Canada thistle

Encyclopedia Article

Canada thistle              Cirsium arvense (L.) Scop.

Family:  Asteraceae (Composite family)
Life cycle:   Perennial reproducing by seed and rhizomes
Native status:  Native to Eurasia, believed to have been introduced to N. America via crop seed inn 1700s
Habitat:  Pastures, prairies, roadsides, crop fields

General description:   Erect plant up to 4 ft tall, typically found in patches due to spreading rootstalks.  Leaves are irregularly lobed, spininess is highly variable.  Small flowers (<1 in diameter) are pink to purple, occasionally white.  Rosettes in spring are 3 to 4 in in diameter before stems elongate.

Key ID traits: Variably lobed leaves up to 5 in long, occurs in patches due to vegetative reproduction.

Similar species:  Canada thistle is smaller than the biennial thistles, and usually is found in distinct patches.

Miscellaneous:  Canada thistle was the first weed declared noxious in the U.S., given this status by Vermont in 1795.  The plant is dioecious, resulting in colonies of either all male or all female plants.  Canada thistle patches with white stems are commonly observed.  The bleaching is due to infection by a bacterium (Psuedomonas syringae) that has been evaluated as a biocontrol agent (unsuccessfully at this time). History of Canada thistle in Iowa.

The majority of Canada thistle stems originate from vegetative rootstocks rather than seed, producing a rosette in the spring.

Canada thistle plant beginning to elongate in spring.

C. thistle invading a disturbed area from perennial rootstocks.


Thistle heads:  Bull (L), Canada (R). Canada thistle flowers range from white to lavender in color, white is relatively uncommon.


Canada thistle flower heads are produced in dense clusters. A musk thistle is in the background of picture.

C. thistle with bleached branches are commonly seen along roadsides.  The bleaching usually is due to a bacteria -  Pseudomonas syringae pv. tagetis (PST). Efforts have been made to use P.s. as a biocontrol agent, but failed to develop a strain virulent enough to kill the plant.