Canada thistle

Cirsium arvense
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Asterids
Order:	Asterales
Family:	Asteraceae
Tribe:	Cynareae
Genus:	Cirsium
Species:	C. arvense
Binomial name
Cirsium arvense (L.) Scop.

A Creeping Thistle with a "cuckoo spit"

Cirsium arvense is a species of Cirsium, native throughout Europe and northern Asia, and widely introduced elsewhere. The standard English name in its native area is Creeping Thistle.^[1][2][3] A number of other names have been used in the past or in other areas (see below).

It is a tall herbaceous perennial plant growing 30-100cm, forming extensive clonal colonies from an underground root system that sends up numerous erect stems each spring, reaching 1–1.2 m tall (occasionally more); the stems often lie partly flat by summer but can stay erect if supported by other vegetation. The leaves are very spiny, lobed, up to 15–20 cm long and 2–3 cm broad (smaller on the upper part of the flower stem). The inflorescence is 10–22 mm diameter, pink-purple, with all the florets of similar form (no division into disc and ray florets). The flowers are usually dioecious, but not invariably so, with some plants bearing hermaphrodite flowers. The seeds are 4–5 mm long, with a feathery pappus which assists in wind dispersal. ^[3][4][5]The plant also spreads underground using Rhizomes.

There are two varieties: ^[3]

• Cirsium arvense var. arvense. Most of Europe. Leaves hairless or thinly hairy beneath.
• Cirsium arvense var. incanum (Fisch.) Ledeb. Southern Europe. Leaves thickly hairy beneath.

Ecology

A Goldfinch feeding on Creeping Thistle seeds

The seeds are an important food for Goldfinch and Linnet, and to a lesser extent for other finches.^[6] Creeping Thistle foliage is used as a food by over 20 species of Lepidoptera, including the Painted Lady butterfly and the Engrailed, a species of moth, and several species of aphids.^[7][8][9]

Status as a weed

The species is widely considered a weed even where it is native, for example being designated an "injurious weed" in the United Kingdom under the Weeds Act 1959.^[10] It is also a serious invasive species in many additional regions where it has been introduced, usually accidentally as a contaminant in cereal crop seeds. It is cited as a noxious weed in several countries; for example Australia, Brazil, Canada, New Zealand, and the United States. Many countries regulate this plant, or its parts (i.e., seed) as a contaminant of other imported products such as grains for consumption or seeds for propagation. In Canada, Cirsium arvense is classified as a primary noxious weed seed in the Weed Seeds Order 2005 which applies to Canada's Seeds Regulations.^[11]

Control methods include:

• cutting at flower stem extension before the flower buds open to prevent seed spread. Repeated cutting at the same growth stage over several years may "wear down" the plant.
• Applying herbicide

Uses

Electron scan micrography of Aceria anthocoptes, a potential biological control agent of the weed, Canada

Like other Cirsium species, the roots are edible, though rarely used, not least due to their propensity to induce flatulance in some people. The taproot is considered the most nutritious. The leaves are also edible, though the spines make their preparation for food too tedious to be worth eating.^[12]

Other names

Other names include Canada Thistle^[13], Canadian Thistle, California Thistle, Corn Thistle, Cursed Thistle, Field Thistle, Green Thistle, Hard Thistle, Perennial Thistle, Prickly Thistle, Small-flowered Thistle and Way Thistle. The first two names are in wide use in North America, despite being a misleading designation (it is not of Canadian origin).^[14]

References

Wikibooks' A Wikimanual of Gardening has more about this subject: Cirsium arvense

Wikiversity has bloom time data for Cirsium arvense on the Bloom Clock

Wikimedia Commons has media related to: Cirsium arvense

• Defra Website on Weed control
• Lowland Grassland Handbook Chapter 7,available in pdf form from Natural England website