Musk Thistle

Weed Description: An erect biennial with spiny leaves and stems that may reach 6 ½ feet in height. Primarily a weed of pastures, hayfields, roadsides, and noncrop areas that can be found throughout the United States.

Seedling: Cotyledons are rectangular to oblong in outline, approximately 7 ½ to 15 mm long and 2 ½ to 6 mm wide. Cotyledons occur with little to no petioles (sessile) and have distinctive white veins on their upper surface. Young leaves are essentially without hairs and immediately take on a rosette growth habit.

Stems: Erect, branched, with spines extending down the stem from the leaf bases.

Roots: Large, thick taproot that is hollow near the soil surface.

Leaves: During the first year of growth a basal rosette of leaves form with the first 2 true leaves being opposite and all subsequent leaves alternate. During the second year of growth, the rosettes elongate and flowering stems are produced. All leaves that occur on the flowering stems are also alternate. All leaves are dark green in color with light green to white midribs and veins. Leaves are lanceolate in outline, deeply lobed and approximately 10 inches long by 4 inches wide. Three to five spines occur along the margins of each lobe, and these white or yellow spines are approximately 2 to 5 mm long. The leaf bases extend down to the stem, and the leaves become progressively smaller up the stem.

Fruit: An achene that is tan to brown in color and approximately 4 mm long. Achenes are oblong in outline and have a white pappus that resembles white hairs.

Flowers: Solitary flower heads are produced at the end of branches. Individual flowers are 1 ¼ to 2 inches wide and are pink to violet or purple in color. Spiny bracts occur below the flower heads and these are often tinted purple in color.

Identifying Characteristics: Erect, spiny biennial with deeply lobed leaves and relatively large flowers that are pink to violet or purple in color. Musk thistle is similar in growth habit and appearance to Bull Thistle (Cirsium vulgare), however bull thistle has many hairs on the upper surface of the leaf blades unlike musk thistle which mostly lacks hairs. Additionally, the flower heads and bracts of bull thistle gradually taper to a point when compared to those of musk thistle. Musk thistle may also be confused with Canada Thistle (Cirsium arvense), but Canada thistle has rhizomes and rarely takes on a rosette growth habit unlike either bull or musk thistle.