Musk Thistle, Carduus nutans, is a biennial, noxious weed that occurs throughout the United States. EDDmaps reports it in Lyon county, where I took these photos. A rosette forms in the first year and the second year the plant will bolt and form a flower heads. The plant is solitary and does not spread vegetatively. The stems and leaves have spines. The flowers are white to pink or purple with spine tipped bracts at the covering the receptical, or base, of the flower. The flower heads often “nod” with the stem bending and the head tilting over to one side. Carduus species can hybridize, so morphological characteristics vary.
Confirm your identification prior to treating an area. Native thistles can appear very similar to musk thistle, but pose no environmental or economic threat in agricultural, grazing or natural areas.
To control musk thistle, it is best to remove the plant in the first year, when it is a rosette and has not flowered. The tufted seeds spread easily with wind. Thistles only reproduces from seed, so mechanical control can be very effective. Mechanical control is most effective when the tap root is disturbed and destroyed. Once the plant goes to seed, the area will need to be monitored every year and any rosettes destroyed. Seeds readily germinate in disturbed soil, so plan mechanical controls in the spring prior to flowering from June to September.
Biological control agents have been released for this weed in Nevada, but are no longer available for distribution and release. If you have a large contiguous patch, where other controls are not an option, you can monitor for the biological control agents by looking for the larvae in the root crown or seed heads. The spines and the detestable flavor of the plant prevent grazing by most animals, so control by grazing is not effective.
This time of year, the plants will have already gone to seed. If you notice the remainder of the plants, mark the location and plan to revisit the area in the spring to scout for rosettes. Prevention of further establishment is key to preventing this noxious weed from causing economic or environmental harm.