Musk or nodding thistle is the most common type of thistle locally. The primary growth period is in the spring and summer. However, most seed germinate in the fall and form a rosette which grows close to the ground, often growing unnoticed until spring.
The most important step in long-term control of thistle is to prevent flowering, and the production and spread of new seed (which is carried by wind). This can be done by mechanical or chemical control.
For mechanical control, mowing, clipping pastures or hand grubbing can be used before flowering. Some regrowth and production of flowers can occur after mowing, but will be notably less than if mechanical control had not been used. Thistle plants mowed or removed by hand after blooming can still produce viable seed.
Broadleaf herbicides (labeled for use in pastures) can be applied in grass pastures and non-cropland areas. However, for herbicides to be effective the timing of the application is critical. Best results can be obtained if herbicides are applied to plants that are in the early rosette stage of growth. Therefore, the best times for herbicide application are in the early spring or fall. These herbicides will also usually kill any clover in the field.
For spring herbicide applications apply when air temperatures are above 55 degrees for two-to-three days. After flower stalk elongation, control will be less effective. Avoid spraying near sensitive or ornamental plantings, and do not spray on windy days or days with extremely high temperature and/or humidity.
The Kentucky Department of Agriculture has a thistle control demonstration program where they supply the chemical and sprayer to spray 10 acres of pasture (you supply tractor and driver). The date for LaRue County is Monday, April 7.
For more information on this program, or for publication AGR 20, Nodding Thistle and It’s Control in Grass Pastures, contact the Extension office.