Scotch Thistle
Scientific Name: Onopordum acanthium

History/Origin
          This native plant of Europe and eastern Asia was introduced in the U.S. late in the 1800's. It is said to have saved Scotland from invading Norsemen when nighttime guards were alerted by cries of pain as the attacking army walked through the thistle with their bare feet.

Biology/Ecology
          This thistle quickly invades sunny areas that have been disturbed, but is suppressed when invading into a healthy system. Once established, it becomes highly competitive and can crowd out even cheatgrass. Its rapid growth and large size reduce available light for smaller plants, and draw away other needed resources. Long spines intimidate animals into eating easier targets. When a scotch thistle dies, it leaves abundant litter that can smother surrounding plants. The scotch thistle plant averages 70-310 flowers per plant. Add to that a potential of 110-140 seeds per flowering head, and you get a bountiful seed producer. Eighty to ninety percent of the seed can be dormant for approximately 5 years. Seeds may be dispersed by water, wind, animals, and human activities.

Known Impacts
Detrimental:
          This plant competes with and decreases desirable forage, and can form a dense monoculture stand. It can act as a living barbed-wire fence, preventing livestock and wildlife access to feed and water.

Beneficial:
          It has been used for food and medicinal purposes.

Potential Locations
          Scotch thistle invades disturbed areas where competition has been reduced. It is best adapted to high soil moisture and is often associated with waterways in the western U.S. Although high soil moisture (especially in dry climates) is preferred, it will occupy dry sites as well. Scotch thistle is often associated with plant communities dominated by annual weedy grasses (downy brome/cheatgrass) and has been known to invade crested wheatgrass sites. It grows along roadsides, fence lines, ditch banks, open dry areas, and in pastures. It is rarely found in gardens and areas cultivated yearly.

Control/Management
          Prevention of this plant's invasion is the best management. Seeding disturbed areas with competitive native perennials is best. Control of these plants must include preventing new seed dispersal for 6 years. The best tools for removing infestations are:
• Digging them out by hand or annual cultivation.
• Use of herbicide on young plants prevents seed set (example herbicides: Curtail, Tordon 22K, Escort, Telar [look at herbicide label: free search]).
• Grazing young plants with sheep.