Onopordum acanthium L. and O. tauricum L.

Keys to Identification

• Up to 8 ft tall
• Large, coarsely lobed, hairy leaves have a velvety-gray appearance
• The rosette forms the first year and can have leaves up to 2 ft long and 1 ft 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 and has no hair on the plant

This information courtesy of the Colorado Natural Areas Program

Family

Sunflower (Asteraceae)

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 in wide, 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 ft long and 1 ft 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 ft tall. 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

Seed.

Seed production

One plant produces 70-100 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 seeds may be carried by wind and water.

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.