Psorothamnus is a genus of subshrubs, shrubs, and rarely small trees in the legume family Fabaceae (order Fabales), native to the desert regions of the southwestern United States and northern Mexico. The genus belongs to the tribe Daleeae and was once treated as part of Dalea; many species are still commonly called daleas or indigo bushes.
Plants in this genus are typically intricately branched, often thorny, and strongly aromatic, with stems dotted by conspicuous resinous glands. Leaves are alternate and deciduous, ranging from simple to odd-pinnate with up to eight pairs of linear to oblong leaflets. The flowers are borne in congested or open racemes and are typically bright purple, violet, or blue — a striking display common to the Mojave, Sonoran, and Chihuahuan deserts. The calyx is campanulate and 10-ribbed, with glands visible in the rib intervals. The fruit is a small, 1–2-seeded pod conspicuously dotted with red glands and tipped by a persistent style beak.
The genus ranges from California to Texas in the US and into northern Mexico, with individual species occupying dry washes, sandy desert flats, and rocky slopes. Psorothamnus spinosus (smoke tree) is the most distinctive member, growing as a small tree in sandy desert washes and producing so many flowers it appears to be on fire or emitting smoke. P. fremontii (Fremont's indigo bush) and P. emoryi (Emory's indigo bush) are widespread across Mojave and Sonoran Desert habitats.
The genus was revised by Rupert C. Barneby (1977) and was later found to be paraphyletic in its broader circumscription; genus Psorodendron was reinstated to accommodate several sections (Xylodalea, Capnodendron, Winnemucca), reducing Psorothamnus to its current narrower scope.
Etymology
The genus name Psorothamnus derives from the Greek words for "scurfy" and "shrub," a reference to the gland-dotted, rough-textured appearance of the stems and pods. Many species are informally called daleas, reflecting the genus's former placement within Dalea.
Distribution
Psorothamnus species occur across the Sonoran, Chihuahuan, and Mojave deserts, ranging from California east to Texas in the United States and south into northern Mexico. Individual species occupy dry washes, sandy flats, and rocky desert slopes, typically at lower elevations.
Ecology
Species grow in arid desert environments including sandy washes, gravelly bajadas, and rocky desert slopes. The gland-dotted stems and pods are a defining ecological trait, producing aromatic compounds that may deter herbivores. Psorothamnus spinosus (smoke tree) is strongly associated with desert wash habitats in the Sonoran Desert, where its roots stabilize sandy substrates.
Taxonomy Notes
Psorothamnus was long treated as synonymous with Dalea and its circumscription has shifted through Barneby's 1977 monograph and subsequent phylogenetic work. The genus was found to be paraphyletic in its broader form; Psorodendron was reinstated to receive sections Xylodalea, Capnodendron, and Winnemucca, leaving Psorothamnus in a narrower, more natural grouping within tribe Daleeae, family Fabaceae.
