Take on Biennial Weeds in Pastures

Warm temperatures are pushing the development of undesirables in those fields.

Published on: Apr 20, 2012

No doubt, warm weather was the talk this spring, but it was also great for biennial weeds that can pop up anywhere, but are trouble in a pasture or along roadsides. Notes Robert Bellm, extension educator, University of Illinois: "Up until now," Bellm said, "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, those weeds are now beginning to bolt in preparation for flowering and seed production. Biennial weeds live for only two years and propagate only through seed production. The seeds germinate during late spring to early summer, and the plants from a prostate rosette of leaves during their first growing season and through the winter.

CLEAN PASTURES: Getting rid of biennial weeds takes a little planning.
CLEAN PASTURES: Getting rid of biennial weeds takes a little planning.

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 during late May through June, but will occur earlier this year.

Bellm noted that there are numerous biennial thistles in Illinois, including plumeless thistle, bull thistle, Flodman thistle, and tall thistle. "However, by far the most common biennial thistle found here is musk thistle, which is sometimes called nodding thistle because of the way its flowers often bend over or 'nod' toward the ground," he said.  

All thistles, because of their aggressive spread and spiny nature, are detrimental to forage production. The Illinois Department of Agriculture lists musk thistle as a noxious weed.

Poison hemlock is a member of the parsley family and looks similar to parsley during the rosette stage. "As Socrates found out, it truly is poisonous when eaten, both by humans and animals," said Bellm.

Successful management of biennial weeds requires an integrated and systematic approach to prevent seed production and spread. Early infestations are often in 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, either in late fall or early spring. However, "Once the plant has begun to bolt and flower, it has the capability of producing viable seed even after being sprayed with a herbicide," Bellm said.

It is important to read and follow all herbicide label directions. There may be use restrictions related to haying, grazing, and animal withdrawal when herbicides are used on hay land or pastures. Bellm suggests using selective herbicides that kill biennials without harming desirable grasses because a thick grass cover will help to suppress germination of seeds later in the season.

Mowing can be beneficial but must be done at least once a month with the mower run 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," explained Bellm. A combination of mowing followed by a herbicide application works better than mowing alone.

Systematic management includes controlling biennials in fence rows 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.