Musk thistle, often called nodding thistle, is the most common biennial invasive thistle in North Dakota. Native to southern Europe and western Asia it was introduced in to North America in the early 1900's. Musk thistle tends to invade overgrazed or otherwise disturbed pastures, rangeland, roadsides and waster areas. Musk thistle spreads rapidly and can form very dense stands the crowd out desirable forages and native species.
Musk thistle is easily identified, yet many people confuse it with either Bull thistle or Plumeless thistle. Musk thistle often grows in excess of 6 feet tall, has very large flowers that tend to droop that nod in the breeze. The flower has very characteristic brown bracts that resemble a pine cone. Flower color is usually a deep rose, solitary and very large, range from 1.5 to 3 inches in diameter. Musk thistle flowers from July to late September. The average musk thistle plant produces in excess of 10,000 seeds per plant and, under favorable conditions, may produce 120,000 seeds per plant. The seed generally germinates in the summer and fall and overwinters as a rosette. The following spring the plants bolts and flowers.
Control in non-crop lands:
Since biennial plants such as musk thistle reproduce only from seed, the key to successful management program is to control the plants before flowering.
Chemical: Fall is the preferred time for applying herbicides for biennial thistle control. Seedlings that emerge in summer will not bolt but remain in the rosette stage. Milestone, Curtail, Tordon, and Overdrive effectively control musk thistle in the rosette stage. Escort and Cimarron Max will control biennial thistles in the spring and control seed production when applied in the bolting to bud growth stages.
Cultural: Repeated mowing will reduce musk thistle infestations. Mow whenever the plants are in the early bud growth state to prevent seed-set. Repeated mowing during the growing season may be necessary and over several years to eliminate all seedlings.
Biological: Two insects have been introduced to control musk thistle. The seed weevil Rhinocyllus conicus reduces seed production. The larva develop in the flower head and consume the seed as it develops. The larva of the thistle crown weevil Trichosirocalus horridus attacks the growing tip of the musk thistle. When both are used in combination effective control is achieved.