Helianthella castanea, commonly known as Mount Diablo helianthella, is a rare perennial herb endemic to the chaparral and grasslands of the San Francisco Bay Area in California, particularly in the hills east of the bay, including Mount Diablo State Park, Wildcat Canyon Regional Park, Briones Regional Park, Las Trampas Regional Wilderness, and surrounding areas. It grows up to 18 inches tall and has leaves that can reach 6 inches in length. The plant is characterized by its solitary yellow flower head per stem, which blooms from May to July. Each head is composed of both ray and disc flowers, with the ray flowers being particularly showy.

Mount Diablo helianthella is valued for its unique presence in the garden, especially in wildflower meadows and native plant gardens. It is adapted to the Mediterranean climate of its native region, requiring little water once established and thriving in well-drained soils with full sun exposure. While not commonly used in cultivation due to its rarity, it can serve as an attractive addition to pollinator gardens, supporting local bee populations. Care should be taken to avoid overwatering and to provide conditions that mimic its natural habitat. There are no major disease issues, but its habitat is threatened by development and invasive species.CC BY-SA 4.0