Thymophylla acerosa

Thymophylla acerosa is a species of flowering plant in the daisy family. Its common names include pricklyleaf dogweed, prickleaf dogweed and Texas dogweed. It is a small, shrubby plant with needle-like leaves and yellow flower heads, native to the southwestern United States and Mexico, where it grows on limestone outcrops and gypsum soils.

Description

Thymophylla acerosa is a green subshrub or small shrub up to tall. It is usually finely hairy, though sometimes nearly or entirely hairless. Its stems are erect and branch from the base. The leaves are mostly opposite and unlobed, linear to needle-shaped, and long.

The flower heads are borne on stalks up to long, or are nearly stalkless. Each head has a bell-shaped to cylindrical involucre long, made up of about 13 phyllaries and surrounded at the base by five narrow bractlets at least half as long as the phyllaries. There are seven or eight lemon-yellow ray florets, with blades long, and 18 to 25 or more pale yellow disc florets with corollas long. The fruits are cypselae long, each topped by a pappus of about 20 scales long, each scale made up of a bundle of three to five bristles. Chromosome counts of 2n = 16 and 24 have been recorded.

Taxonomy

The species was first described by Augustin Pyramus de Candolle in 1836 as Dyssodia acerosa, in the fifth volume of his Prodromus Systematis Naturalis Regni Vegetabilis. In 1986, the botanist John L. Strother transferred it to the genus Thymophylla, publishing the new combination in the journal Sida.

Distribution and habitat

Thymophylla acerosa is native to Mexico and the southwestern United States, where it has been recorded in Arizona, Nevada, New Mexico, Texas and Utah. It grows on calcareous outcrops and gypsum-rich soils at elevations of about or more. It can flower throughout the year, but mostly does so in summer and autumn.