Canada Thistle 

(Cirsium arvense)

Description:

California thistle, creeping thistle, field thistle

Key Features:

Canada thistle can reach heights of four feet and forms deep and extensive root systems, making it difficult to control. Creeping horizontal roots sprout new plants and can reach more than nineteen feet in one season. These roots can also go as deep as twenty two feet. Rosettes are smooth, have irregular lobes and have spine tipped edges. Thistle leaves are distinctive as they are thorny, alternate and deeply lobed. Leaves are lance-shaped and  the edges have yellowish spines, making hand pulling a thorny proposition. Stems can grow tall, are often branched, slightly hairy, and lack spines. Flowers resemble spotted knapweed in that they are pink to purple and ray-like. Canada thistle produces both female and male flowers and both genders must be present for the plant to go to seed. Male flower heads are globe- shaped while female flower heads are flask-shaped. Canada thistle flowers tend to be smaller than other thistle species. Each female flower produces a single, tan curved seed which has a papery covering and is easily tossed about by the wind. One plant is capable of producing over 3,000 seeds annually. This plant reproduces by both seed and root, but primarily from the roots.

Habitat:

Canada thistle is found in open areas with a moderate amount of moisture but does poorly on wet soils lacking sufficient oxygen.  It can grow on many different soil types but it does not grow well in shade and is rarely found within wooded sites, except in clearings. It is commonly found in abandoned fields or lots, abandoned gravel pits, pastures, right-of-ways , roadsides, railway embankments, lawns, gardens, and agricultural fields. It also invades wet areas with fluctuating water levels such as stream banks or irrigation ditches and sloughs.

Currently found in the following counties:

Canada Thistle has been found in all counties in Montana.

IWM:

• Herbicide

• Biocontrol/Insects

• Cutting

Interesting Facts:

Contrary to the name, Canada thistle did not come into the U.S. from our northern neighbor, but rather was introduced in the 17th century from the Mediterranean region and southeast Europe. Tea made from Canada thistle leaves has been used as a diuretic as well as for treatment of tuberculosis.

Commonly Confused Plants:

• Bull thistle

• Plumeless thistle (Carduus acanthoides) has floral bracts that are covered with sharp spines. Flowers tend to be solitary.

• Wavyleaf thistle (Cirsium undulatum) is most common on grassland and dry forests.  Flower heads are usually large and the plant looks more like bull thistle than Canada thistle.

Photo Credits: Ohio State Weed Lab Archive, The Ohio State University, Bugwood.org;Steve Dewey, Utah State University, Bugwood.org; Jan Samanek, State Phytosanitary Administration, Bugwood.org; Jan Samanek, State Phytosanitary Administration, Bugwood.org