Photographer: Steve Dewey Affiliation: Utah State University Source: Bugwood.org Copyright: (CC BY 3.0)
Onopordum acanthium, is a multi-branched biannual plant that can grow up to 6-8 feet tall and wide. It can sometimes act as a short-lived perennial. Stems have spines and leaves are oblong and prickly with toothed margins. Leaf length varies between 4 and 20 inches in length. Flowers bloom from July-October and are terminal, 1-2 inches in diameter and dark pink to purple in color.
Scotch thistle is listed as a noxious weed in 14 states, meaning it is designated for control and is prohibited and banned. Its dense stands compete with native plants for resources and can form a physical barrier to water and grazing for animals. Additionally, it can compete and decrease the populations of desirable forage for the animals and virtually prohibit land use for livestock.
Scotch thistle reproduces only by seed which can be dispersed by animals or wind. Each flower can produce between 110 and 140 seeds; and depending on the bummer of flowers a plant can produce up to 40,000 seeds. Less than 20% of the seeds are ready to germinate, and the rest lie dormant in the soil anywhere from 7 to 20 years. Dormancy is broken by moisture and light can prevent germination so seeds must be buried in the soil in order to sprout.
Scotch thistle was introduced from the Mediterranean region, to the United States as an ornamental in the 1800s.
Native Origin: Eurasia
U.S. Habitat: Disturbed areas, such as roadsides, ditch banks, pastures (especially when overgrazed), campgrounds, and burned areas.
U.S. Present: AL, AZ, CA, CO, CT, DC, DE, FL, IA, ID, IL, IN, KS, KY, MA, MD, MI, MN, MO, MT, NE, NJ, NM, NV, NY, OH, OK, OR, PA, RI, TX, UT, VA, VT, WA, WI, WV and WY
Canada thistle (Cirsium arvense), bull thistle (Cirsium vulgare) and Musk thistle (Carduus nutans) may be confused with Scotch thistle. Canada thistle is different by having small unisexual flower heads and smooth stems and plumose bristles. Bull thistle is a coarse biennial with upper leaf surfaces covered with stiff bristly hairs that are rough to touch. Musk thistle has leaves that are dark green, coarsely lobed, with a smooth waxy surface and a yellowish to white spine at the tip. Flower heads will droop to a 90-degree angle from the stem when mature.
Perennial grasses that cover the soil surface appear to inhibit successful establishment of Scotch thistle. Grasses that have proved competitive include tall fescue, orchard grass, and smooth brome grass. Other grasses may also be effective so long as they cover the soil surface. Several herbicides used in range, pasture, and non-crop areas control Scotch thistle.
Thankfully, this plant does not reproduce vegetatively or by rhizomes so cutting the plant below the soil surface will kill the plant. Mowing can be effective if it occurs before the plant has flowered. Yearly surveys are necessary because of the long dormancy period of seeds, but a combined strategy of mechanical management, competitive grasses, surveying and herbicides if needed, can be very effective
Ashigh, J., Wanstall, J., & Sholedice, F. 2010. Troublesome Weeds of New Mexico. New Mexico State University, College of Agricultural, Consumer and Environmental Sciences, Cooperative Extension Service.
Cattaneo, M., Ashigh, J., & Carrillo, T. 2011. Certified Noxious Weed Free Program. NM State University, Cooperative Extension Service.
Young, J. A., & Evans, R. A. 1969. Control and ecological studies of Scotch thistle. Weed Science, 60-63.