Bull, musk and plumeless thistle (Cirsium vulgare, Carduus nutans and acanthoides) germinate in the summer and fall and overwinter as a rosette. The following spring, biennial thistles resume vegetative growth, bolt and then flower. Purple flowers are the identifying factor among the three species. Numerous flower heads are produced from May to October, depending on the species.
Bull thistle is erect, 2 to 5 feet tall with many spreading branches, spiny wings from leaves. Rosette formed first year; flowering stem elongates second year. Leaves have short prickles on the surface and are cottony below. Flowers are dark purple and somewhat clustered.
Musk thistle is erect, freely branching, up to 7 feet tall. Leaves are dark green with light midrib, hairless on both sides, with long, sharp spines. Flowers are deep rose to purple, up to 3 inches in diameter. Branching plant with spiny wings that extend up to flowers.
Plumeless thistle stems grow 1 to 4 feet tall. Rarely flower the first year. Flowers are purple to pink, 1 to 2 inches in diameter, solitary or in clusters of two to five. Bracts are sparsely to densely hairy.
Bull, musk and plumeless thistle are native to Eurasia and were introduced into North America as seed contaminants. Spreading by seed only and producing 8,400 seeds per plant with plumeless thistle to 20,000 seeds per plant from musk thistle. Biennial thistle species can be found across the United States invading overgrazed or otherwise disturbed pastures, rangeland, roadsides and waste areas.
Since biennial thistles reproduce only from seed, the key to a successful management program is to control the plants before flowering. For best control, apply 12 fluid ounces per acre of DuraCor™ herbicide during the rosette stage early in the season. Or apply 1.5 to 2.5 ounces per acre of Chaparral™ herbicide to rosettes or bolting plants or in the fall to seedlings and rosettes before the ground is frozen.
For late-season application (bolting to early flower), apply 12 fluid ounces of DuraCor plus 0.25% NIS when plants are at the late bolt through early flowering stage.
Label precautions apply to forage treated with DuraCor or Chaparral and to manure from animals that have consumed treated forage within the last three days. Consult the label for full details.
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