05-27-2009 Cirsium arvense
Canada thistle was probably introduced to North America by early colonists in the 17th Century, through the importation of crop seed. Control legislation was enacted in Vermont in 1795 and by New York in 1831. It was not reported west of the Allegheny Mountains until after 1835. C. arvense is found in almost every plant community disturbed by man. It is common to roadsides, railway embankments, lawns, gardens, abandoned fields, sand dunes, agricultural fields, margins of forests, and waterways. It grows poorly in shaded conditions and produces few flowers. Canada thistle is a perennial herb with a deep-seated complex system of roots. The seedlings of Canada thistle develop a fibrous taproot, and within a few months, the main root thickens and develops lateral roots. After growing 6-12 cm, the horizontal roots bend downwards, growing towards the water table. A new horizontal root develops at this point of bending and continues the horizontal spread. Aerial shoots develop from the original vertical root or from buds on the arching branches of the horizontal system. The weedy nature of this plant is also due to the ability of the root to regenerate from small pieces.
Reference: Moore, R.J. 1975. The Biology of Canadian Weeds. 13. Cirsium arvense L. Scop. Can J. Plant Sci. 55: 1033-1048.
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