Scotch Thistle
Onopordum acanthium L. and O. tauricum L.
Keys to Identification
Can reach a height of 8 feet- Large, coarsely lobed, hairy leaves have a velvety-gray appearance
- The rosette forms the first year and can have leaves up to 2 feet long and 1 foot wide
- The spiny-edged, alternate leaves form leaf wings that extend down onto the stem
- This branching plant has reddish-purple to violet flowers and a large, fleshy taproot
- It is found primarily along roadsides and railroads, but can become an impassable obstacle to livestock on rangeland and pastures. It is increasing in densities throughout Colorado
- In the Pueblo area a different species of Scotch thistle (Onopordum tauricum) is found. This species is bright green in color and has no hair on the plant like the 0. acanthium species
This information courtesy of the Colorado Natural Areas Program
Family: Asteraceae (Sunflower)
Other Names: cotton thistle, winged thistle
USDA Code: ONAC, ONTA
Legal Status: Colorado Noxious Weed List B
Identification
Lifecycle: Perennial
Growth form: Forb
Flower: Flower heads are numerous, 1-2 inches in diameter, with spine-tipped bracts. Flowers are violet to reddish.
Seeds/Fruit: One-seeded fruit (achene) is wrinkled, brown to grayish-black, tipped with a plume (pappus) of slender bristles (Stubbendieck et al. 1995).
Leaves: Leaves are alternate, large, irregularly lobed, and have sharp yellow spikes. Rosette leaves may be up to 2 feet long and 1 foot wide (Whitson et al. 1996). Upper and lower leaf surfaces are covered with a thick mat of cotton-like or woolly hairs, giving the foliage a gray-green color (Dewey 1991).
Stems: Mature plants can grow up to 12 feet tall, and have a large, fleshy taproot. Stems are numerous, branched, and have broad spiny wings.
Roots: Thick fleshy taproot
Seedling: Forms rosette
Similar Species
Exotics: Onopordum acanthium is the predominant Scotch thistle species in the western United States and is characterized by its hairy leaves (Beck 1991). A hairless species, Onopordum tauricum, also occurs but much less frequently, mostly in the Arkansas River drainage in Colorado (Beck 1991).
Natives: There are many native thistle species (in the genus Cirsium). The natives generally do not have leaves clasping the stem all the way from node to node (strongly decurrent leaves), and many have hairy upper and lower leaf surfaces and are blue-green or gray in color.
Impacts
Agricultural: Scotch thistle is an aggressive plant that is competitive with desirable native forage species. It can form dense stands that are impenetrable to livestock.
Habitat and Distribution
General requirements: Scotch thistle is often found along roadsides, irrigation ditches, waste areas and on rangelands. It is especially fond of areas that are adjacent to riparian or sub-irrigated deeper soils along stream courses, lower alluvial slopes and bottomlands.
Distribution: Occurs sparsely over much of the United States. It is increasing in densities throughout Colorado.
Historical: Native to Eurasia
Biology/Ecology
Life cycle: Scotch thistle is a biennial that produces a large, ground level rosette the first year, and a tall, spiny plant the second. Flowering occurs from mid-June to September.
Mode of reproduction: Scotch thistle reproduces by seed
Seed production: One plant produces70-100 flowering heads containing 100-140 seeds per head (Young and Evans 1969).
Seed bank: Seeds may remain viable in the soil for over 30 years
Dispersal: Plumed seeds can be dispersed by attaching to clothing and animal fur. Seeds may be transported in hay and machinery, and seed heads may be carried by wind and water.
Integrated Management Summary
Scotch thistle is best controlled in the rosette stage. Scotch thistle can be controlled by severing its taproot 1-2 inches below the ground. Control can be enhanced by a follow-up application of herbicides to the surviving rosettes. One integrated approach to Scotch thistle management involves 1) managing livestock grazing to increase grass vigor and reduce bare ground; 2) spray rosettes with clopyralid or 2,4-D; 3) re-seed treated ground with competitive desirable plants in the fall after spraying; 4) follow-up with spot cutting of entire plants when first flowers appear annually for several years to deplete the seed bank in the soil.
References
Beck, G. K. 1999. Biennial thistles. In: R.L. Sheley and J.K. Petroff (eds.) Biology and management of noxious rangeland weeds. Oregon State University Press, Corvallis. pp. 145-161.
Beck, G.K. L.F. James (Ed.). 1991. Biennial thistle control with herbicides. Noxious Range Weeds. Boulder: Westview Press. pp.254-259.
Dewey, S.A. L.F. James (Ed.). 1991. Weedy thistle of the western United States. Noxious Range Weeds. Boulder: Westview Press. pp.247-253.
Stubbendieck, J., G.Y. Friisoe and M.R. Bolick. 1995. Scotch thistle. Weeds of Nebraska and the Great Plains. Nebraska Department of Agriculture, Bureau of Plant Industry, Lincoln, Nebraska. pg. 169.
Whitson, T.D.(ed.), L.C. Burrill, S.A. Dewey, D.W. Cudney, B.E. Nelson, R.D. Lee, R. Parker. 1996. Scotch thistle. Weeds of the West.
Western Society of Weed Science, in cooperation with the Western United States Land Grant Universities Cooperative Extension Services, Newark CA. pg. 164.
Young, J. A. and R. A. Evans. 1969. Germination and persistence of achenes of Scotch thistle. Weed Science 20: 98-101.
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