Cirsium arvense
Canada thistle
Family: Asteraceae · Type: Herb · Not Native
Conservation status: Cal-IPC Yes
Cirsium arvense (Canada thistle) is a non-native California herb in the Asteraceae family, recognized by colonial habit from runner roots and dioecious flowers.
Canada thistle is a naturalized perennial herb found in northwestern California, the Cascade Range, Sierra Nevada, San Francisco Bay Area, northern and southern coastal regions, and the Great Basin in disturbed areas at elevations below 2,450 meters. Flowering from June to September, this plant produces purple (occasionally white or pink) flowers in rounded or flat-topped clusters with heads 1 to 2 centimeters wide. Growing with colonial stems 50 to 100 centimeters tall, it spreads through runner roots and has green to gray-tomentose herbage. Its leaves are 3 to 30 centimeters long, mostly along the stem, with spiny edges and gradually reduced in size toward the stem tip. The plant forms dense colonies in disturbed landscapes, with each stem producing multiple flower heads and spreading aggressively through underground root systems.
Identification and appearance
Key characters: 39 sympatric species; colonial habit from runner roots and dioecious flowers are diagnostic
Habitat and range
Habitat: Disturbed areas
Elevation: < 2450 m
Outside California: North America, native to Europe
Bloom period
Cirsium arvense blooms Jun-Sep.
Conservation status
Cal-IPC Yes
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 arvense native to California?
No. Cirsium arvense is not native to California (recorded as not native).
When does Cirsium arvense bloom?
Cirsium arvense blooms Jun-Sep.
What plant family does Cirsium arvense belong to?
Cirsium arvense is in the Asteraceae family, genus Cirsium.
Is Cirsium arvense invasive in California?
Cirsium arvense is listed by Cal-IPC with an invasiveness rating of Yes.
What habitat does Cirsium arvense grow in?
Cirsium arvense grows in Disturbed areas.
Related species
Other California species in the same genus, family, or bioregion.
Sources: Jepson eFlora · Index of California Plant Names · CalPhotos
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.