Scotch Thistle

Onopordum acanthium

What is it?

Scotch Thistle is designated as a “List B” species in the Colorado Noxious Weed Act. It is required to be either eradicated, contained or suppressed depending on the local infestations. It is a native of Europe and Asia, introduced in the 19th century as an ornamental. It is a biennial that with spiny leaves in a rosette during the first year and flowering shoots in the second. Typical plants contain 70-100 flowers, violet to reddish in color. It reproduces solely by seed and produces up to 14000 seeds per plant. It can grow upwards of 12 feet tall.

Why is it a problem?

Scotch Thistle can grow so thick that, due to it’s spiny nature, it can form natural barricades to livestock, wildlife and humans. Campsites and trails can become inaccessible or no longer enjoyable. Access to stream banks and fishing areas can be cut off. It invades rangelands, overgrazed pastures, roadsides and ditches and reduces the carrying capacity of livestock. The seeds are easily dispersed by humans, livestock, wildlife, wind and water. They are viable for up to 10 years and can germinate in nutrient deficient soils.

Management Techniques:

Mechanical control is most effectively used to eliminate small patches.Treatments that sever the roots below the surface will kill the plant. Mowing or chopping has proven effective when the plant is in full bloom, as long as the seeds are properly disposed of.
Biological controls include a fly, Urophora stylata, that lays its eggs in the seed head. The maggot consumes the seeds in the flower. Distribution of the fly is limited in Colorado by cold temperatures. Goats will only consume the plant when it is in its early rosette stage.
Infestations can be reduced or eliminated by planting competitive grasses. This method is especially effective after mechanical treatments have been applied.
Chemical applications have proven effective at reducing and eliminating scotch thistle. They are most effective when applied during the rosette stage of the plant’s growth, in the spring or early fall.Herbicides include Tordon 22K, Milestone and Cimaroon X-tra. Retreatments of the area infested by scotch thistle will be necessary for 4 to 6 years until the seeds are exhausted.

What can we do?

Maintain healthy pastures and rangelands
Limit seed production by collecting plants with buds or flowers and disposing of them immediately.
The public is encouraged to stop the spreading of scotch thistle by steering clear of weed infested areas by not driving through them
Monitor your land and destroy single plants or new infestations before they spread on your’s and your neighbors land.
Establish desirable forage that can compete with scotch thistle and help prevent its spread

General Information on Scotch Thistle:

National Forest Service

PDF, field guide for managing invasive thistles

National Forest Service

PDF, identification,biology and integrated management of scotch thistle

Colorado State University Extension

PDF, identification and management of scotch thistle