Roots: This weed called Canada thistle has roots that are extensive, fleshy, have creeping root-stocks, and well separated roots connected by a deep and extensive lateral root system. (2)  There roots extend up to three and a half feet into the soil. The Canadian thistle's root pattern grows horizontally into the ground. (3)

Stems/Leaves: The stems and leaves grow 1.2 to 1.5 m tall and are ridged and branched. (1)  The above ground portion of the shoot dies during the winter, but the under ground part generally survives to produce new shoots the following season.  New shoots are also developed from lateral root buds.  The leaves are spiny, and the edges are serrated and ruffled. (2)  The stems also be grooved and can branch at the top and then during maturity is when the spines begin to form.  For the leaves they are oblong and are irregular triangular indentations. The surfaces is dark green and doesn't contain any spines. (3)

Flowers: The flowers are unisexual, which means they contain male and female parts.  The flower head is urn-shaped (like a vase) and the bracts are spineless.  The color of the flowers may vary from plant to plant, either purple, pink, or white.  Flowering occurs during June through August. (2)  The heads of these flowers can range from 3/4 to1 1/4 inches in diameter. (3)

Fruits and Seeds: The fruit of the Canada thistle are 2.5 to 4 mm long and a straw to light brown color. (3) 

Reproduction: This weed reproduces by seed, but it mostly spreads by lateral roots sending up new shoots each year.  Cutting the roots with cultivation implements only produces more plants unless cultivation is repeated frequently enough to kill roots. (2)  

Life Style/Habits/Life duration: A colony-forming, aggressive perennial. (1)

Environments Favorable to Infestation: This weed grows in cultivated fields, pastures, range land, forests, and along roadsides, ditches, and river banks. (2)

Methods of Control:
Biological Control: The weevil Ceuthorhynchus litera is a larvae that bore into the stems of the thistle and weakening the plants, sometimes killing them. (1)

Herbicides: They remove or suppress the tops of Canada thistle in crops which will grow rapidly enough to provide a closed canopy and shade the ground before the new thistle shoots recover. (2)

2) Rees, Norman, et. al., Ed., Biological Control of Words in the West, Western Society of Weed Science in cooperation with USDA ARS, MT Dept. of Ag., and MT State University, Color World printers, Bozeman, MT, Feb., 1996.