The presence of poison hemlock (Conium maculatum L.) in pastures, fencelines, and field edges (Figure 1) is a frequent concern in many parts of Indiana. This plant can be noticed throughout the year, but it is most noticeable when emerging in the fall or coming out of dormancy in the spring, as it is typically one of the first weeds to green up, usually in late February to early March if temperatures are favorable. The appearance of poison hemlock on roadsides and fencerows of Indiana is not new as we can find articles in the Purdue Weed Science database dating back to 2003. The largest threat of this weed is the toxicity of its alkaloids if ingested by livestock or humans, but it can also reduce the aesthetic value of landscapes and has been reported to creep into no-till corn and soybean fields as well.
Figure 1. Poison hemlock infestation on roadside (Photo Credit: Travis Legleiter)
Biology and Identification
Poison hemlock is a biennial weed that exists as a low-growing herb in the first year of growth (Figure 2) and bolts to three to eight feet tall in the second year, when it produces flowers and seed (Figure 3). It is often not noticed or identified as a problem until the bolting and reproductive stages of the second year. The alternate compound leaves are pinnate (finely divided several times) and are usually triangular in outline. Flowers are white and occur in an umbel inflorescence. Poison hemlock is often confused with wild carrot but can be distinguished by its lack of hairs and the presence of purple blotches on the stems.
Figure 2. Poison hemlock rosette (Photo Credit: Travis Legleiter)
Figure 3. Flowering poison hemlock plant (Photo Credit: Purdue Plant and Pest Diagnostic Lab)
Toxic Properties
Poison hemlock contains five alkaloids that are toxic to humans and livestock and can be lethal if ingested. The plant’s alkaloids may also be absorbed through the skin, so if you find yourself hand pulling poison hemlock, it would be a good idea to wear gloves. All parts of the plants contain the toxic alkaloids with levels being variable throughout the year. Symptoms of toxicity include nervousness, trembling, and loss of coordination followed by depression, coma, and/or death. Initial symptoms will occur within a few hours of ingestion or exposure.
Cases of poisoning due to poison hemlock ingestion are rare as the plants emit a mousy odor that makes them undesirable and unpalatable to livestock and humans. Consumption and toxicity in animals usually occur in poorly managed or overgrazed pastures where animals are forced to graze poison hemlock because of lack of desirable forages.
Control
Control of poison hemlock with herbicides is most effective when applied to plants in the fall/first year of growth. Fall applications should be timed when weeds are likely beginning to enter their winter survival stage, which generally occurs once average high temperatures are around 60°F for at least a week. These fall applications are beneficial as the plant begins sending sugars and reserves to the large taproot system to survive the winter. If systemic herbicides are applied at this time, the herbicide is translocated to the taproot to completely kill the weed. Applications prior to bolting and flowering in the second year can also be effective; however, the closer to reproductive stages, the less effective the herbicide. In roadside ditches, pastures, and waste areas, herbicides containing triclopyr (Remedy Ultra, Garlon, many others) or triclopyr plus 2,4-D (Crossbow, Crossroad) are most effective in controlling poison hemlock. Other herbicides that provide adequate control when applied at the proper timing are dicamba (Clarity, many others), metsulfuron-methyl (Escort XP), metsulfuron-methyl plus dicamba plus 2,4-D (Cimarron Max) and clopyralid plus 2,4-D (Curtail).
For no-till fields, mixtures of 2,4-D plus dicamba will be most effective for fields going to soybean. If applications are delayed until spring, be sure to pay attention to preplant intervals when these herbicides are used. Preplant intervals will vary based on the soybean herbicide-resistance trait to be planted and whether or not 2,4-D and dicamba were used together to control the weed. For fields going to corn, mesotrione (CallistoTM and other names) and mesotrione premixes + 2,4-D or dicamba have been effective in reducing infestations along field edges.
For further information on toxic plants in Indiana refer to the Purdue University Weed Science Guide to Toxic Plants in Forages (https://www.extension.purdue.edu/extmedia/WS/WS_37_ToxicPlants08.pdf).
