Scotch Thistle Facts and Fables

Scotch thistle is a robust non-native thistle that is becoming a big issue for Musselshell and Golden Valley Counties. Scotch thistle is not on the Montana State noxious weed list, but has been listed as an invasive species that need to be controlled by both counties Weed Districts. Scotch Thistle was introduced into the United States during the 1880s possibly as an ornamental and /or as a medicinal plant. It is now abundant and problematic in the western U.S. with infestations that can form tall, dense stands that impede livestock and wildlife access to desirable forage plants. Scotch thistle is generally considered a biennial weed, but can also grow as an annual or short-lived perennial. The plant reproduces exclusively by seed that can germinate throughout the year depending on moisture and temperature. Large Scotch thistle plants can produce from 20,000 to 40,000 seeds that can remain viable in the soil for at least seven years. Seed are 0.2 inches in length, smooth, slender, and plumed. A water-soluble germination inhibitor contained in the seeds suggests that germination will not occur without optimum soil moisture. The first year, Scotch thistle forms a large spiny rosette with leaves that can be 12 inches or more in width. Flowering stems are normally produced during the second growing season and can grow from 6 to 12 feet tall. Stems have vertical rows of spiny ribbon-like wings that extend to the flower bases. Leaves are oblong and prickly, upper and lower leaves are covered with a thick mat of cotton-like hairs that gives the plant its gray-green color. The dark pink to lavender flower heads measures 1 to 2 inches in diameter. The flowers bloom July-October.

Prevention is the best and cheapest management option. Having well-established perennial grasses and forbs on a maintained pasture or rangeland can go a long way to prevent establishment. Management of seed production is the key to keep this plant from spreading. There are no biological control methods available at this time, other than early grazing with sheep or goats that can reduce the seed production. Small infestations can be dug out. Dig out the rosettes by severing the plants taproot with a shovel below the soil’s surface. Mowing has limited effectiveness for controlling Scotch thistle, usually only prevents seed production. Mowing too early and the plant can recover and flower. Flowerheads that are cut when already in bloom may still release seeds if left on the ground, so bag up and trash flowerheads. Please remember to use gloves when dealing with Scotch thistles, Herbicide treatments are available and very effective if used properly. Contact the Weed District for help finding the most effective way to control Scotch thistle at 204 8th Ave East Roundup Mt 59072 or call 406-323-3740. As Smokey Bears used to say only you can prevent the spread of Scotch thistle in our counties.