Managing Poison Hemlock in No-till

April 24, 2009
ICM News

 By Bob Hartzler and Mike Owen, Department of Agronomy

Poison hemlock, a biennial in the carrot family (Apiaceae), is becoming more common in crop fields as no-till acres increase.  Infestations typically start in fencerows or other less-intensively managed areas and creep into production fields.  Like other biennial weeds, its tolerance to control tactics increase rapidly as the plant moves from a vegetative rosette to reproductive stages in early spring.

The simplest method of controlling poison hemlock is to eliminate seed sources in areas adjacent to the field.  Fall applications of 2,4-D + dicamba are highly effective against the plant.  Applications to these areas can be made following corn or soybean harvest, and infested areas of crop fields should be treated at the same time.

Poison hemlock will not be controlled consistently by the standard burndown program of glyphosate + 0.5 lb/A  2,4-D LVE (1 pt 4 lb a.e./gal).  Increasing the rate of 2,4-D LVE to 1 lb will improve control, but this requires a 15 day planting interval for soybean and 7 to 14 day interval for corn. 

Alternatives to the standard glyphosate +2,4-D burndown treatment include dicamba  (14 day planting interval is required before planting soybean following an 8 oz application of Clarity) or  Basis (corn only).   Field observations suggest that mesotrione (Callisto, Lumax, etc.) has activity on poison hemlock, but we are unaware of any research documenting this product's effectiveness on poison hemlock.

In summary, poison hemlock in no-till fields may have reached a stage where consistent control will not be achieved by any registered treatment.  However, the competiveness of the weed should be greatly reduced with a traditional burndown herbicide followed by an early postemergence application of glyphosate, dicamba, Ignite or Callisto.  In future years, targeting seed sources of poison hemlock in areas adjacent to the field and making applications in the fall or early spring will minimize problems with this weed.


Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities. He can be contacted by email at or phone (515) 294-1164. Micheal Owen is a professor of agronomy and weed science extension specialist with responsibilities in weed management and herbicide use. Owen can be reached by email at or by phone at (515) 294-5936.


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 24, 2009. The information contained within may not be the most current and accurate depending on when it is accessed.