Attractive Roadside Flower or Obnoxious Weed?

Canada Thistle (Cirsium arvense)

The small purple flowers of the Canada Thistle can be seen on many road sides, fields and pastures across Canada. Their attractive nature fools you, their alluring blooms swaying in the warm summer breeze, their sweet fragrance tickling your nose, but these flowers hold a dark secret.

The Canada Thistle grows from one to five feet in high which helps to distinguish it from other thistle as it is smaller in size.  Its slender stalks have small hairs that increase with maturation. They possess green, shiny leaves that are tapered around the edges. Canada Thistle sprout small purple flowers blooming in from July to September, that grow to be ¾ of an inch.

Two theories exist as to how this plant was introduced to Canada. One states that it was introduced by the French from Europe as swine food, though there is no record of this found, the other stating that it was brought in by poorly monitored grain crops from Europe and settled in farmers fields in  Ontario and Quebec and since then has spread rapidly through Canada and the Northern states (Hodgeson et. Al  1968).  

The flowers are able to invade larges areas by their ability to produce large amounts of seeds in favorable conditions which can be easily spread by transport in affected crops, attaching to animals, drifting in rivers and streams and attaching to farm equipment and other vehicles. It was thought to be introduced to Newfoundland through vehicle traffic first asserting itself on roadsides then branching into new habitats. They also have massive root clusters that reach far beneath the surface of the soil which allow the plant to increase in mass. One thistle plant can produce many clones due to its extensive root system and its ability to absorb great amounts of nutrients from the soil. This gives them their characteristic circular patters and allows them to easily invade and squeeze out nearby plants by surrounding and starving them of needed nutrients. The root systems also harbour underground buds well below the surface. When removal attempts and made by removing the stalks of the plant it stimulates the growth of new buds underground creating a larger plant.  (Hodgeson et. Al  1968).

The Canada thistle has caused many problems with agriculture across Canada including Newfoundland. The thistle invades farmland squeezing out valuable crops posing a large threat to farmer’s fields. In a study conducted by agriculture Canada they investigated the estimated yield loss of barley due to invasive Canada Thistle populations. The study concluded that high levels of Thistle in a barley field can drastically reduce the yield and quality of barley gained (O’sullivan P.A et al. 1982.) This can drastically affect the livelihood of many Canadian farmers, as the Canada Thistle does not just affect Barley fields but many other grains and vegetable crops. Thistle can thrive in many types of habitat from pastures and field to wetlands and forests, affecting not only farmer’s fields but many ecosystems by pinching out native species and being an obnoxious nuisance to many gardeners.

Root system of Canada Thistle.

http://www.btny.purdue.edu/pubs/ws/canadathistle/CanadaThistle.html

Canada Thistle is notoriously hard to eradicate, as removing the surface plant does not kill it. Many types of removal have been tried such as presides, use of competition plants such as alfalfa and grasses as well as biological control with the use of Ceutorhyncus litura a weevil that feeds on the thistle resulting in death of the plant.

Sources:

Jesse M. Hodgson. The Nature, ecology and control of Canada Thistle. United States. Agricultural Research Service, 1982.  http://books.google.ca/books?hl=en&lr=&id=Z6MoAAAAYAAJ&oi=fnd&pg=PA1&dq=canada+thistle&ots=wCkRduf5JJ&sig=paBaXuLlhRVB7almKiV_7wAm8VQ#v=onepage&q&f=false

Nelson, Rob. "Canada Thistle." Exotic, Invasive and Problem Plants. 2007. Web. 11 Mar. 2012. <http://www.thewildclassroom.com/biodiversity/problemplants/species/Canada%20Thistle.htm>.

P. A. O'SULLMN, V. C. KOSSATZ, G. M. WEISS, and D. A. DEW.  AN APPROACH TO ESTIMATING YIELD LOSS OF BARLEY DUE TO

CANADA THISTLE Research Stution, A,qriculture Cunudu 23 Feb. 1982 http://pubs.aic.ca/doi/pdf/10.4141/cjps82-105

"Canada Thistle." Colorado State University. Colorado State University, 29 Aug. 2011. Web. 11 Mar. 2012. http://www.ext.colostate.edu/pubs/natres/03108.html

“Canada Thistle (Cirsium Arvense)." Invasive Species. 3 Sept. 2004. Web. 11 Mar. 2012. <http://dnr.wi.gov/invasives/fact/canada_thistle.htm>