Common Name: Canada Thistle, California thistle, Corn thistle, Creeping thistle, Cursed thistle, Green thistle, Hard thistle, Perennial thistle, Prickly thistle, Small-flowered thistle, Way thistle - The many sharp spines of the thistle are the basis for the etymology of the name; thistle is thought to originate from the Sanskrit word tejate, which meant "it is sharp."
Scientific Name: Cirsium arvense - The generic name is from the Greek word for thistle kirsion which in turn is derived from kirsos, which means "swollen veins." Thistles were at one time used to treat a medical condition that resulted in the veins being enlarged. The species name arvense is from the Latin arvus which means "plowed land" or "field." This is to indicate the habitat of the thistle.
The Canada thistle is native to Europe, Asia and North Africa and is invasive to North America. The most frequently applied common name of Canada thistle is thus a misnomer; its perceived North American provenance may have been from northern habitations so that it appeared to be more prevalent there by those who noticed its spread southward and named it accordingly. It was probably introduced as a seed contaminant early in the colonial period; by the middle of the 20th Century, it had been declared a noxious weed in forty-three states. It has long been considered an economic problem for agricultural interests but has only more recently been considered as a more widespread problem in crowding out native species.
The ubiquity of the Canada thistle and its consequent designation as a noxious weed is due to its propagative success and due to the fact that it is a perennial, unlike most thistles which are biennial. There are several additional factors that account for its spread. It is a prodigious seed producer in spite of the fact that the male and female flowers are on different plants and must be pollinated by insects or the wind. Each thistle flower can produce over 600 seeds that mature in about a week after first flowering and are subsequently widely dispersed by the wind that carries them away due to a tan-colored bristled plume called a pappus that extends from each seed. On distribution, the seeds can remain viable for up to 20 years to allow for wide variance in the weather conditions necessary to promote germination.
When the seeds germinate, they send out a taproot that extends downward to take advantage of low water table conditions. Once established, the original plant sends out an anastomotic system of horizontally spreading underground roots that can extend up to 3 feet underground. Root extension is punctuated by shoots that sprout at regular intervals to create dense stands of erect stems. Since the plant can regenerate from a root fragment as small as an inch in length, the Canada thistle is almost impossible to eradicate once it is established. A single plant can multiply to cover over half an acre in just 3 years.
The Bull Thistle (C. vulgare – from the Latin vulgus, meaning the common people and in general having the connotation of “common”) is almost as pervasive as C. arvense and is also an introduced plant that occupies similar habitat. The name bull likely is in reference to the large size of the flower though it may also derive from its prevalence in open fields where bulls prevail. It is also known as spear thistle, plumed thistle, and roadside thistle. It can be distinguished from the Canada thistle by a number of features: the flower head is generally larger (1-2 inches compared to about ¾ inch); the flower head is surrounded by spiny bracts; the leaves are pubescent, or hairy, on both sides; and the leaves and stems are covered with spines.
The Bull thistle is not as pernicious a weed as the Canada thistle due to a number of factors. It is biennial vice perennial; it consists of a large rosette of spiny leaves in the first year and a 3 to 5 foot tall plant with branching, erect, spiny stems in the second year. It propagates solely by seed and not by vegetative horizontal root expansion. Accordingly, it is generally not a problem in cultivated fields, but can pose a problem in overgrazed pastures, as its opportunistic characteristics can result in significant infestations that may impede livestock grazing. It is only classified as a noxious weed in a number of states, notably Maryland where oligophagous flies (Urophora stylata) have been released as a biological control agent; the larvae form galls that preclude seed development.
Thistles are members of the tribe Carduceae of the family Asteraceae and originate primarily from the Eastern Hemisphere. However, 3 of the 13 genera contain species that are native to North America; the genus Cirsium includes some 100 native species of which about 20 are indigenous to the Eastern United States. Some native species of thistle are threatened or endangered; caution must therefore be exercised in any eradication effort to insure that an erroneous identification does not lead to extirpation.
Thistles are celebrated in lore and legend and have been at various times been ascribed to have medicinal and gustatory properties. The adaptation of the thistle as the national emblem of Scotland is a prime example. According to the most popular story, the army of King Haakon of Norway landed on the coast of Largs to mount a surprise attack on the Scots. On removing their footwear to promote stealth, they proceeded across a field of thistles; the ensuing cries of pain alerted the Scottish defenders. The ascension of the thistle to the national symbol is attributed to King James V who established the Order of the Thistle in 1540 with the motto “Nemo me impune lacessit” or “Touch me who dares.” In Norse mythology, the thistle is known as the lightning plant; those who wore the thistle were protected by Thor, the god of thunder.
Thistles of the genus Cirsium are generally edible, notably the young shoots harvested in the spring, which are considered to be something of a delicacy in Scotland. The root is high in inulin, an indigestible starch that passes into the digestive system and ferments, producing flatulence. The genus of the Scottish thistle is Onoperdon which is from the Greek words ono meaning “ass” and perdon meaning “passing wind,” as donkeys that ate the thistle were subject to this effect. Thistles contain chemicals that have some anti-inflammatory properties. They have historically been used in a number of medicinal applications, including a palliative for toothache and a treatment for worms; the genus name Cirsium attests to its use in an astringency application.
The thistle is appreciated by many for its tenacity and abrasiveness. The poem "Thistles" by Louise Erdrich is one example:
Under ledge, under tar, under fill