Repeated tillage of these areas for one growing season often eliminates the infestation arising from roots, but this is seldom a practical approach in pastures. Areas where thistles are controlled with repeated tillage must be watched carefully for new plants that start from seeds. These should be removed when they are small so that they do not form roots with buds. Bull thistle seldom forms dense stands but musk and plumeless thistles are quite invasive and can render large areas unsuitable for grazing if not controlled. Biennial thistles form a rosette of leaves the year seeds germinate and must undergo winter to shift from vegetative to reproductive growth.
Plants flower the following summer and die at the end of summer or with the first frost.
Preventing seed production is the key to managing biennial thistles. Mowing Mowing is an option for weed control in pastures. Mowing annual weeds once will usually control them if the pasture is healthy and exhibits rapid regrowth. Serious annual weed problems are rare in pastures and are indicative of declining health of desirable species. These paddocks may be candidates for improvement or renovation.