Tetradymia Genus

Tetradymia, commonly known as horsebrush, is a genus of about 10 species of shrubs native to western North America and northwestern Mexico. It belongs to the tribe Senecioneae within the sunflower family (Asteraceae), and was first formally described by Augustin Pyramus de Candolle in 1838.

Plants in this genus are shrubs typically 30 to 200 cm tall, with one to several erect or spreading stems that are variously tomentose (woolly), pannose (felt-like), or glabrous (hairless), and often armed with spines. The leaves are cauline and alternate, usually lacking stalks, with blades that are linear to filiform (thread-like), narrowly lanceolate, or oblanceolate with entire margins. A distinctive feature is the presence of fascicles (clusters) of secondary leaves in the axils of the primary leaves; primary leaves themselves sometimes harden into rigid spines.

The flower heads are discoid — lacking ray florets entirely — and appear singly or in small groups in the distal leaf axils, or in larger corymbiform clusters of three to eight. Each head bears four to nine bisexual, fertile disc florets with cream to bright yellow corollas. The phyllaries number four to six and are arranged in one to two series. Fruits (cypselae) are prismatic to obconic or fusiform and variously hairy; the pappus, when present, consists of 70 to 150 bristles or 20 to 30 subulate to setiform scales. The base chromosome number is x = 30.