Olneya tesota is a perennial flowering tree of the family Fabaceae, legumes (peas, beans, etc.), which is commonly known as ironwood, desert ironwood, or palo fierro in Spanish. It is the only species in the monotypic genus Olneya. This tree is part of the western Sonoran Desert complex in the Southwestern United States. Description The desert ironwood grows as a bush or tree, reaching heights of about and average trunk diameters of about . Exceptionally, in larger protected washes it can reach greater height and a more massive trunk. In younger trees, the bark is gray, shiny, and smooth; in older trees the bark is broken open. The tree is evergreen, but can lose its leaves if temperatures fall below . In continual drought conditions the leaves will be lost. The leaves are bluish-green and pinnately compound. They are arranged on a petiole, long, with 6–9 leaflets (or variously up to 15, with 7 opposite and one terminal), each measuring . At the base of each pinnate leaf petiole grow two thorns, each about long. Bloom time occurs in late April/May to June. Flowers are of 5 unequal petals, in colors of medium purple, magenta-red, or white to pale pink. Seedpods are long and light reddish brown when seedpods have ripened. Two other species, Parkinsonia florida (blue palo verde) and Acacia constricta (catclaw acacia), have similar light red/brownish seedpods. Catclaw acacia's seedpods are shorter and . Taxonomy The genus name of Olneya is in honour of Stephen Thayer Olney (1812–1878), who was an American manufacturer and botanist with expertise in the genus Carex. The genus was circumscribed by Asa Gray in Pl. Nov. Thurb. page 328 in 1854. Distribution and habitat The species is native to the Southwestern United States and extreme northwestern Mexico in the Baja California Peninsula and the Sonoran Desert. Within Mexico its range includes the states of Baja California Sur and Baja California, on the Gulf of California side east of the cordillera ranges, and Sonora state west of the Sierra Madre Occidental cordillera, in the south approaching the northern border of northern Sinaloa state. In the Southwestern US its range includes the Colorado Desert of southeast Southern California, a part of the Sonoran Desert, and western and southern Arizona. Olneya does not range into the higher-elevation, colder, southeast of Arizona's Sonoran Desert region, nor into the sky islands of the Madrean Sky Islands region. Ecology