Musk thistle has been a nuisance in Kansas for many years. It is on the noxious weed list and landowners with the weed on their property are obligated by law to get it under control. Musk thistle, which is also known as nodding thistle because of the way it moves in the wind, is usually a biennial or winter annual but can also occur as a summer annual. As a biennial, it produces a rosette the first year and then develops flowers and seeds the following year. Seedlings can emerge any time during he growing season, but the plants usually remain in the rosette for about 90% of their life cycle.
Musk thistle usually starts flowering in May and may remain blooming for several weeks. The flowers are usually deep rose to violet or purple. The leaves are coarsely lobed and dark green with a light green midrib. They can have a sliver-green leaf margin. The leaves are alternately arranged on the stem and are smooth and hairless on both sides. Each lobe has three to five lobes that ends in a whitish or yellowish spine. Seed development is very quick with seed dispersal beginning within seven to ten days after the head begins to bloom. Musk thistle seed is generally dispersed by the wind but can be spread by other methods as well.
Light infestations can be controlled fairly well with cultural practices. The plant can be removed by hand by digging below the crown of the rosette. Be sure to remove flowers and seed heads from the field and burn them. Nebraska research shows that mowing is most effective at full bloom but needs to be done repeatedly to destroy new stems and flowers that develop after the initial mowing. Remember that an escaped flower represents seed that is produced and distributed with the end result being more thistle plants.
Chemical control can be accomplished either in the fall or the spring. Ed Field, Crawford County noxious weed director indicates that in his experience that spring control can be accomplished with the use of 2,4-D alone while the plant is still in the rosette stage. The rosette stage is when the plant is growing in a circular pattern without any elongation or stem present. After the plant begins to bolt or elongate, Milestone should be mixed with the 2,4-D in order to get satisfactory control. After the plant begins to bloom Escort needs to be added to the mixture of 2,4-D and Milestone in order to prevent the flowers from producing seed. Applications over 4/10 oz. of Escort may temporarily reduce fescue production. Spraying should not be done unless the air temperature is over 50 degrees F.
