Iowa State University

It's Time to Control Biennial Thistles
by Bob Hartzler

  March 28, 2003 -  The biennial thistles (musk, bull, tall, plumeless, etc.) are some of the most problematic weeds of Iowa pastures.  Biennials require two years to
  complete their life cycle.  Seeds germinate in the spring, and during the first year produce a basal rosette of leaves (photo).   In the second year the flower stalk
  emerges from the basal rosette, seed is produced, and the plant dies.  A small percentage of musk thistle (and probably other biennials also) behave as annuals in
  Iowa.  These plants flower and die the same year they emerge, typically flowering 1-2 months after the individuals that act as true biennials.  Unlike Canada thistle, the biennial thistles only reproduce by seed, and because of this, are considerably easier to manage.

Fall or early spring is the best time to control musk thistle and other biennials in pastures. Biennials are much more difficult to kill once the flower stalk has begun to elongate. Several options are available that will provide acceptable control of most biennials, including:

• 1.5-2 quarts 2,4-D,
• 1 quart 2,4-D + 0.5 pint Banvel/Clarity,
• 0.2-0.3 ounces Ally/Cimmaron,
• 2-4 quarts Remedy,
• 2-4 pts Grazon P&D, and
• 8-12 ounces Tordon 22K + 1 quart 2,4-D.

These treatments also are effective on other biennial weeds, such as wild carrot and wild parsnip, that frequently invade pastures.  Tordon, Grazon P&D and Remedy all contain picloram.  This product is more effective on thistles that have begun to bolt (elongation of flower stalk) than 2,4-D or Banvel/Clarity.  The lower rate should be used on rosettes, whereas higher rates are required on bolted plants. 

Mowing can be used as an alternative to herbicides, with the intent of preventing flowering and production of seed.  Biennial thistles have numerous basal buds that will break dormancy when the main flower stalk is severed by mowing.  Because of this, several mowings are required to completely prevent seed production.

Many products registered for use in pastures have restrictions concerning haying or grazing, which should be considered prior to product selection (see table). Most perennial weed species (e.g. Canada thistle) will not reach an appropriate growth stage for treatment for several weeks after the optimum time for treating biennial weeds.

Grazing and haying restrictions for pasture herbicides.

¹Interval animals must be removed from treated area prior to slaughter.
²Restrictions vary on 2,4-D labels. Check label of product used for specific restrictions.

While herbicides occasionally are needed to manage specific weed problems in pastures, promoting vigorous growth of the pasture grasses should be the foundation of the weed management program. Many weed problems are due to overgrazing, poor fertility programs, or other management problems. Eliminating the cause of the problem, rather than treating the symptom, should be the focus of weed management in pastures.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University