Scotch Thistle   

a.k.a. – Cotton thistle, Heraldic thistle, Scotch cotton thistle  

Scientific name:  Onopordum acanthium L.

Family:  Asteraceae (Sunflower family)

Description

Origin:  Eurasia.  Introduced into the United States as an ornamental plant in the 1800s. 

Scotch thistle is a non-native biennial forb but can behave as an annual or short-lived perennial.  It reproduces/spreads from seed.  Scotch thistle is a prolific seed producer.  Each thistle plant can produce up to 40,000 seeds.  Less than twenty percent of the seeds initially produced are ready to germinate.  The remainder of the seeds has a water-soluble coating that serves as a germination inhibitor that requires moisture to break dormancy.  Light can also serve as a seed germination inhibitor; therefore, seeds need to be in the soil or covered to germinate.  This allows seeds to remain viable in the soil up to twenty years.  Water, livestock, wildlife, and humans disperse seed. 

Scotch thistle forms a rosette the first year and then bolts the second year to produce flowering stalks and has a taproot.  Plants are usually two to six feet tall but can grow to a height of twelve feet with a width of five feet.  Plants have a blueish-gray look to them because of the thick hairs covering the leaves.  Leaves are arranged on an alternate pattern from the stalk and can be twenty inches long.  Leaves are oblong, lobed with yellow spines.  Stems have spiny wings and become rectangular with plant age.  Flowers are purple to white in color.  There can be one to seven flower heads per branch.  Seeds are small brown to black in color.

Habitat

Scotch thistle is found in any type of habitat but normally establishes quickly in disturbed areas dominated by annual plants such as cheatgrass.  It can also be found in over-grazed sites, roadsides, and riparian areas.  Scotch thistle is found across most of North America.  It can invade healthy, undisturbed sites as well, out-competing desirable forbs and grasses in pastures and rangeland and reducing biodiversity.  The sharp spines deter wildlife and livestock from grazing.  Scotch thistle is considered a noxious weed in some counties of Nebraska and some neighboring states. 

Management

Prevention is the best and cheapest management option.  Having well-established perennial grasses and forbs on a maintained pasture or rangeland with proper grazing and rotational grazing techniques can go a long way to prevent its establishment.  Scouting, monitoring and proper identification are key factors for management.  Infestations of this weed can occur very rapidly.  Management of seed production is the key to keep this plant from spreading.  Several different management options will need to be utilized to manage this weed.

There are no biological control methods available at this time, other than early grazing with sheep or goats that can reduce seed production.  A chemical follow-up treatment may be needed to manage surviving plants.  Pulling and/or digging up the plants below the crown is effective if there are a few plants. 

Mowing can be done but will have to be repeated for the regrowth.  Mowing will not kill the plant.  Mowing plants with visible seed heads will not prevent seed production.  Chemical treatment should follow the mowing to prevent seed formation. 

There are numerous chemical treatment options available to manage Scotch thistle.  Products containing aminopyralid, clopyralid, chlorsulfuron, dicamba, metsulfuron, picloram (Restricted Use), triclopyr, glyphosate (non-selective) and 2,4-D have been shown to work.  Spring or fall applications, especially in the rosette stage, prior to the pre-bud stage are best.  Fall treatments are better after a light freeze.  Tank mixes of several of these compounds may provide better control.  The addition of a non-ionic surfactant to the herbicide mix will aid in control.  Re-treatment is usually necessary 3 to 5 years or until the seed in the soil is exhausted.  Spray early, plants treated with visible seed heads will still produce viable seed.  Be sure to select a product labeled for the site.  Read, understand and follow all label instructions when using any pesticide

Nebraska Extension Publications has a number of publications on thistle management and other invasive species.  These publications and much more are found at http://extensionpubs.unl.edu/ .  Search “thistle” or “invasive”.

References

Kadrmas, T. et al, “Managing Scotch Thistle”, University of Nevada, Fact Sheet 02-57

Schuster, M. and Prather, T. S., “Scotch Thistle”, University of Idaho, PNW 569

USDA / NRCS Plant Profile, “Scotch Thistle, Onopordum acanthium L.”

Interviews with the authors of BeefWatch newsletter articles become available throughout the month of publication and are accessible at https://go.unl.edu/podcast.