Cirsium arvense (weed)

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2 bees on a creeping thistle (Cirsium arvense) (click to enlarge the image)
Author: Richard Bartz
Source:Wikimedia Commons

Cirsium arvense (weed) (Linnaeus) Scop. - (Canada thistle)

This is an important weed in North America and Europe which affects crops like cereals, soybean or alfalfa. It is also a serious environmental weed in various open habitats and is common in rangelands and pastures. The species is believed to be native to the Mediterranean region and western Asia, but now has a nearly worldwide distribution. It forms dense stands which cause substantial crop losses. At a density of 10-20 shoots per m2 yield losses have been estimated at 10-20% in wheat and approach 50% in alfalfa. Additional damage results from contaminations by the seeds. In autumn, dried plants become a fire hazard.

Canada thistle spreads through its seeds which disperse by wind and farm equipment as well as through its horizontal rhizomes (up to 6 m in diameter). Female plants can produce 2-5 thousand seeds. These survive in the soil for up to 20 year. Control can be difficult, due to the extensive root system of the plant and its ability to reproduce from small root fragments. Management options include mowing, controlled burning, frequent cultivation of the infested area and the use of herbicides.

Vernacular names
• Deutsch: Acker-Kratzdistel
• English: Canada thistle
creeping thistle
Californian thistle
• Español: cardo cundidor
• Français: chardon des champs

The plant grows to a height of 50-150 cm. It is diecious, which means male and female flowers occur on separate plants. The leaves are glossy and have irregular lobes with strong spines at the tips, up to 5 mm long. Stems and flowerheads are not spiny. The flowerheads are small (compared to other thistles) and have a diameter of 1-2 cm, with pinkish (sometimes whitish) to purple flowers. The seeds are 4-5 mm long with a large feathery pappus.

For details see the respective page in BugwoodWiki.