Remove any seed bearing weeds from the site immediately after cutting. At this stage, the prairie plants are still small and have not yet gained full control of the soil environment. Burning in spring of the second year often encourages germination of dormant wildflower seed, but also exposes the surface soil, which can cause increased weed growth. Mowing tends to facilitate germination of dormant seed and enhances the growth of prairie plants, without potentially increasing weeds. If weeds remain a problem in the second year, they may have to be mowed in late spring or early summer.
Biennial weeds can be very competitive in the second year.
Mowing them back to about one foot when they are in full bloom will kill them or set them back severely, with minimal damage to your prairie plants. This must be controlled because the seeds of Sweet Clover are stimulated to germinate by fire and can become a long-term management problem if not handled at the outset. If it reappears in the third year, it will likely be on a limited basis and can be hand-pulled. Do not let Sweet Clover make seeds, as it can be a most pernicious weed. Burning or mowing is usually, but not always, conducted in mid-spring.