Invasive Watch: Scotch thistle (Onopordum acanthium)
WHAT TO LOOK FOR.
Scotch thistle is a biennial or annual forb that can grow up to 8 feet tall and 6 feet in width with branching, spiny leaf wings that extend down onto the stem and are covered with dense, fine hairs. Leaves are large, closely lobed, hairy and lined with sharp spines. Flowers are numerous with spine-tipped bracts and are violet to reddish-colored. The plant typically produces a large, ground-level rosette in the first year and becomes a tall, spiny plant in its second.
Scotch thistle reproduces by seed, and each plant is capable of producing 8,000 to 40,000 seeds, which may remain viable in the soil for 30 years. Plumed seeds can be dispersed by attaching to clothing and animal fur, by being transported in hay and machinery, or by being carried by wind and water.
WHERE IT’S FOUND.
Scotch thistle is often found along roadsides, irrigation ditches and waste areas, in pastures and on rangelands. The plant is native to Europe and Asia and was introduced to the United States in the late 19th century as an ornamental plant. It’s now considered a major noxious weed and has spread to most states across the United States. It spreads rapidly into dense stands that prohibit foraging by wildlife and livestock and is difficult to eradicate because of its drought resistance.
HOW TO TREAT IT.
Apply 5 to 7 fluid ounces per acre of Milestone® specialty herbicide in spring and early summer to rosette or bolting plants, or in fall to seedlings and rosettes before the ground is frozen. An additional option is an application of 2.5 to 3.3 ounces per acre of Opensight® specialty herbicide, again, in spring and early summer to rosette or bolting plants, or in fall to seedlings and rosettes before the ground is frozen.