From Amanda Homans, Agronomist with Decisive Farming.
Common name: Canada Thistle
Latin name: Cirsium arvense
Canada thistle is a creeping perennial weed that can be found everywhere, in crop land, hay fields, rangelands, irrigation, roadsides, and waste areas. This plant has deep, horizontal, creeping roots and often forms in dense patches. It is so hard to control this weed because of its underground root system. The leaves have spiny-toothed margins but the leaf itself is smooth.
When Canada thistle has reached its maturity, the disk flowers are usually pink-purple but occasionally white, and are surrounded by narrow, flattened bracts ending is short, weak prickles. This weed is sometimes confused with bull thistle. The Canada thistle has male and female heads; the male head are globe shaped in contract to the more flask shaped female heads. Each female flower is capable of producing a single seed that is easily dispersed by wind.