Biennial thistles start as a flat rosette of leaves, then flower, seed, and mature.
The most effective control will target applications before the flower stalks form. These early-season biennials are local noxious weeds in many counties, which puts them under South Dakota law, said Wrage. Musk, plumeless, and bull thistle are the most common biennials while Scotch thistle is less widespread. Musk and plumeless thistle have a distinct white mid-rib. Leaves are dark green and may be frosted along the edge.