Tuesday, January 10, 2012

Botany A to Z: Cirsium

by Karin Decker

is for Cirsium

Cirsiums are thistles, and are members of the sunflower family (Asteraceae). Although the thistle is the national emblem of Scotland, most people probably regard them as unwelcome pest plants. You are probably familiar with troublesome weed species like Scotch thistle, Canada thistle, and musk thistle. These non-native thistles are often able to grow and spread into disturbed environments where their natural predators are absent, and may form large single-species stands that crowd out other plants.

Scotch thistle (Onopordum acanthium), a long way from Scotland.
“What are you looking at me for? I’m not even a Cirsium!”


Thick stand of Canada thistle (Cirsium arvense).
Who wants to go pick up the other end of the measuring tape?

Native thistle species do not typically pose problems because they are co-evolved members of naturally occuring plant communities, and are kept in check by natural processes. Colorado has several dozen native species of Cirsium, and most are fairly common. Unfortunately, all of our native thistles are potentially threatened by being mistaken for non-natives during weed control efforts.

Mountain thistle (Cirsium scopulorum), thriving amidst the talus and columbines.

Barnaby's thistle (Cirsium barnabyi)

CNHP keeps documented location records for three of Colorado’s rarest Cirsium species: Ownbey’s thistle (Cirsium ownbeyi), adobe thistle (Cirsium perplexans), and mountain-slope thistle (Cirsium scapanolepis).

Adobe thistle

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