Warm May weather promotes rapid growth of many pasture plants. While this is great for forage grasses and legumes, it is not so great if you have biennial thistles in your pastures. These thistles will start to become more evident in pastures in the upcoming weeks, and because of their aggressive spread and spiny nature, they are detrimental to forage production as well as animal and human well-being.

"Most thistles that we encounter in pastures have a biennial life cycle," explains Robert Bellm, University of Illinois Extension crop systems educator. "They live for only two years and propagate only through seed production. Their seeds will germinate during late spring to early summer, forming a prostrate rosette of leaves during the first growing season and through the winter. During the second growing season, typically around early May, the plant will assume a more upright growth pattern and begin to bolt a flowering stalk. Flowering and seed production will occur during late May through June."

Common biennial species include musk, tall, bull and plumeless thistle. Musk thistle is probably the most common species, and it is officially listed as "noxious" by Illinois law because of its highly invasive nature.

Bellm says thistle control in pastures requires an integrated and systematic approach to prevent seed production and spread. Early infestations often consist of small patches that should be eliminated as quickly as possible. A systematic approach also includes controlling thistles in fencerows and roadways to prevent new seed introductions, avoiding overgrazing so that your forages will out-compete the weeds, and re-seeding forage species into overgrazed and disturbed areas to provide competition to thistles as they germinate.

Herbicides are most effective when applied during the rosette stage of growth, either in late fall or early spring. Once the plant has begun to bolt and flower, it has the capability of producing viable seed even after being sprayed with a herbicide. Herbicides used to control thistles may have restrictions on haying or grazing after application, and some may have animal withdrawal restrictions prior to slaughter. These restrictions will be on the herbicide label, so carefully read and follow instructions.

"Unfortunately, all herbicides labeled to control thistles will also kill any forage legumes that they come in contact with," says Bellm. "Control thistles by spot treating early infestations. If things have gotten out of hand and the entire pasture has to be treated, you will need to re-seed the legumes after the appropriate waiting period indicated on the herbicide label."

Along roadsides and in other non-pasture areas, mowing can be beneficial. However, it must be done at least monthly, with the mower run as close to the ground as possible. If you only mow once during the season, basal and root buds will often break dormancy and produce new flowering stalks.

"A combination of mowing followed by a herbicide application works better," advises Bellm. "If mowing is delayed until flowers have formed, it is highly likely that viable seeds will be produced and that the mowing will spread the seeds."