Home | Past Issues

Issue No. 3, Article 7/April 20, 2012

Yes, It's Time Again to Control Biennial Weeds in Pastures

This spring's above-average temperatures have speeded progress on fieldwork and planting. So it shouldn't be too surprising that they have also accelerated the development of biennial weeds found in pastures and along roadsides. A problem that we would typically think about dealing with in mid-May is rearing its ugly head already.

If you haven't noticed the biennial weeds yet, you soon will as they become more evident along roadsides and in right-of-ways, waste areas, and pastures. They have been overwintering in their 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.

Biennial weeds live for only two years and propagate solely 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, the plant assumes a more upright growth pattern and begins to bolt a flowering stalk. Flowering and seed production normally occur in late May through June, but the timeframe will be earlier this year.

Numerous biennial thistles grow in Illinois, including plumeless thistle, bull thistle, Flodman thistle, and tall thistle, but by far the most common is musk thistle, sometimes referred to as nodding thistle because of the way its flowers often bend or "nod" toward the ground. All thistles, because of their aggressive spread and spiny nature, are detrimental to forage production as well as to animal and human well-being. Musk thistle is listed as a noxious weed by the Illinois Department of Agriculture.

Musk thistle rosette beginning to bolt.

Musk thistle (aka "nodding thistle") flower.

Poison hemlock is a member of the parsley family, and during the rosette stage it does indeed look similar to the parsley you would put on your salad. Consuming it has an unfortunate side effect, however, as Socrates found out to his sudden demise a couple of thousand years ago. Poison hemlock truly is poisonous when eaten, whether by humans or animals.

Poison hemlock: Note the parsley-like leaves and round stem (which is hollow) with purple splotches.

Poison hemlock forms dense stands if left uncontrolled.

Successful management of biennials 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.

Herbicides are most effective 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 biennials in pastures may have restrictions on haying or grazing or on animal withdrawal prior to slaughter. Remember to always read and follow all herbicide label directions. Selective herbicides that kill biennials 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 at least monthly, with the blade as close to the ground as possible. If you mow only once during the season, basal and root buds will often break dormancy and produce new flowering stalks. Mowing followed by an herbicide application works better than mowing alone.

Systematic management includes controlling biennials 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.--Robert C. Bellm

Robert Bellm

Click here for a print-friendly version of this article

Return to table of contents