus, lower than that of Fouquieria splendens but slightly higher than that of Fouquieria diguetii. Specialist horticultural sources indicate a USDA zone of 9b to 11, with cold tolerance to about −5 °C for well-established adult specimens in completely dry soil. In the colder upland sites of its native range, particularly on the eastern fringes of Sonora and in the foothills of the Sierra Madre Occidental, occasional minima down to −7 to −9 °C have been reported on adult specimens, suggesting a tolerance somewhat above the conservative nursery ranges typically published.

Tolerance to occasional frost and critical threshold

Adult, well-established specimens, planted on perfectly drained soil, may tolerate brief episodes around −5 °C, provided the frost is nocturnal, short, and accompanied by completely dry soil. Young specimens, recently transplanted plants, or specimens grown on moist substrate are damaged from the very first humid nighttime frosts, sometimes even at slightly above-zero temperatures associated with high atmospheric humidity.

This intermediate cold sensitivity is explained by the biogeographic origin of the species: the natural populations of Fouquieria macdougalii experience mild winters in most of their range, with frosts limited to brief and rare events at altitude. The plant tissues have therefore developed a moderate but real cold-resistance capacity, intermediate between the strictly subtropical species (Fouquieria diguetii, Fouquieria burragei) and the more cold-tolerant northern species (Fouquieria splendens).

Aggravating factors

Several factors substantially aggravate the cold sensitivity of Fouquieria macdougalii:

  • Winter atmospheric humidity, which compounds frost damage and promotes secondary rot.
  • Prolonged frost, which penetrates deeper into the tissues than brief overnight events.
  • Substrate moisture in winter, which dramatically aggravates cold sensitivity.
  • Mode of transplantation: bare-root specimens often take more than a year to rebuild a functional root system, during which their cold tolerance is greatly reduced.
  • Age of the plant: young specimens, with less developed reserve tissues, are markedly more sensitive than adults.

In humid Mediterranean climates such as those of southeastern France, in-ground cultivation is feasible only in the most sheltered microclimates of the coastline, with reliable winter protection. Container cultivation with overwintering under a dry, bright shelter remains the most reliable approach for the majority of European situations.

Traditional and modern uses

Traditional uses

The traditional ethnobotanical uses of Fouquieria macdougalii are particularly well documented in the rural cultures of northwestern Mexico, where the species has been used by various Indigenous and mestizo communities for several centuries.

The most emblematic use is the preparation of a natural soap from the bark. Crushed and mixed with water, the bark of Fouquieria macdougalii produces a saponaceous lather useful for laundering and personal hygiene, an effect linked to the saponin content of its tissues. This use, still practiced today in some rural areas of Sonora, directly inspired the Spanish vernacular name jaboncillo (“little soap”) and the synonym Fouquieria jaboncillo coined by Loesener in 1911. Fouquieria macdougalii is, in this respect, one of the few Fouquieria species whose practical economic use by rural communities is still observed today.

The Indigenous peoples of northwestern Mexico, particularly the Yaqui, Mayo, and Seri, also used the species in traditional pharmacopoeia. The bark and roots have been employed in decoctions against fluid retention, joint pain, and skin disorders, on indications partially overlapping with those of Fouquieria splendens. The flowers, edible and rich in nectar, have been occasionally consumed fresh or dried as an infusion. The straight branches were sometimes used for the construction of light fences and rural enclosures.

The Spanish vernacular name ocotillo macho (“male ocotillo”) reflects the local perception of the species as a more robust and arborescent version of the more familiar Fouquieria splendens, while torote verde (“green torote”) refers to the bright yellow-green color of the bark, similar to that of certain Bursera species (the torotes of the Sonoran Desert).

Contemporary uses and research

Fouquieria macdougalii attracts a renewed scientific interest in several fields. In ecology, the species is a model system for the study of the nectar corridors used by migratory hummingbirds along the western Sierra Madre Occidental. Recent studies have notably documented the dependence of Selasphorus rufus populations on the spring flowering of Fouquieria macdougalii, Ipomoea arborescens, and a few other key species, with implications for the conservation of long-distance migratory routes.

In plant physiology, the bright yellow-green photosynthetic bark of Fouquieria macdougalii offers a particularly interesting model for the study of stem photosynthesis in xerophytic plants. The unusually high chlorophyll concentration of the bark and the regular renewal of the photosynthetic surface through papery exfoliation make this species one of the best candidates for the analysis of mechanisms of cortical photosynthesis in arid environments.

In phytochemistry, the saponins responsible for the soap-like properties of the bark have been the subject of a few targeted studies, but no comprehensive analysis of the phytochemical profile of Fouquieria macdougalii is widely available in the accessible literature. The species likely shares the typical phenolic compounds and specific terpenes of the genus (ocotillol, fouquierol).

Ornamental and xeriscape applications

In landscape design, Fouquieria macdougalii is increasingly used in the xeriscape gardens of southwestern North America, where its tree habit and bright yellow-green exfoliating bark give it a strong sculptural value, complementary to that of Fouquieria splendens and Fouquieria columnaris. The species is particularly appreciated for its relatively rapid growth (compared with the rest of the genus) and for the early appearance of the photosynthetic exfoliating bark, which gives juvenile specimens an ornamental appeal that develops within a few years of cultivation.

The species is cultivated in several reference botanical gardens: Boyce Thompson Arboretum (Arizona), Desert Botanical Garden of Phoenix, Huntington Botanical Gardens (California), San Diego Botanic Garden, Arizona-Sonora Desert Museum (Tucson), Jardín Botánico Helia Bravo Hollis (Tehuacán), and a few specialist European Mediterranean gardens. Its availability on the international horticultural market has gradually improved over the past decade, and seed-grown specimens are produced regularly by specialist nurseries.

FAQ for Fouquieria macdougalii

Can Fouquieria macdougalii be grown in temperate Europe? Yes, in southern France, Spain, Italy, or Portugal, in a dry Mediterranean climate, on condition that three strict requirements are met: full southern exposure, perfectly drained soil, and protection from winter humidity. Fouquieria macdougalii is one of the Fouquieria species best suited to Mediterranean cultivation, second only to Fouquieria splendens. Container cultivation, with overwintering under an unheated shelter, remains the safer option in the cooler Mediterranean situations.

Is Fouquieria macdougalii frost-hardy? Moderately. Established adult specimens tolerate occasional short frosts down to about −5 °C in dry soil, with exceptional reports of −7 to −9 °C on mature specimens in their native upland range. The cold hardiness is intermediate between Fouquieria splendens (the most cold-hardy) and Fouquieria diguetii (one of the least cold-hardy). Winter humidity is more dangerous than cold itself.

Why is the bark of Fouquieria macdougalii yellow-green? The bright yellow-green coloration of the bark of Fouquieria macdougalii reflects an unusually high concentration of chlorophyll in the cortical parenchyma, which makes this species a champion of stem photosynthesis within the family Fouquieriaceae. This pigmentation, visible to the naked eye, allows the plant to maintain an active carbon balance during the long defoliated periods of the dry season. The papery exfoliation of the bark continually renews the photosynthetic surface and prevents the accumulation of corky tissues that would compromise light penetration.

Why does the bark of Fouquieria macdougalii peel in papery sheets? The papery exfoliation of the bark is a characteristic ecological adaptation: it allows continuous renewal of the photosynthetic surface and prevents the accumulation of corky tissues that would compromise light penetration. The exfoliated bark falls in beige-coppery papery sheets that gradually accumulate at the base of the trunk. This feature is unique within the genus Fouquieria and gives Fouquieria macdougalii a particularly distinctive ornamental appearance.

What is the difference between Fouquieria macdougalii and Fouquieria splendens? The most reliable difference is the growth form: Fouquieria macdougalii develops a true main trunk surmounted by a branched crown (small-tree habit), whereas Fouquieria splendens produces a strictly multi-stemmed candelabra without an apparent trunk. The bark of Fouquieria macdougalii is bright yellow-green and exfoliates in papery sheets, whereas that of Fouquieria splendens is grey, longitudinally furrowed, and never exfoliates. The geographic ranges also differ, with Fouquieria macdougalii strictly endemic to northwestern Mexico, while Fouquieria splendens extends widely into the southwestern United States.

Why is Fouquieria macdougalii nicknamed jaboncillo? The Mexican Spanish name jaboncillo (“little soap”) refers to the traditional use of the bark as a natural soap. Crushed and mixed with water, the bark produces a saponaceous lather useful for laundering and personal hygiene, an effect linked to its saponin content. This use, still practiced in some rural areas of Sonora, also inspired the synonym Fouquieria jaboncillo coined by Theodor Loesener in 1911.

Where exactly does Fouquieria macdougalii grow? The species is strictly endemic to northwestern Mexico, with a range covering mainly the state of Sonora and extending southward to northern Sinaloa and westward to western Chihuahua. It is particularly common along the foothills of the Sierra Madre Occidental, where it is integrated into the tropical deciduous forest and the Foothills Corridor used by migratory hummingbirds.

How can I get Fouquieria macdougalii to flower in cultivation? Flowering is more reliable in Fouquieria macdougalii than in Fouquieria diguetii under European cultivation, but still requires a mature specimen (more than five to seven years old), maximum sunlight, moderate but well-spaced summer waterings, and a dry winter rest. Maintaining a consistent seasonal regularity is probably the most important factor for inducing regular flowering.

Can I buy Fouquieria macdougalii in Europe? Yes, the species is offered by several European nurseries specializing in succulent and Mediterranean or desert plants. Availability has gradually improved over the past decade, particularly through international seed exchange programs. Seed-grown specimens are strongly preferred, both for ecological reasons and for a better establishment rate.

Reference websites

Bibliography

  • Nash, G. V. (1903). A revision of the family Fouquieriaceae. Bulletin of the Torrey Botanical Club, 30: 449–459.
  • Loesener, T. (1911). Fouquieria jaboncillo. Verhandlungen des Botanischen Vereins der Provinz Brandenburg, 53: 31–32.
  • Henrickson, J. (1972). A taxonomic revision of the Fouquieriaceae. Aliso, 7(4): 439–537.
  • Henrickson, J. (1969). An introduction to the Fouquieriaceae. Cactus and Succulent Journal (Los Angeles), 41: 97–105.
  • MacDougal, D. T. (1908). Botanical Features of North American Deserts. Carnegie Institution of Washington, Washington D.C.
  • Felger, R. S., Johnson, M. B., & Wilson, M. F. (2001). The Trees of Sonora, Mexico. Oxford University Press, New York.
  • Búrquez, A., & Martínez-Yrízar, A. (2007). Conservation and landscape transformation in northwestern Mexico. Acta Botánica Mexicana, 78: 1–24.
  • Schultheis, L. M., & Baldwin, B. G. (1999). Molecular phylogenetics of Fouquieriaceae: Evidence from nuclear rDNA ITS studies. American Journal of Botany, 86(4): 578–589.
  • De Nova, J. A., Sánchez-Reyes, L. L., Eguiarte, L. E., & Magallón, S. (2018). Recent radiation and dispersal of an ancient lineage: the case of Fouquieria (Fouquieriaceae) in North American deserts. Molecular Phylogenetics and Evolution, 126: 92–104.
  • Eggli, U. (ed.) (2004). Illustrated Handbook of Succulent Plants: Dicotyledons. Springer, Berlin.
  • Shreve, F., & Wiggins, I. L. (1964). Vegetation and Flora of the Sonoran Desert. Stanford University Press, Stanford.
  • Calder, W. A. (2004). Rufous and broad-tailed hummingbirds: Pollination, migration, and population biology. In Conservation of Migratory Pollinators and their Nectar Corridors in North America. University of Arizona Press, Tucson.
  • Nevárez Prado, L. O. et al. (2021). El género Fouquieria: una revisión de aspectos etnobotánicos, fitoquímica y actividad biológica. TecnoCiencia Chihuahua, 15(3): 76–94.
  • Rzedowski, J. (1978). Vegetación de México. Limusa, México.
  • POWO (2026). Plants of the World Online. Royal Botanic Gardens, Kew. Accessed 2026.
  • Govaerts, R., Nic Lughadha, E., Black, N., Turner, R., & Paton, A. (2021). The World Checklist of Vascular Plants, a continuously updated resource for exploring global plant diversity. Scientific Data, 8: 215.