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Issue No. 4, Article 4/April 29, 2011

Be on the Lookout Again for Biennial Thistles

If you haven't noticed them yet, you soon will. As we approach early May, biennial thistles will become more evident along roadsides and in right-of-ways, waste areas, and pastures. Until now they have been overwintering in the rosette stage of growth, mostly hidden in the taller grasses surrounding them. After several weeks of warm spring weather, they are beginning to bolt, in preparation for flowering and seed production.

There are numerous biennial thistles in Illinois, including plumeless thistle, bull thistle, Flodman thistle, and tall thistle. But by far the most common is 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. Musk thistle is listed as a noxious weed by the Illinois Department of Agriculture.

Musk thistle, rosette stage.

Biennial thistles live for only two years, and they propagate only through seed production. 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 assumes a more upright growth pattern and begins to bolt a flowering stalk. Flowering and seed production occur from late May through June.

Musk thistle, bolting.

Successful management of thistles 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 to reduce seed production.

Musk thistle, flowering.

Herbicides are most effective against thistles when applied during the rosette stage of growth, in either late fall or early spring. Once the plant has begun to bolt and flower, it can produce viable seed even after being sprayed with an herbicide. Herbicides used to control thistles in pastures may have haying or grazing restrictions or animal-withdrawal restrictions before slaughter. Always read and follow all herbicide label directions. Selective herbicides that kill thistles without harming desirable grasses are preferable, since a thick grass cover will help suppress germination of thistle seeds later in the season.

Mowing can be beneficial, but it must be done on at least monthly, with the mower run as close to the ground as possible. If you mow only once, basal and root buds often break dormancy and produce new flowering stalks. Mowing followed by an herbicide application works better than mowing alone.

Systematic management includes controlling thistles in fencerows and roadways to prevent new seed introduction, avoiding overgrazing so that forages compete with the weeds, and reseeding forage species into overgrazed and disturbed areas.--Robert C. Bellm, Extension educator, crop systems

Robert Bellm

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