Rhinotropis lindheimeri aka Shrubby Milkwort
Taxonomy ID: 13334
Rhinotropis lindheimeri, commonly known as Lindheimer's milkwort or shrubby milkwort, is a semi-woody, multi-stemmed perennial shrub in the family Polygalaceae. The plant typically grows to about 35 cm in height while its stems can sprawl horizontally up to 1 meter. It was originally described as Polygala lindheimeri by the botanist Asa Gray in 1850 from specimens collected near the Guadalupe and Pedernales rivers in Texas by German botanist Ferdinand Lindheimer, for whom the species is named. In 2011, J. Richard Abbott reclassified it into the newly erected genus Rhinotropis based on phylogenetic analysis.
The flowers are a striking feature of this plant — typically pink to purple, occasionally white — with a distinctive keel petal that forms a curved sac enclosing the stamens and style, tipped with yellow and terminating in a pointed cylindrical beak. This beak structure gives the genus its name, from the Greek rhinos (snout) and tropis (keel). Unlike true legumes, the fruits are capsules rather than pods.
The species is native to a range stretching from southern Arizona east to south-central Texas in the United States, and south through much of northern and central Mexico, including Chihuahua, Coahuila, Tamaulipas, Aguascalientes, Querétaro, and Veracruz. It grows on rocky substrates — limestone, sandstone, shale, gypsum, and granite — in juniper-oak woodlands, grasslands, desert scrub, thorn scrub, and canyon brush, typically at elevations up to 1,600 meters. Three varieties are recognized: var. lindheimeri (the typical variety), var. eucosma, and var. parvifolia (which extends to higher elevations up to 2,400 m).
Common names
Shrubby Milkwort, Polygala Arizonae, Polygala Arizonae Tenuifolia, Polygala Blepharotropis, Polygala Emoryi, Polygala Lindheimeri Lindheimeri, Polygala Texensis, Polygala TweedyiMore information about Shrubby Milkwort
How big does Shrubby Milkwort get?
Rhinotropis lindheimeri is a compact, semi-woody shrub. Upright stems reach approximately 35 cm (about 14 inches) in height, while the sprawling stems can extend up to 1 meter (about 3.3 feet) in length. It is multi-stemmed and generally low-growing in form. No rapid or vigorous growth rate is indicated by sources; habitat conditions (thin, rocky soils in arid environments) suggest a slow to moderate rate.
Where is Shrubby Milkwort native to?
Rhinotropis lindheimeri is native to the southwestern United States and northern-to-central Mexico. Its US range spans from southern Arizona east to south-central Texas, with records also from New Mexico and Oklahoma. In Mexico, it occurs in Chihuahua, Coahuila, Nuevo León, Tamaulipas, Durango, Zacatecas, San Luis Potosí, Aguascalientes, Guanajuato, Querétaro, Hidalgo, and Veracruz. The type specimens were collected in Texas along the Guadalupe and Pedernales rivers in the mid-19th century.
What soil does Shrubby Milkwort prefer?
In the wild, Rhinotropis lindheimeri grows primarily on rocky, well-drained soils derived from limestone, caliche, sandstone, shale, gypsum, and occasionally granite or igneous rock. These substrates tend to be alkaline, reflecting the limestone-heavy geology of its core Texas and Arizona range. Good drainage appears essential; the plant is not documented in heavy clay or poorly drained situations.
What do Shrubby Milkwort flowers look like?
The flowers of Rhinotropis lindheimeri are typically pink to purple, though white forms occur. The most distinctive feature is the lowest petal (keel), which curves into a sac enclosing the stamens and style, has a yellowish tip, and bears a pointed cylindrical projection called a beak — the feature that gives the genus its name. The blossoms superficially resemble pea-family flowers but differ fundamentally in their petal and stamen arrangement. Fruits that follow are capsules, not the legume pods typical of closely related families.
Are there different varieties of Shrubby Milkwort?
Three varieties of Rhinotropis lindheimeri are currently recognized: var. lindheimeri (the typical variety, found across the main range at elevations up to 1,600 m), var. eucosma, and var. parvifolia (a smaller-leaved form that tends to grow at higher elevations up to 2,400 m and tolerates a somewhat wider range of substrates including clay soils). Most wild and cultivated specimens in Texas belong to var. lindheimeri.
How do I grow Shrubby Milkwort outdoors?
Rhinotropis lindheimeri is suited for outdoor cultivation in the warm, arid to semi-arid climates of the southwestern US and similar Mediterranean-type climates. In the wild it grows on rocky, well-drained substrates — limestone, sandstone, shale, gypsum, and granite — in full sun habitats including grasslands, desert scrub, juniper-oak woodlands, and canyon brush. It is adapted to low-water conditions typical of its native range and is not suited to sustained wet soils. Plants are found at elevations up to 1,600 m.
Is Shrubby Milkwort toxic to humans/pets?
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