According to observations of naturalists and beekeepers.
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Cirsium arvense is a perennial species of flowering plant in the family Asteraceae, native throughout Europe and western Asia, northern Africa, and widely introduced elsewhere. The standard English name in its native area is creeping thistle. It is also commonly known as Canada thistle and field thistle.
Cirsium arvense was accidentally introduced to North America in the 1600s and is designated as a noxious weed in 43 states. It occurs from Alaska east to the Northwest Territories, Quebec, and Newfoundland and south to California, New Mexico, Kansas, Arkansas, and North Carolina.
This plant is present in at least 54 states/provinces in this country.
Cirsium arvense (Canada thistle) has a deep and wide-spreading root system with a slender taproot and far-creeping lateral roots. It often forms large patches, and individual clones may reach 35 m in diameter. Most Canada thistle roots are in the top 0.2-0.6 m of soil, but roots can extend as deep as 2-6.75 m.
Canada thistle has slender aerial shoots with leafy stems reaching 0.3-2 m tall. Leaves are 3-18 cm long and 0.5-6 cm wide. Canada thistle leaf morphology (texture, hairiness, loving, and spininess) can vary considerably, even within a geographical region. Canada thistle has numerous above-ground branches that bear several, small flowerheads (1-2 cm in diameter) in clusters. Seeds are 2.4-5 mm long, and 1 mm in diameter with a pappus of feathery bristles.
Plants develop new roots and underground shoots in January and begin to elongate in February. New shoots from established Canada thistle plants begin to emerge when the average weekly temperature reaches 41 degrees Fahrenheit (5 °C), with optimum emergence after temperatures are at least 46 degrees Fahrenheit (8 °C). Plants remain short until long days trigger flowering and stem elongation, normally in May and June, or about 3 weeks after emergence. When soil is warm and temperatures are moderate (as in fall) Canada thistle grows vigorously.
