Cirsium hydrophilum var. vaseyi
Mount tamalpais thistle, Mount Tamalpais Thistle
(c) Arvel Hernandez, some rights reserved (CC BY), uploaded by Arvel Hernandez · licence
Family: Asteraceae · Type: Herb · Native
Conservation status: CNPS 1B.2
Cirsium hydrophilum var. vaseyi (Mount tamalpais thistle) is a native California herb in the Asteraceae family, recognized by Mount Tamalpais serpentine seeps, lower stem branching, larger heads.
Mount tamalpais thistle is a rare (CNPS 1B.2) California native perennial found in northern San Francisco Bay region on Mount Tamalpais in Marin County, specifically within serpentine seeps at elevations of 300 to 450 meters. Flowering from June to September, this thistle produces purple flowers in heads 3 to 3.5 centimeters long with distinctive, densely clustered blossoms. Growing with stems generally branched in the lower half, it develops erect and robust vegetative structures that support its distinctive flowering heads. Its leaves are likely divided and textured, typical of thistles, with potential serrated edges and a rugged appearance characteristic of serpentine habitat plants. The small fruits are oblong or elliptic, measuring 4 to 5 millimeters in length, completing its reproductive cycle in the unique microenvironment of Mount Tamalpais.
Identification and appearance
Key characters: 16 sympatric species; Mount Tamalpais serpentine seeps, lower stem branching, larger heads
Habitat and range
Habitat: Serpentine seeps
Elevation: 300-450 m
Bloom period
Cirsium hydrophilum var. vaseyi blooms Jun-Sep.
Conservation status
CNPS 1B.2
How to tell it apart
vs Carduus: Cirsium has fluffy, feathery seed tufts; Carduus has rough bristles or no pappus and continuous spiny wings down the stem.; vs Silybum: Cirsium leaves are plain green; Silybum has bright white marbling along the leaf veins.; vs Sonchus: Cirsium has thistle-type flowers with spine-tipped bracts; Sonchus has dandelion-like flowers and milky sap.; vs Cynara: Cirsium heads are 1-6 cm across; Cynara heads are massive (4-15 cm, tennis ball sized) with fleshy bases.; vs Centaurea: Cirsium has linear bracts without appendages; Centaurea bracts have fringed or comb-like tips.
Frequently asked questions
Is Cirsium hydrophilum var. vaseyi native to California?
Yes. Cirsium hydrophilum var. vaseyi is native to California (recorded as native).
When does Cirsium hydrophilum var. vaseyi bloom?
Cirsium hydrophilum var. vaseyi blooms Jun-Sep.
What plant family does Cirsium hydrophilum var. vaseyi belong to?
Cirsium hydrophilum var. vaseyi is in the Asteraceae family, genus Cirsium.
Is Cirsium hydrophilum var. vaseyi rare or endangered?
Cirsium hydrophilum var. vaseyi carries California Rare Plant Rank 1B.2, state rank S1.
What habitat does Cirsium hydrophilum var. vaseyi grow in?
Cirsium hydrophilum var. vaseyi grows in Serpentine seeps.
Related species
Other California species in the same genus, family, or bioregion.
Sources: Jepson eFlora · CNPS Rare Plant Inventory · Index of California Plant Names · CalPhotos · CDFW CNDDB
Last updated 2026-10-07
Data from The California Species Project — 14,000+ California species with verified data from CNPS, CDFW, USFWS, Cal-IPC, and more.