In May, spiny horsebrush (Tetradymia spinosa) was flowering in profusion on the alkali flats along California Highway 299 east of Cedarville CA near the Nevada border (Modoc County). Spiny horsebrush is common throughout the arid regions of North America from the Rockies westward, except it is not found in Washington or Arizona. Although widespread, spiny horsebrush does not occur in pure stands.
A native plant, spiny horsebrush is a low rigid shrub growing from 2 to 4 feet in height. The herbage is covered by a dense, matted tomentum (soft, downy hairs). The plant arises from a taproot. Rhizomes also extend from the parent plant and can sprout to form new plants.
The primary leaves of spiny horsebrush are modified into rigid, usually recurved spines. The secondary leaves are green, narrow and linear, forming clusters in the spine axils.
Also in the axils of the spines are one or two bright yellow flower heads. This member of the Sunflower Family has discoid (composed only of disk flowers) flower heads on short stalks. There are 6 or 7 disk flowers per spiny horsebrush flower head. Each head is surrounded by 4 to 6 overlapping, short, woolly bracts.
Spiny horsebrush fruits are achenes (single dry seeds) with soft woolly hairs and pappus bristles.
Native Americans used the sharp spines as tattoo needles and as piercing instruments.
All parts of spiny horsebrush are poisonous to livestock, especially sheep. Ingestion of the plant can cause liver injury. Pigments from spiny horsebrush can get into peripheral circulation and, particularly in light-colored animals, may result in photosensitization. The acute sunburn and swelling in the head and neck resulting from the photosensitization in combination with liver damage can lead to death in extreme cases.
The genus name, Tetradymia, refers to the heads containing 4 flowers in the first species of the genus to be described (in Latin tetra is four and dymos is together). The spines are the origin of the species name.
Other common names for T. spinosa are catclaw horsebrush, cottonthorn and shortspine horsebrush.