Bear Lodge Project – Bull Hill Mine
January 2013
Weed Management Plan
7
Canada thistle
Canada thistle is a colony forming perennial that reproduces from seed and creeping rootstalks and
can grow up to 4 feet tall. Leaves vary from light to dark green, oblong or lance shaped with deeply
grooved spiny toothed margins. Flowers form in small bristly clusters that are light lavender to deep
rose purple. Seeds are smooth, light to dark brown, tipped with a cupped conical point, and are
approximately 1/8 inch long.
Management Objective
: Containment
Integrated Management
Chemical: Clopyralis, aminopyralid, chlorsulfuron, glyphosate, 2,4-D, picloram, dicamba,
and curtail are all effective control agents.
Biological: Canada thistle stem weevil (
Ceutorhynchus litura)
, musk thistle crown weevil
(
Trichosirocalus horridus
), and Canada thistle stem gall fly (
Urophora cardui
)
are all approved, effective biocontrol agents.
Cultural: Increased competition from desirable plants is effective, proper management of
perennial grasses helps greatly.
Manual/Mechanical: Hand pulling is not effective. Repeated cultivation is successful if
conducted every 3 to 4 weeks.
Musk thistle
Musk thistle is a biennial or winter annual that grows erect up to 7 feet tall and reproduces only by
seed. Rosette forms the first year with a flowering stem forming the second year. Leaves are dark
green with a light midrib, hairless, with long sharp spines. Flowers are solitary, 1 ½ to 3 inches in
diameter, and deep rose to violet to purple in color. Roots are fleshy taproots that are hollow near
the ground surface. Seeds can be produced in excess of 20,000 per plant with 90% of seeds being
viable. Seeds may germinate after 10 years in the soil.
Management Objective
: Containment
Integrated Management
Chemical: Clopyralid, dicamba + difluferzopyr, clopyralid + 2,4-D, aminopyralid,
metsulfuron, picloram, and chlorsulfuron are all effective control agents on rosette
stage.
Biological: Thistle stem hover fly (
Cheilosia corydon
), musk thistle leaf beetle (
Pslliodes
chalmomera
), musk thistle crown weevil (
Trichosirocalus horridus
), and musk
thistle seed head fly (
Urophora solstitialis
) are approved, effective biocontrol
agents.
Cultural: Increased competition from desirable plants is effective proper management of
perennial grasses helps greatly.
Manual/Mechanical: Digging and pulling is effective when most of the tap root is removed.
Scotch thistle
