Stems/Leaves: The stems and leaves grow 1.2 to 1.5 m tall and are ridged and branched.  This weed will grow 1' to 4' tall and will branch in the upper part of the plant.  The stems are a light green color and are ridged.  The leaves are normally around 3 to 4'' long and 2 to 3'' wide, and have the capability of being quite a bit larger.  The leaves also have yellowish thorns on them.  The stems and leaves may also have hair on them, which varies from plant to plant. (5)(6)(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)

Flowers: The flowers will grow from the upper stems, and have one, to several flowers growing from different stems.  The flowers are unisexual, which means they contain male and female parts.  Each flower can grow to approximately 1/2'' to 1'' across. (3)(4)  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, being purple, pink or white.  Flowering occurs during July and August.  The plant can still bloom later if it was disrupted in any way (such as mowed). (2) 

Fruits and Seeds: The seeds of the Canada thistle are 2.5 to 4 mm long and a straw to light brown color.  Each of the fruits is small and only contains one seed. (1) Canada thistle will make many bristly seeds that can be spread many ways, the wind being the easiest way. (5)  Most of the seeds will grow within a year of being spread, but some seeds can stay dormant, and eventually sprout after 20 or more years. (7) 

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, due to the fact that the weed can grow from small segments of cut roots. (5)  When using this method, cultivation it must be repeated until the nutrients in the roots are depleted. (3)

Habits/Life duration: A colony-forming, aggressive perennial. (1)  Canada thistle's habits are mainly that it seeds easily, grows large underground root systems, and also grows rapidly which make it a hard weed to control.  It out competes the natural plants in its area and can become a major problem.  It has been estimated that the plant will live for about 2 years, but it is continually growing new roots that replace the old shoots. (8) Environments Favorable to Infestation: This weed grows in cultivated fields, pastures, range land, forests, and along roadsides, ditches, and river banks. (2)  This weed grows best in areas where the soil has been disturbed and worked up.  Canada thistle needs much sunshine, so it is not often found in shady areas.  Also, this weed doesn't do as well in wet soils that are not well aerated. (4) 

Methods of Control:
Biological Control: The larvae of the weevil Ceuthorhynchus litera bore into the stems and weaken the plants, sometimes killing them. (1)  Also, there is Larinus planus which is a seed head feeding weevil that lay its eggs in the seed head.  Its larvae feed on the flower parts and developing seeds reducing the plants seed production and stressing the plant. (7)   The stem galling fly Urophora cardui has been released in the Whitehall, MT area several times but has not established.  It may be too cold here for this fly. (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. There are many different types of herbicides that will work. The main result you want from an herbicide is to destroy the top portion of the plant, so it can be out competed. (2)  Some people have sprayed vinegar onto the foliage to kill the tops of the plants or injected it into the stems with a syringe to kill that plant and some of its root system. (1)

Cultural: Burning does not kill the roots and may even stimulate the plant to send up many more stems.  Tilling if repeated until all the root fragments are killed, can be effective. (1)(5)