Canada thistle has long been one of the most problematic weeds in Iowa and the northern corn belt. While modern herbicides have greatly reduced problems caused by Canada thistle in crop fields, it is still a formidable foe in pastures, roadsides and other less intensely managed areas. This article will present information from a paper published in 1934 by Ada Hayden, Department of Botany, Iowa State College (Hayden, A. 1934. Distribution and reproduction of Canada thistle in Iowa. Amer. J. Bot. 21:355-373).
Canada thistle (Cirsium arvense) is native to Europe and nothern Africa, and was introduced to North America soon after the arrival of European settlers. Its invasiveness was soon recognized, and in 1795 Vermont lawmakers created the first noxious weed law in the United States in an attempt to stop the spread of Canada thistle. Iowa's first noxious weed law was written in 1868 by the 16th General Assembly and stated: "Be it enacted by the General Assembly of Iowa, that if any resident owner of any land in this state after having been notified in writing of the presence of Canada thistles on his or her premises, shall permit them or any part of the root to blossom or mature, he or she shall be liable to a fine of five dollars and cost of collection for each offense". Today's Noxious Weed Law empowers county weed commissioners to enter private land and control noxious weeds and bill the landowner for the expense of the treatment. The Iowa Noxious Weed Law now includes sixteen weed species, including Canada thistle.
Louis Pammel, another early botanist at Iowa State College, conducted the first studies of Canada thistle in Iowa. In an 1898 bulletin titled 'Weeds of corn fields' he wrote: I have repeatedly examined the thistle in Wisconsin, Iowa, Illinois, and Missouri, and with few exception seeds have not been found'. Based on these observations, Pammel concluded Canada thistle would not be a serious problem in Iowa: " It will not do, however, to lay too much stress on the fact that it is not likely to become troublesome in the West since it does not 'seed'."
So how did this renowned botanist misjudge the weedy potential of Canada thistle in Iowa? At the time that Pammel was studying Canada thistle, it was not known that this species was dioecious (plants are either male or female). Since Canada thistle was just beginning to invade Iowa, patches typically were initiated by a single seed brought in with crop seed or some other human transport mechanism. Thus, a patch would consist of either male or female plants originating from the rootstocks of the plant developed from the initial seed. Seed would not be produced because both sexes of plants were not present to result in pollination and the production of viable seed. A map showing the distribution of Canada thistle in Iowa in 1898 indicated isolated infestations in only 25 counties. Pammel assumed the lack of seed was due to Canada thistle being poorly adapted to the environment of the western Corn Belt. The dioecious character of Canada thistle was first reported in the 1920's.
Hayden developed a map of Canada thistle distribution in Iowa in the 1930's, and numerous infestations were reported in every county. Thus the species rapidly spread across Iowa in approximately 30 years. In contrast to Pammel, Hayden found that seed production in Canada thistle was quite common in Iowa. She examined the relationship between seed production and the distance between male and female patches. In Cerro Gordo county she found that 9 out of 10 patches were comprised exclusively of male or female plants. If male and female patches were less than 25 ft apart, individual seedheads contained 50 to 100 seeds. Female plants that were 25 to 100 ft from a male plant contained 20 to 30 seeds per head, whereas female plants 500 to 600 ft from male plants averaged only 2 seeds per head. Canada thistle is primarily pollinated by honeybees, thus the farther a female flower is from a male flower the less likely it is to be visited by a bee carrying pollen. Waterhemp is another dioecious weed of Iowa; however, waterhemp is a wind pollinated species and therefore pollination occurs at much greater distances than with Canada thistle.
Hayden concluded that the introduction of Canada thistle to new areas was largely due to human activities, but that once established local dispersion was due to wind transport of the seed. At the time it was thought that ducks were a major source of long-distance seed transport, but examination of the intestines of ducks that had been feeding in the vicinity of Canada thistle found no evidence of Canada thistle seeds. A survey of crop seeds found that a high percentage of seed was contaminated with Canada thistle seed. The rapid spread of Canada thistle across the state was attributed to the 'widespread failure of human agency to destroy the plants before fruiting occurs'.
Footnote: Those familiar with Ames may recognize the names of the two botanists cited in this article. Pammel Courts were temporary housing built after World War II that provided inexpensive (some might say rundown) lodging to thousands of students until the 1990's. While the Pammel Courts have been eliminated, Pammel Drive still remains on the north edge of campus. A new park north of Ames (formerly Hallet's Quary) was recently named after Ada Hayden. Perhaps in the future a landfill or similar landmark will be named after one of the current ISU weed scientists.
Prepared by Bob Hartzler, extension weed management specialist