The issue: Scotch thistle (Onopordum acanthium L.), is a non-native plant from Eurasia. This biennial, which can reach 12 feet, forms a rosette with large leaves and a deep taproot the first season. The leaves and stems are gray-green in color. Scotch thistle has very prominent stem “fins” all the way to the seed head.
It was introduced as an ornamental in the 1800s and escaped cultivation. One plant can produce up to 40,000 seeds in spiny flowers. The 1- to 3-inch flowers are mostly pink to purple.
Scotch thistle spreads only by seed via irrigation water, wind, hay, animals, equipment and people. It invades Idaho rangeland, recreation sites, rights-of-way, dryland farm ground, pastures and disturbed areas.
Integrated pest management options:
· Prevention: Keep ditch banks free of seed-producing plants; clean all recreational, construction and farming equipment and vehicles of weed chaff; feed weed-free hay; manage pastures for good competition; establish desirable plants in disturbance areas.
· Mechanical: Mowing is not effective. Tillage and hand pulling below the crown before flower head development is effective.
· Cultural: Establish and encourage desirable vegetation.
· Biological: None. Early grazing by sheep, horses, or goats may reduce seed production.
· Chemical: Spring and fall application on rosettes is usually the best timing — products containing 2,4-D, clopyralid, dicamba, glyphosate, aminopyralid, chlorsulfuron, metsulfuron, picloram (restricted use), or triclopyr are effective. Many of these products have significant soil activity and may damage non-target plants (especially trees and shrubs) with roots in the application area. Be sure to select a product labeled for the site. Always read and follow herbicide label directions.
Scotch thistle seeds can remain dormant up to twenty years—a long-term program is essential. Combine different IPM options over a period of years to help improve the effectiveness of your efforts.
For more information, contact Ron Patterson, University of Idaho Extension Horticulture/Agriculture Educator in Bonneville County, at 208-529-1390 or email@example.com