Scotch thistle (Onopordum acanthium) is a robust non-native plant well established throughout much of the United States and Canada (Figure 1). The plant was introduced into the United States during the 1880s possibly as an ornamental and/or as a medicinal plant. It subsequently escaped from cultivation and is now abundant and problematic in the western U.S. Severe infestations can form tall, dense stands that impede livestock and wildlife access to desirable forage plants, impacting wildlife habitat and limiting carrying capacity of infested rangeland and natural areas.
Scotch thistle is generally considered a biennial weed, but can also grow as an annual or short-lived perennial. The plant reproduces exclusively by seed that can germinate throughout the year depending on moisture and temperature. Large Scotch thistle can produce from 20,000 to 40,000 seeds that can remain viable in soil for at least seven years. Seeds are 4 to 5 mm (0.2 in.) in length, smooth, slender, and plumed. A water soluble germination inhibitor contained in seeds suggests that germination will not occur without optimum soil moisture.
The first year, Scotch thistle forms a rosette of large, spiny leaves that can be 12 inches or more in width. Flowering stems are normally produced during the second growing season and can grow from 6 to 12 feet tall. Stems have vertical rows of spiny ribbon-like “wings” that extend to flower bases. The plant is highly branched and gray-green in appearance. Leaves are oblong and prickly, and toothed or slightly lobed along the margins. Upper and lower leaf surfaces are covered with a thick mat of cotton-like or woolly hairs, giving the foliage a gray-green color. The dark pink to lavender flower heads measure 1 to 2 inches in diameter. The whorl of bracts beneath the flower is tipped with flat, pale, orange-colored spines. Flowers stand alone on branch tips and bloom July-October. Stout taproots anchor the plant. Scotch thistle is distinguished from other invasive thistles by the very dense, white woolly covering on stems and leaves.
Scotch thistle favors habitats with high soil moisture and is often associated with waterways (swales, gullies, roadsides, and other moist sites) in the western United States. Disturbed areas and plant communities dominated by annual grasses are susceptible to invasion.
Improving the desirable plant community by seeding competitive grasses or implementing grazing management practices that favor desirable vegetation may be necessary to provide long-term control of Scotch thistle. On disturbed sites, integrating the use of herbicides with reseeding is likely to decrease Scotch thistle populations more effectively than either control method used alone. Buried seed may persist for more than seven years, and re-infestation is likely without follow-up management.
Several herbicides are recommended for managing Scotch thistle on grazed rangeland and natural areas including Milestone® (aminopyralid), Opensight® (aminopyralid + metsulfuron methyl), and Transline® (clopyralid) specialty herbicides, and 2,4-D and dicamba (Banvel and others). A field study conducted in Nebraska compared the effectiveness of various herbicides applied post-emergence in the spring for Scotch thistle control. Herbicide treatments were applied in May to Scotch thistle plants at the rosette growth stage. Visual evaluations of Scotch thistle were made following application. In late May and early July, Scotch thistle control with all herbicides was excellent. By late August, Scotch thistle seedlings were emerging in some of the herbicide treated plots (Table 1).
The best time to apply herbicides is from rosette to early bolt growth stages when Scotch thistle is actively growing. Field studies conducted in California showed that Transline at 2/3 pints per acre provided 100 percent control when applied at the rosette growth stage and only 65 percent control when applied at late bolt growth stage.
Optimum herbicide rates to control Scotch thistle include:
- Milestone (aminopyralid): 5 to 7 fluid ounces of product per acre (fl oz/A).
- Other premix herbicide formulations of aminopyralid include Opensight at 2.5 to 3.3 ounces/A, and GrazonNext HL at 1.5 to 2.1 pints/A. Herbicides containing aminopyralid applied to rosettes in fall will control Scotch thistle seeding germination through the following spring.
- Transline (clopyralid): 2/3 to 1 pint/A.
- 2,4-D at 2 quarts (2 lbs ai/A) does not control large bolting plants and has minimal soil activity to control Scotch thistle seedlings that germinate from seed.
- Dicamba (Banvel) at 1 pint to 1 quart per acre should be mixed with 2,4-D to improve Scotch thistle control.
Small infestations of Scotch thistle can be controlled by digging/cutting the crown of the plant a few inches below the soil surface. Cutting in late bud to flowering stage will reduce seed production, but may require repeated treatment because populations typically exhibit a wide range of developmental stages among individual plants. Plants should not be cut following seed set, as this increases chances for dispersal. Plants that are cut by hand should be bagged, removed from the site and destroyed if they are flowering.
No classical biological control agents directly targeting Scotch thistle have been released in the United States. Biocontrol agents (e.g. Rhinocyllus conicus and Trichosirocalus horridus) released against other exotic thistles that also utilize Onopordum spp. appear to have little or no impact on Scotch thistle. Native and adventive insects and pathogens that may feed on Scotch thistle are not causing any appreciable damage. Thus, biological control is not currently an option for Scotch thistle management in the United States.
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Dewey SA and LF James (Ed.). 1991. Weedy thistle of the western United States. Noxious Range Weeds. Boulder: Westview Press. pp.247-253.
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Hansen R. 2009. Current status of, and future prospects for, biological control of musk thistle (Carduus nutans) and Scotch thistle (Onopordum acanthium) in the US. USDA-APHIS-PPQ “white paper”.
Henry C (ed). 2006. Performance of herbicides applied in spring for Scotch thistle. Techline. Winter. pp. 4-5.
Kadrmas T and WS Johnson. 2002. Managing Scotch Thistle. University of Nevada Cooperative Extension Service. Fact Sheet 02-57.
Wilson R. 2005. Scotch thistle control on rangeland with herbicides applied at the rosette and bolting stage. Western Society of Weed Sci. Research Progress Reports. pp.12-13.
USDA, NRCS. 2016. The PLANTS Database (http://plants.usda.gov, 23 February 2016). National Plant Data Team, Greensboro, NC 27401-4901 USA.
Young JA and RA Evans. 1969. Germination and persistence of achenes of Scotch thistle. Weed Science 20: 98-101.
®Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. Milestone and GrazonNext HL are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Milestone and Opensight: When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details. State restrictions on the sale and use of Milestone, Opensight, Transline apply. Consult the label before purchase or use for full details. Always read and follow label directions.
Active ingredients for products mentioned in this article. Product (active ingredient): Milestone (aminopyralid), Opensight (aminopyralid + metsulfuron), Transline (clopyralid).