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.

The clouded sulphur, also known as common sulphur, (Colias philodice) is a butterfly that belongs to the family Pieridae, which includes other yellows, whites and sulphurs. The clouded sulphur is native to Canada and can be found throughout North America. The adults are best adapted to open areas such as moist meadows, lawns, and alfalfa and clover fields, where they feeds on nectar of various plants, including alfalfa, clovers, milkweed, dandelions, thistles, goldenrods, common selfheal, wild teasel. Adults also extract moisture and minerals from mud puddles or animal excrements. The larvae feed on members of the legume family.

The clouded sulphur produces several generations each year. To mate, the male typically flies towards the female and after the female lands the male will repeatedly hit his wings against the female's body. This results in pheromones being released. The female may then lower her abdomen and mating will occur. Their eggs are cream to yellow in colour when first layed, turning red shortly after, and hatching within about 5 days.. The larvae are green with light stripes on their sides and a dark stripe on the back. They also have a smooth texture. The larvae will construct a chrysalis that is green and pointed on both ends where they will remain overwinter. . The adults have yellow wings though this varies throughout the season, with them being green-yellow in the spring and fall, whereas they are yellow in the summer. Their wings have a black border on the upper side. This feature on the females is interspersed with yellow spots. The underside of their hindwings have a silver spot surrounded by two red rings. Some females can be white. Adults live for 2 to 7 days.

Clouded sulphurs can hybridize with orange sulphurs (Colias eurytheme) as they are closely related species. This happens when the females are young (less than one hour old) and are unable to distinguish between the pheromones of the two species, resulting in offspring that may be sterile.