Flora and fauna of the Athens County area, and occasionally habitats outside Ohio. Subject matter will consist of both interpretive material and taxonomic issues in plant and animal identification.

Friday, September 17, 2010

Thistles do not tickle

While the bloom itself is inviting, the flower head, stem, and leaves can be quite irritating. Here is my key to separating some of the common thistles in Ohio.

1a) Main stem covered with prickly spines, either growing directly out of the stem, or on leaf-like wings. (All spiny species are non-native)...2
1b) Main stem, for the most part, without prickly spines...3

2a) Flower head large, several inches in diameter. Phyllaries (bracts) broadly shaped and sharp. Flower usually bent or nodding...Nodding Thistle, Carduus nutans
2b) Flower head smaller than above, bloom growing erect, bracts thin, sharp but needle-like...Bull Thistle, Cirsium vulgare

Nodding Thistle, surrounded by a multiple crown of thorns. Not true thorns of course, but still sharp. Recognized by the way the flower leans to the side instead of growing upward. It has a very large head, and when driving by they look like purple sunflowers. This has become highly invasive in northern Ohio.

The needle shaped spines are quite obvious on Bull Thistle
Here is an example of the spiny stems the exotic species have. All business and no bull from Bull Thistle.

3a) Most leaves moderately lobed or completely unlobed...4
3b) Most leaves deeply lobed, dissected, or divided...5

4a) Plant 2-3 foot tall, flower heads numerous but small, one inch or less across...Canada Thistle, Cirsium arvense
4b) Plant 3-10 feet tall, flowers over one inch, leaves pale beneath...Tall Thistle, Cirsium altissimum

Small flowered but plentiful. Canada Thistle can dominate an open field in a short time. This is the one exotic species that has a smooth stem.

Tall Thistle is easy to recognize by the usually unlobed leaves.
Tall Thistle? Here's a ten footer.

5a) Plant 2-3 ft. tall, flower heads 1" or less, plant often in clumps...Canada Thistle. (pictured above) So why did I put it in the key again? Because often portions of the plant have divided leaves.
5b) Plants 3-8 feet tall, flowers over one inch, leaves white wooly beneath...6

6a) Upper leaves surround flower head, protruding spines obvious on flower head, common in disturbed sites... Field Thistle, Cirsium discolor
6b) Few to no leaves around flower head, protruding spines absent, flower head covered in spider web like fuzz, stems hollow, common in wetlands...Swamp Thistle, Cirsium muticum

Spines on Field Thistle are distinct
Notice how the last group of leaves surrounds the flower head
Both Field and the next species have this white fuzzy underside.
Flowers of Field Thistle can be pink or light purple. Another common name is Pasture Thistle.

Notice the lack of leaves under the flower head, and that the spines are appressed up against the flower head and do not protrude outward. This is Swamp Thistle.

This one stumped me. I originally thought swamp thistle, but the flower bloom is kind of strange. The stem seems to indicate an exotic species. The spines were extremely numerous. I am wondering if this might not be the Spiny Plumeless Thistle, Carduus acanthoides, or a hybrid mix.  While shot in Michigan, this species is known from several counties in Ohio.  I often post unknowns in hopes I'll get a response at some point.  Update:  I want to thank Scott Namestnik from 'Get Your Botany On' for viewing this and informing me this is most likely European Marsh Thistle, Cirsium palustre, an invasive known to occur in upper Michigan.


  1. This is one of many very useful posts on this blog Thank you!

  2. This is one of many very useful posts on this blog Thank you!

  3. I have photos from two different Ohio thistle plants from the same general area. The first is pretty much spine free on the stem, but the leaves are very very spiny and very lobed. I concluded, from your page, that it was the Canada or Creeping thistle (Cirsium arvense.) The second plant had the same kind of flower structure, and the stem was the same, but the leaves were only slightly thorny and not generally lobed. I cannot seem to identify it. Since this post is from 2010, I do not know if you still monitor the page. I am hoping you do. Thank you, Karen