Cirsium arvense

It is a tall herbaceous perennial plant, forming an extensive underground root system that sends up numerous erect stems each spring, reaching 1–2 m tall; 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 1–2 cm diameter, pink-purple, with all the florets of similar form (no division into disc and ray florets). The seeds are 4–5 mm long, with a downy pappus which assists in wind dispersal.

It is a nuisance weed in its native area, and a very serious invasive species in many other areas of the world.

Cirsium species, are, for the most part, not toxic; however, the leaves are considered bitter and therefore rendered inedible; the taproot is considered the most nutritious. In Scotland (especially Fife) they are considered a delicacy, albeit a somewhat unusual one.

This plant is extremely difficult to control, as it can regrow from fairly small root fragments and is resistant to many herbicides.

• Pre-emergents can help prevent the plant from becoming established, but it is generally more often encountered as a perennial problem.
• Contact herbicides are only partially effective. Glyphosate in particular needs to be repeatedly applied.
• Mowing is effective over time, if done regularly.
• Goldenrods are allelopathic to this plant. Smothercropping of buckwheat and winter rye will also weaken the plant considerably.
• Pulling must be done repeatedly over a long period, but will eventually kill the plant. Gloves should be worn.
• Smothering techniques such as paper mulch barriers are quite effective.
• Canada thistle should never be added to a cold-composting system. All parts of the plant should be burned, thrown away, or put into high-temperature composting systems.