Although Poison hemlock has been troublesome in pastures and rangeland for quite some time, it has typically sat beyond the borders of corn and soybean fields, being content to watch from railway tracks and the ditch. This may no longer be the case. The adoption of no-till has promoted a weed shift favoring some of the perennials and biennials. Bill and I receive calls regarding control of hemlock in row crops, particularly in soybean. Most often growers and applicators were concerned that glyphosate alone just prior to planting or as the 1st postemergence spray did not provide adequate control. We noticed that poison hemlock is actively growing in ditch banks and along field edges. So the purpose of this article is to provide some guidance on how to manage this weed.

Poison hemlock is a biennial, meaning that it takes two years for it to complete its life cycle. The first year it exists as a low lying rosette, then it will bolt after over wintering and be three to eight feet tall at maturity. Poison hemlock flowers in June or July and once seed is produced generally dies late July and August. We generally receive calls regarding the control of poison hemlock when it has reached maturity and is flowering out. Biennials are often more susceptible to chemical control in the first year of growth when they are rosettes.

Control

Poison hemlock historically has not been a problem in corn and soybean. Because of this there is not a large body of research done on poison hemlock’s control in corn and soybean. If you have poison hemlock in your no-till field it is a good idea to add either dicamba or 2,4-D in your burndowns and to target poison hemlock in the first years growth, while it is still a rosette. Below are some of the options available to suppress or control poison hemlock in corn and soybean situations.

Corn and Soybean

Burndown applications of glyphosate plus 2,4-D (1 lb ai/A) in the fall can control rosettes in the fall or in the early spring. Applications of 2,4-D of rates higher than 0.5 lb ai/A require a 30 days waiting period before planting soybean and 7 to 14 days before planting corn (see specific label for details). Glyphosate labels recommend applications from bud to flower.

Glyphosate can also be used POST in RR soybean and corn.

Corn

Burndown or PRE applications of Basis (0.5 oz/A) plus 2,4-D LVE (1 pt/A) or Basis (0.3 to 0.5 oz/A) plus 2,4-D (1 pt/A) plus atrazine at 0.5 to 1 lb ai/A. There is a 7 to 14 day planting restriction when using 2,4-D to planting corn, see specific product label for details. Basis will provide some residual control of germinating poison hemlock.

Burndown or POST applications of dicamba (0.5 pt/A) or 2,4-D can suppress to control poison hemlock. Dicamba provides good control where 2,4-D can provide fair control. Dicamba can be applied before planting and postemergence from spike to 36 inch tall corn or until 15 days before tassel emergence. Risks of injury increases after corn is eight inches tall, the use of drop nozzles are suggested. Drop nozzles should be used when applying 2,4-D (0.17 to 0.25 lb ai/A) after the corn is eight inches tall for added safety.

Soybean

Burndown applications of glyphosate (1 lb ae/A) plus 2,4-D (1 lb ai/A) ether in the fall or early spring on rosettes of can provide good control of poison hemlock. There is a 7 day waiting period after 2,4-D applications of 0.5 lb ai/A or less, but a 30 day waiting period with applications above 0.5 lb ai/A to plant soybean. Glyphosate can be used POST in RR soybean.