If you haven't noticed them yet, you soon will. Biennial thistles will become more evident along roadsides, rights-of-way, waste areas, and pastures in the next few weeks, says University of Illinois Extension crop systems educator Robert Bellm.
Numerous biennial thistles grow in Illinois, including plumeless thistle, bull thistle, Flodman thistle, and tall thistle. However, the most common biennial thistle found here by far is the musk thistle, also referred to as nodding thistle because of the way its flowers often bend over or "nod" toward the ground.
"All thistles, because of their aggressive spread and spiny nature, are detrimental to forage production as well as animal and human well-being," Bellm says. "Musk thistle, in particular, is listed as a noxious weed by the Illinois Department of Agriculture."
Biennial thistles live for only two years, and propagate through seed production only. The seeds germinate during late spring to early summer, and plants form a prostrate rosette of leaves during their first growing season and through the winter. During the second growing season, beginning in late April to 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.
"Successful management of thistles requires an integrated and systematic approach to prevent seed production and spread," he adds. "Early infestations often consist of small patches that should be eliminated as quickly as possible in order to reduce seed production."
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 in pastures may have haying or grazing restrictions, or animal withdrawal restrictions prior to slaughter.
"Remember to always read and follow all herbicide label directions," he notes. "Selective herbicides that kill thistles without harming desirable grasses are preferable, since a thick grass cover will help to suppress germination of thistle seeds later in the season."
Bellm says mowing can be beneficial, but must be done on at least a monthly basis, with the mower run as close to the ground as possible. If mowing only occurs 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 than mowing alone.
"Systematic management is the best approach," Bellm concludes. "This includes controlling thistles in fencerows and roadways to prevent new seed introduction, avoiding overgrazing so that forages will compete with the weeds, and reseeding forage species into overgrazed and disturbed areas."
For more information about weeds and other crop-related information, read The Bulletin at http://bulletin.ipm.illinois.edu.