Canada thistle is a member of the Aster or Sunflower family and was introduced from Europe. It is a creeping perennial, which reproduces by seeds and fleshly, horizontal roots. The erect stem is hollow, smooth and slightly hairy, one to five feet tall, simple, and branched at the top.
The leaves are set close on the stem, slightly clasping, and dark green. Leaf shape varies widely from oblong to lance-shaped. Sharp spines are numerous on the outer edges of the leaves and on the branches and main stem of the plant. The flowers are small and compact; about 3/4 inch or less in diameter. Color ranges from light pink to rose-purple. The seeds are oblong, flattened, dark brown, and approximately 1/8 inch long.
Canada thistle emerges in April or May in most parts of Colorado. It is one of the most widespread, and economically damaging noxious weeds in Colorado. Infestations are found in cultivated fields, riparian areas, pastures, rangeland, forests, lawns and gardens, roadsides, and in waste areas. Because of its seeding habits, vigorous growth, and extensive underground root system, control or eradication is difficult. It is distributed across Colorado from 4,000 to 9,500 feet.