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Canada thistle (Cirsium arvense) is a member of the sunflower family (Asteraceae). It is native to North Africa, Asia and southern Europe. We are told that it came to North America in the 1600’s as a contaminant in crop seeds. Since then it has spread all over the continental US and southern Canada; it is a Montana category 2B noxious weed. Some characteristics of Canada thistle are:

  •  Perennial lifespan. Plants live for more than two years overwintering in the root then emerging in the spring as rosettes. They reproduce both by seed and by production of new independent plants from horizontal root structures called creeping rhizomes.
  • Dioecious: Male and female flowers are produced on separate plants.
  • Produces relatively few seeds per (female) plant, one to five thousand, compared to spotted knap-weed which can produce over 40,000 seeds per plant.
  • Seedlings grow slowly and are sensitive to competition. However, once established, a seedling can begin to develop the ability to reproduce through its own rhizomes in seven to eight weeks.
  • Most of the plant’s reproductive energy is dedicated to vegetative propagation through the roots. A mature plant may develop horizontal roots extending 15 feet or more and vertical roots six to fifteen (some say more) feet deep.
  • Canada thistle will colonize or invade rangeland, pasture, cropland, roadsides, ditches and disturbed areas. It is known to reduce wheat yields by 15 to 60% in affected areas and can decrease production of alfalfa seed crops by 40% or more.
  • Considered a poisonous plant because it can accumulate nitrates and may contain high levels of sulfates in sulfate-rich soil.

Canada thistle is difficult to control because it is able to draw upon energy stored its extensive root system to recover from damage to its above-ground structures.

Treatments such as tillage may actually spread Canada thistle as even very small pieces of root (one quarter of an inch long) can generate new plants.

Chemical control is typically recommended as part of a long-term, integrated control strategy.  The success of chemical treatment depends on using the right herbicide in the right amount in the right place at the right time.  Spring applications should target seedlings and rosettes or early growth prior to the bud stage.  Chemical control during the vegetative stage is less effective than earlier application.  Treatment of mature plants is not considered feasible since the above-ground plant structures are not actively translocating nutrients and will eventually die back without moving the herbicide to the root.  Fall treatment of regrowth is effective because the plants are actively transporting nutrients to the root systems and will translocate the herbicide to the root system as well.

MSU Extension, Montana Weed Control Association and the NRCS have information about and can assist with developing a plan to control Canada thistle.

Note: Canadian bacon, Canada thistle.

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