Plant of the Month: Canada Thistle (Cirsium arvense)
Cirsium arvense is commonly referred to as the Canada thistle. It is considered an invasive weed species as it causes significant crop loss and out-grows native plants along disturbed areas, such as roadsides, riverbanks and logged forests. It can reproduce by seeds or by its extensive root system, which can spread horizontally and produce new shoots. It contains thick, deep growing roots that make this weed very hard to control. The root starts as a single tap root that grows downward (about 1.8 to 3 meters on average) until it finds moisture, then lateral roots start forming. This thick system of roots allows for regeneration after top-growth removal. Additionally, the Canada thistle can reproduce through lateral roots. This type of reproduction is more common than reproduction through seed production, which leads to large groups of individuals that are genetically identical.
When seed production does occur it results in an average of 1500 parachute-type seeds being created, and is the result of insect pollination. Solitary bees, bumble bees, flies, butterflies, beetles, and honey bees are commonly observed visiting plants in the Cirsium genus within Calgary.
While the Canada thistle is considered invasive within Canada, there are many native species of thistle (Cirsium sp.) that provide nectar to pollinators and whose seeds can act as a food source to birds and other wildlife. The Canada thistle can be distinguished from other thistle species as the Canada thistle does not have spikes on its stem. The Canada thistle also has white to purple pom-pom like flowers that grow in groups of up to five at the ends of the stems, and are typically smaller than other thistle species. They also have leaves with sharp, spiny edges and white hairs covering their undersides.
The plumeless thistle (Carduus acanthoides), marsh thistle (Cirsium palustre), purple-flowered knapweeds (Centaurea sp.) are non-native species that look similar to the Canada thistle.
In Canada, a number of mechanical, chemical or biological controls have been used to attempt to control this weed. As mentioned above, mechanical controls do not work well because of root regeneration, however correct timing of tillage and repeated efforts can make this method more effective. Herbicides (chemical controls) tend to be effective in only controlling topgrowth of the Canada thistle, however systemic herbicides are more effective. A combination of tillage and herbicide use can improve the effectiveness of control. Additionally, biological controls in the form of stem gall flies, stem weevils, and defoliating beetles have been used to control Canada thistle in Canada.
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