Most people don't grow thistles as ornamentals but they could and you have to admit the flower is nice and is very attractive to bees and butterflies, but if you do want to grow them, this one should be avoided. It and its aggressive cousin the Bull Thistle, Cirsium vulgare, are imports from Eurasia and have almost ruined the reputation of the other species of Cirsium.
Stems: Plants can reach be 1 to 4 feet in height with occasional branching on the upper stem. Stems are light green, ridged, and usually without a lot of hair. The plant is usually perennial.
Leaves: Leaves are alternate, lanceolate (broadest below the middle) in shape with indentations on the margins or almost cut to the central vein (pinnatifid), up to 8 inches long but only about 1 inch wide. They either touch the stem or clasp it, but some longer lower leaves may taper to a short stalk-like base. The leaf has hard white needle-like prickles on the margins which point in all directions; the surface is smooth and green on the upper side, the lower side may have some fine hair giving it a whitish appearance.
The floral array consists of single or multiple flower heads on erect stems branching from the top of the plant. These form somewhat flat-topped clusters. There is a small green leafy bract at the base of the cluster.
Flowers: Most plants of this species are usually dioecious, that is, the male and female flowers occur on separate plants, some may have perfect flowers. C. arvense is the only species of Cirsium in North America from the Great Plains eastward that is dioecious. The flower heads are only up to 1/2 inch wide and are densely filled with small pinkish 5 lobed tubular disc florets. The staminate florets have shorter tubes than the pistillate florets. Styles of the pistillate florets are greatly exserted and have a short divided tip. Five stamens are grouped in a column in the staminate florets, and if the florets are perfect, then they surround the style. The outside base of the flower head has a 6 to 8 series of phyllaries that are green to purple in color and are tightly appressed with sharp pointed purple tips that are slightly spreading, but no spines. The phyllaries have a center ridge line, like a small keel. Florets have a fragrance somewhat resembling Lilac.
Seed: The flowers mature to a dry cypsela, 2 to 4 mm long, which has light brownish-white pappus for wind dispersal.
Habitat: This invasive plant forms large colonies that spread outward in a circle from creeping horizontal roots and by deep underground lateral shoots from the taproot itself. These jointed rhizomes lie deep enough underground that they are usually avoid dryness in the upper soil. It has been known to expand as much as 10 to 12 feet in one season. A small section of root can form a new plant. Seeds easily germinate within a year and are viable in soil for 20 plus years.
Bleached tops: Members of the Aster family, but particularly Canada Thistle, can be infected with a bacteria called Pseudomonas syringae pv. tagetis, PST for short. The visual effects are a bleached top on the plant, which reduces development of the plant with reduced flowering and seed production. Wet springs tend to increase the presence of the bacteria - natures form of pest control. Here is a 2014 article about the disease.
Names: An older scientific name for the plant is Carduus arvensis. The genus Cirsium is from the Greek word kirsion - for a particular thistle and has been adopted for a number of plants generally not appreciated by gardeners. The species name arvense means 'growing or pertaining to cultivated fields.' The author name for the plant classification are: First to classify was '(L.)' which refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. His named the plant Serratula arvensis in 1753. His work was updated in 1772 to the current name by ‘Scop.’ which refers to Giovanni Antonio Scopoli (1723-1788), Tyrolean naturalist who studied and published on plants, insects, animals and birds of the Tyrol. The genus Scopolia is named for him.
Comparisons: Here is a comparison guide to the 7 thistles found in Minnesota.
Above: The outside of the flower head has a series of of phyllaries that are green to purple in color and are tightly appressed with sharp pointed purple tips that are slightly spreading, but no spines. Drawing from Dr. Otto Thome, courtesy of Kunt Stüber's Online Library.
Below: 1st photo - Canada Thistle plants are not more than 4 feet tall, frequently less. 2nd photo - Flower heads can be single or in small clusters. 3rd photo - Leaves touch the stem and sometimes begin to clasp it. Note the ridges on the stem.
Below: 1st photo - Flower heads are about 1/2 inch wide, stalked, with a small leafy bract at the base of a cluster. Flowers in a cluster are closely spaced. 2nd photo - Flower heads have many small 5-lobed disc flowers, pistillate flowers shown here.
Below: 1st photo - a flower head of staminate florets in pollen stage. 2nd photo - staminate floret. 3rd photo - pistillate floret.
Below: 1st photo - Lower stem leaf with indents that are cut almost to the central rib (Pinnatifid). 2nd photo - Upper stem leaves are not as deeply lobed.
Below: The flowers mature to a dry cypsela which has light brownish-white pappus for wind dispersal. Note in 1st photo that the darker color of the tips of the phyllaries is still noticeable as the seed head opens.
Below: This photo shows how Canada Thistle can form large colonies that crowd out other plants.
Below: An example of a plant affected by the bacteria Pseudomonas syringae pv. tagetis which weakens the plant and reduces reproduction. The whitish bleached top is characteristic of the infection.
Notes: Eloise Butler had catalogued four thistles in her plant index as present in the Garden area. Canada Thistle was not one of them. But, by the time Martha Crone made her 1951 Garden census, Canada Thistle was present. While not native to the state, it is found in 90% of Minnesota's counties and in North America it is everywhere except certain parts of Canada's far north and the gulf coast of the U.S. Like most ecological stowaways it probably began to arrive with the earliest settlements in the 1600s. While named for Canada, it is a native of Europe and Northern Asia.
Noxious: Minnesota lists Canada Thistle on the "Prohibited noxious weeds list" and in similar such restrictive lists in 43 States. As early as the 1890s most states had weed laws that covered this plant, but as Ada George writes in 1914: "Therein it is shown that all but three of the states having laws for the suppression of weeds make it an offense for their citizens to permit the Canada Thistle to mature and scatter its seeds. Penalties are also provided in the case of seedsmen who sell grain, grass, or clover seeds contaminated by its presence - but the thistle marches on, bidding defiance in every prickle to such attempts at its extermination. The laws are very good but enforcement is neglected." (Ref. #6b)
Eradication can be done by repeated pulling but late spring burns are most effective. Chemically, a spot application of glyphosate works.
Eloise Butler wrote about Thistles: "The Field Thistle, Cirsium discolor, is particularly lovely by reason of its pale pink, or sometimes white flowers, and long, drooping leaves. The Bull Thistle [Cirsium vulgare] has larger heads and still more formidable prickles; while the tall Swamp Thistle is less stout and spiny. [C. muticum Michx.]. These species are not undesirable for a garden, if one has space enough to keep them at arm’s length. But no good word can be said for the Canada Thistle, an emigrant from Europe that multiplies apace, although allowed no rights of citizenship. It seems useless to legislate against it; for it has a running root stock that spreads while we sleep, and the seeds fly over the country to sow discomfort everywhere. It is a pest because it is so difficult to keep within bounds. If you wish to know just how Theophilus Thistlewaite thrust three thousand thistles through the thick of his thumb (too low an estimate by far!), clear by hand a plot of land that has been overrun by Canada Thistles." Published August 27, 1911, Minneapolis Sunday Tribune
Thoreau wrote in his journals: "What is called the Canada Thistle is the earliest, and the goldfinch, or thistle-bird, (Carduelis tristis), for he gets his name from his food (carduus being the Latin for ‘a thistle’, knows when it is ripe sooner than I. So soon as the heads begin to be dry, I see him pulling them to pieces, and scattering the down; for he sets it a-flying regularly every year all over the country, just as I do once in a long while. The thistle seed would oftener remain attached to its receptacle till it decayed with moisture or fell directly to the ground beneath if this bird did not come like a midwife to release it -- to launch it in the atmosphere and send it to seek its fortune, taking toll the while by swallowing a few seeds."
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"