Canada Thistle

Cirsium arvense L.

Asteraceae (Sunflower Family)

▲ ▼ seedlings

▲ ▼ young shoots from creeping roots

▲ ▼ abundant, deep, fleshy creeping roots

▲ ▼ flowering plants are relatively short, and have thin, upright, slightly branched stems and small inflorescences (usually 1" diameter or smaller); the following flowering plants from around the U.S. show an array of flower colors and leaf characteristics

▲ ▼ flowering plants

▲ ▼ large 30+ foot diameter clonal colony of Canada thistle in Minnesota

▲ ▼ inflorescences/flowers

▲ inlforescence in bud and flowering

▲ ▼ closer view of inflorescences/flowers

▲ ▼ inflorescences after flowering and beginning to release seed

▲ ▼ fruit of Canada thistle

Cirsium arvense (L.) Scop., Canada thistle: (Bayer Code: CIRAR; US Code CIAR4)

A creeping perennial noxious weed in mostly northern U.S. (northern Midwest and northern Missouri) that can produce dense, large colonies
Is a short thistle, growing from 6 inches to 4 feet tall at flowering, averaging around 2-3 feet tall, often of varying heights in colonies
Usually does not have leafy wings on stems
Leaves can be quite variable, from nearly hairless to quite hairy; leaf undersides can be green to white; leaf margins usually lobed, with spines along margins; margins may be flat or wavy-edged
Flower heads are at tips of stems in panicle-like arrangements
Individual flower heads are smaller than many thistles, usually 0.5 to 1 inch diameter, and disk flowers are pinkish-purple (occasionally white)
Bracts below flower head are not cobwebby and do not have spines, outer bracts taper to long tip
Stems are solid, and can be green to purplish-brown, usually ridged, not winged
Creeping roots are fleshy and easily broken, allowing spread by tillage; roots can go to about 10 feet deep, sometimes deeper; 60% of roots are over 2 feet deep
Creeping roots can spread horizontally 10-20 feet diameter per year
Their wide genetic variability produces a wide range of susceptibility in response to herbicide applications
Tends to be dioecious, with separate male and female plants
Produces many seeds which can remain viable for over 20 years
Seedlings grow slowly at first, produce creeping roots at 7-9 weeks
Shoots from creeping root shoots emerge in March-May
Blooms May-August
Inactive after bloom until late summer to early fall, they put on new growth up into November
Various biological control agents are being evaluated for its control--a gall fly, a stem-mining weevil and a seedhead weevil
Can distinguish Canada thistle from most other thistles (tall thistle, bull thistle, musk thistle or field thistle) by its small stature and flower size and that it often appears in colonies due to its vast creeping root system

Some white flowering forms of Canada Thistle:

(Updated January 23, 2019)

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