Mt. Diablo Helianthella

Helianthella castanea is known only from the San Francisco Bay area.

Plants 15–45 cm. Cauline leaves: largest proximal to mid stems; blades sometimes ± 3-nerved, oblong or elliptic to broadly-oblanceolate, 10–15 cm, faces sparsely hirsute (mostly abaxially and near margins). Heads borne singly, erect. Involucres hemispheric, 25–40 mm diam. Phyllaries (usually outer 3–4+ foliaceous, 3–10 cm, curling over heads) lanceolate to lance-oblong (margins usually ciliate, abaxial faces densely pubescent or glabrate). Paleae chartaceous, relatively firm. Ray florets 12–21; laminae (10–)20–30 mm. Disc corollas yellow. Cypselae weakly compressed (plump in middles, edges thin, faces, glabrous); pappi 0 or of 2 ± subulate scales. 2n = 30.

Helianthella castanea is a rare plant endemic to California, and is found only in the San Francisco Bay Area, mostly in the hills east of the bay, including in Mount Diablo State Park, Wildcat Canyon Regional Park, Briones Regional Park, Las Trampas Regional Wilderness, and surrounding areas. Its common names include Mount Diablo helianthella, Mount Diablo sunflower, and Diablo rockrose.

Helianthella castanea is a herbaceous plant up to 45 cm (18 inches) tall. Leaves are up to 15 cm (6 inches) long. The plant usually produces one yellow flower head per stem. Each head contains both ray flowers and disc flowers.