Please try our next iteration of the Michigan Flora Online here. Beginning on February 1, 2023, michiganflora.net will point to this new site.
The new site offers several benefits over the existing website, including real coordinate mapping, giving a clearer view of the density of documentation as well as more precision about plant distributions and their link to landforms. We will also have the ability to update species pages more regularly, both in terms of new collections and as more existing Michigan specimens are georeferenced. In addition, we have a better photo display, and offer indented keys.
Coefficient of Conservatism:
Coefficient of Wetness:
CANADA THISTLE, FIELD THISTLE
B. S. Walters
Found at roadsides, ditches, railroads, fields, clearings, shores; disturbed swampy ground but not usually in very wet places; disturbed sites including filled land, parking areas, utility rights-of-way, dumps, neglected gardens, etc.
Widespread across Eurasia and northern Africa; introduced into North America before 1800. D. Cooley collected Canada thistle in Macomb Co. in 1851, noting “This plant has not yet become troublesome here, but is increasing.” This is a variable species, although most Michigan specimens are the spiny, glabrous var. horridum Wimmer & Grab. as described in the key. A few collections are var. arvense with the upper leaves unlobed and with small spines (up to 2 mm long). We have two collections, from Lenawee Co. (R. W. Smith 747, 4047, MICH), with leaves densely tomentose beneath, at most shallowly lobed, and also with small spines; such plants have been called var. vestitum Wimmer & Grab. White-flowered plants are rare, but have been collected in Baraga, Cheboygan, and Emmet Cos. [f. albiflorum E. L. Rand & Redfield]. This species is dioecious. Plants with only staminate flowers will not produce viable seed (although they may have a reduced pappus).