Scotch Thistle 
(Onopordum acanthium)

County Management Strategy: Suppression above approximately 6,000 feet elevation. Eradication below approximately 6,000 feet elevation.

Identification: A biennial, tap rooted weed that grows to 12 feet. Rosette leaves are very large (1 to 2 feet long and 6 to 12 inches wide), spined and with a dense coating of white hairs that give the leaves a silvery green appearance. The numerous flowers are magenta and 2 inches or more in diameter. Stems are winged, including stems under the flowerheads. Truly an enormous plant.

Other names: Cotton thistle, Scotch cotton thistle, heraldic thistle

Similar Species: No other thistles reach this size or have leaves or flowers this large.

Control Timing: In the rosette stage in spring and fall.

Control target: Prevent seed production.

Control Methods: Mechanical control is very effective in the rosette stage, if the root is severed at least 2 inches below the soil line. Roots are very thick in the second year and may need to be chopped. Herbicides can be effective but must be applied to the young rosette. The older the plant gets, the more difficult it is to control with herbicides because the hairiness of the leaves prevents the herbicide from landing on the leaf surface. The addition of a surfactant or adjuvant is recommended. No biocontrol agents are available at this time. Extensive feeding by painted lady butterfly larvae may be seen some years, but is not a reliable control method.

Status in Mesa County: A serious problem in the Collbran and Mesa areas. Increasing number of infestations are showing up on Highways 65 in Plateau Valley and on I-70. An isolated infestation was found on Glade Park. A few plants are found occasionally in lower elevations of Mesa County.

Photos:
http://extension.usu.edu/weedweb/ident/Scotch_id.htm
http://www.agf.gov.bc.ca/cropprot/weedguid/scotchth.htm

Fact sheets and control:
http://www.nwcb.wa.gov/weed_info/scotchthistle.html
http://www.douglas.co.us/DC/PublicWorks/Weeds/scotch_thistle.htm

Additional Links:
http://www.ceris.purdue.edu/napis/pests/scth/