Travis Legleiter, Weed Science Professional Assistant, Botany & Plant Pathology Department, Purdue University
Poison hemlock seems to stay disguised most of its life and is not recognized or complained about until the second year of its biennial life. This weed is present in the ditches and banks of most public roadways and is usually recognized by its unsightly tall purple-blotched stems and white umbel flowers in mid summer. The ugly plant doesn’t just magically appear in the mid summer months, rather most of the public simply doesn’t recognize it or notice it in the low growing rosette stage. Although, and quite curiously, it will be one of the few weeds that is still green in the winter months while other weeds are brown and yellow
Identifying poison hemlock in the rosette stage is crucial to controlling the weed, as most herbicides are only effective when applied at this stage. Producers should take advantage of this time when poison hemlock seems to expose itself and identify plants to be controlled the following spring prior to bolting and reproduction. Another characteristic that makes it easy to identify along roadsides is that Poison hemlock is a weedy invader that has a tendency to grow in patches. Upon closer inspection you will notice that the individual plants have finely pinnate compound leaves with a triangular outline that is very similar to wild carrot, although poison hemlock will lack the pubescence that is characteristic of wild carrot.
Take a look around the next time you are on the road this winter and you will likely see the exposed green patches of poison hemlock in various road banks and ditches.
Further detail of Poison hemlock biology, identification, and control can be found in the links below.
Poison Hemlock (Conium maculatum) - A Mini Review (pdf file)
Poison Hemlock Control in Corn and Soybean (pdf file)
Guide to Toxic Plants in Forages (pdf file)
Click image to enlarge
Pic 1. A large green patch of poison hemlock along a roadside in Lafayette, IN (Photo taken 12/19/19)
Pic 2. Poison hemlock rossettes with finely pinnate divided leaves growing among dormant grass in early winter (Photo taken 12/19/12)