War on Weeds — Weeds are Everybody’s Problem

Canada Thistle

Canada thistle (Cirsium arvense) is a creeping perennial broadleaf native to the Mediterranean area. It spreads rapidly through underground stems called rhizomes and by producing seeds. It invades pastures, rangeland, roadsides, waste areas, cultivated fields and other disturbed sites. Its presence reduces biodiversity, and it has significant negative impacts on crop production and grazing systems.

Canada thistle can grow up to 3 feet tall, has an extensive root system and can reproduce vegetatively and by seed. Its stems stand erect, are hairless and slender. Canada thistle leaves are oblong to lance shaped, the edges are slightly lobed and toothed with spiny margins. The upper side of leaves are dark green while the underside is covered in woolly hairs giving it a lighter appearance. Flowers are pink to purple in color, disk shaped, and formed in clusters. Each plant can produce approximately 2,000 seeds per year. Seeds have a pappus attached, allowing them to be transported easily by wind, water, humans and animals. Undisturbed seeds that get buried in the soil can remain viable for up to 30 years, but most seeds germinate within a year or are not viable to start with.

Integrated Pest Management (IPM) options

• Prevention — Learn to identify this plant. Never transport unknown plant material. Make sure to clean equipment and clothing when leaving infested areas.
• Mechanical — Mechanical control alone is never an effective form of control for Canada thistle. Tillage will cause this weed to spread; each cut rhizome can become a new plant. Mowing encourages the growth of rhizomes. If mechanical control methods are used, they should be used in conjunction with chemical control methods..
• Chemical — There are several chemical control options available for use. For more information visit https://pnwhandbooks.org/ then search “Canada” in the weed section. Two options will be given, non-selective control options for non-cropland areas and selective control options for use in crops. Select the appropriate option. Always read and follow herbicide label directions.

Idaho is a wonderful place with awesome natural resources, landscapes, agriculture and recreational opportunities. Weeds threaten all the things that we love about Idaho. To detect, contain and control weeds in Idaho we have declared “War on Weeds.” These short articles are designed to give a brief description of each species, an explanation about why it’s a problem, insights into identification and Integrated Pest Management control options. — Justin Hatch

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• Canada thistle, 3-22