Iowa State University

leftbar.JPG (7146 bytes)rightbar.jpg (2335 bytes)

 

Now's the time to control biennials  
by
Bob Hartzler

blueline.jpg (1822 bytes)

October 03, 2007April 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.

Herbicide Biennial
thistles
Biennial
Apiaceae
Grazing Restrictions Classification
1.5-2 lb 2,4-D ester
G-E
G-E
7 days for dairy animals
General use
2,4-D+ dicamba
G-E
G-E
Rate dependent for dairy

General use

Cimarron (metsulfuron)
G
P
None General use
Cimarron Max
(metsulfuron + 2,4-D+dicamba)
G-E
G-E
7 days for lactating dairy animals General use
Grazon P&D (picloram + 2,4-D)
E
E
7 days for lactating dairy animals Restricted use
ForeFront (aminopyralid + 2,4-D)
E
G
None General use
Milestone (aminopyralid)
E
P
None General use
Overdrive (dicamba + diflufenzopyr)
G-E
G-E
None General use
Redeem (triclopyr + clopyralid)
E
G-E
14 day for lactating dairy General use

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

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
Submit questions or comments here.  

Copyright 1996-2006, Iowa State University, all rights reserved  

Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.