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Scotch Thistle Onopordum acanthium L. and O. tauricum L. |
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Keys to Identification:
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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
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