Iowa State University

Now's the time to control biennials  
by Bob Hartzler

October 03, 2007 - April is an excellent time to treat pastures and other non-tilled areas for biennial weeds. The biennial thistles (musk, bull, tall, etc.) are all classified as noxious weeds by the Iowa Noxious Weed Law, but musk thistle is by far the most invasive and troublesome of this group. In recent years, several biennials in the Apiacae (carrot) family have become much more prevalent across Iowa, including wild carrot, wild parsnip and poison hemlock. Wild carrot and poison hemlock are on the Iowa noxious weed list. Biennials have a two year life cycle: in the first year a basal rosette (circular cluster of prostrate leaves) is produced, in the second year a central flowering stalk elongates, and the plant dies after seed maturation (Figure 1).

Management of biennials is a two-step process. The first involves controlling existing plants to prevent seed production. The second step requires identifying, and eliminating, factors that allow the biennials to become established.

Step 1: The most common method of controlling biennials is the use of herbicides. Biennials are more susceptible while in the rosette stage, so fall or early spring applications are the preferred treatment. Herbicides used in pastures for biennial control are listed in Table 1. The optimum time for treatment of biennial weeds differs from that for most perennial or annual weeds found in pastures. Scattered plants can be controlled by severing the root below the crown with a spade or similar tool. Mowing is another effective tactic, but timing is critical. The first mowing should be done before flowers fully expand in order to prevent seed production. Biennials have dormant crown buds that often initiate growth after removal of the primary stalk, thus a second mowing 3-4 weeks after the initial mowing often is required.

Step 2: Biennials are most commonly found in habitats containing low-maintenance, perennial grasses (pastures, roadsides, etc.). Biennial seedlings are relatively weak compared to the perennial sod, thus they are at a competitive disadvantage during the establishment phase. Enhancing the vigor of the sod can greatly reduce biennial infestations, therefore overseeding, proper fertility management, and avoiding over-grazing can greatly reduce problems with biennial weeds.

Table 1. Herbicides for control of biennial weeds in pastures.

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