Musk Thistle

Carduus nutans

 

Musk thistle is a large, biennial herb. Leaves are dark green with a light-green midrib, smooth and hairless on both sides, coarsely lobed, slightly wavy, and arranged alternately on a stem that appears winged. Each lobe ends with a prominent spine. First year rosette leaves can be up to 24 inches long. Plant height varies from two to seven feet. The terminal flower of musk thistle is large--1 1/2 to 3 inches in diameter--solitary, and usually nodding or slightly bent over. The stem is freely branched.

Plumeless thistle is similar, but with many leaf-like spines on the stem, and hair on the underside of the leaf. In addition, the red to purple flowers of plumeless thistle are much smaller--usually about one third the size of musk thistle flowers. Plumeless thistle flowers are single or clustered, erect on the stems, and usually do not droop or nod.

These two thistles can hybridize, and both are very similar in the rosette stage.

Bull thistle is also a biennial with prickly, winged stems. The leaves are coarse and spiny above with wooly white hairs below. A purple "brush" of flowers emerges from a spiny green ovoid base.

Eliminating seed production is the most effective mechanical control technique. Thistles mowed in bud or early bloom stage will produce new branches from buds in the axils of the basal leaves. However, close mowing or cutting twice per season will usually prevent seed production. This can be done at any time during the growing season, although cutting is easier when the thistles are smaller. Mowing once flowering has begun may result in the spread of viable seeds with the mower. For effective selective control, plants should be cut with a sharp shovel at 1" to 2" below the soil surface before flowering.

Tilling, hoeing, or hand pulling should be done before flowering. Cut the plant below the ground or as close to the ground as possible to prevent regrowth. Cutting or mowing is more effective later in the season when the stem core is hollow, but before flowering. At that time, the plant is least likely to regrow. If cut in the rosette stage, they will regrow easily. Mowing can wait until two days before blooming to prevent seed production. Mowing or hand cutting only four days after flowering will allow some seeds to mature. Plants cut after the flowers open should have the flowers removed. Put the flowers in a tight container and destroy them.