Thorn skeletonweed (Pleiacanthus spinosus) belongs to a monotypic genus, that is, there is only only species in the genus. It is called a subshrub because, although it resembles a shrub, thorn skeletonweed does not produce woody tissue. A member of the Aster Family, this native perennial is found below 8,000 feet in Oregon, California, Arizona, Nevada, Utah, Idaho and Montana. It grows in mostly dry habitat, such as gravely washes, slopes and sagebrush scrub.
Several slender stems arise from the thorn skeletonweed’s caudex (rootstock) topping a taproot. The stems divide many times into short, rigid branches and can grow to about 3 feet in height. The branches narrow into sharp, thin thorn tips. At ground level thorn skeletonweed has light-brownish, woolly tufts. otherwise the plant is hairless. All the herbage contains a milky sap.
The alternate thorn skeletonweed leaves are small and linear on the lower stem and are reduced to scaly growths on the upper branches.
Thorn skeletonweed flowers occur singly near the ends of the branches. The flower head consists of three to five purple-pink ray flowers. The cylindrical flower head is wrapped in two series of phyllaries (modified calyx). The ligule (strap part of the ray flower) is squared and notched on the top.
The fruits of thorn skeletonweed are brownish achenes (dry and contain a single seed) topped by fifty or more light tan, minutely barbed pappus bristles.
Other colloquial names for P spinosus include spiny skeletonweed, thorny skeletonweed, wirelettuce and prickly wirelettuce. The scientific name emphasizes the thorny nature of thorn skeletonweed. Pleiacanthus means “many thorns” and is from the Greek – “pleios” for many and “akantha” for thorn. The species name is from Latin and also means “thorny”. Thorn skeletonweed formerly belonged to the genus Stephanomeria and is often still seen in the literature as S spinosus.
These thorn skeletonweed specimens were growing in July amid rocky outcrops and mine tailings along Cold Springs Road north of Reno NV.