Canada Thistle
(Cirsium arvense L)
Canada Thistle, (Cirsium arvense L) is a member of the Aster or Sunflower family. It is a perennial that reproduces by seeds and fleshy, creeping roots that may go sideways 15" and down more than 15’. It has an erect hollow, smooth and slightly hairy stem, 1 to 5 feet tall and branched at the top. The leaves are dark green to grayish green, and crinkly, with a large center vein and set close to the stem of the plant. Sharp spines are numerous on the outer edges of the leaves and on the branches and main stem of the plant. The flower is a small bristly cluster, 3/8" to 5/8" in diameter. They are light pink to rose on longish bulb (involucre) and seed forms a narrow paint-brush-type head until it opens. The seed is along, flattened, dark brown, and approximately 1/8" long.
Canada thistle emerges in April or May in most parts of Colorado. It is a damaging noxious weed. Infestations can be found in areas such as cultivated fields, riparian areas, pastures, rangeland, forests, lawns, gardens, roadsides, and in waste areas.
Control or eradication is difficult because of its vigorous growth, and extensive underground root system.
Stress the plant to use store energy. Mow at least 3 times at 2 week intervals; do not let it produce seeds. Spray emerging spring plants and those after first frost. Plant dry-land foliage, irrigate and plant perennial grasses in the fall and fertilize. Tilling is not recommended because it breaks up the roots and stimulates new plants without photosynthesis.