Poison hemlock is in full bloom across the state and continues to make the news, unfortunately. Poison hemlock is in the Apiaceae or parsley plant family, which also includes wild carrot (Queen Anne’s lace), wild parsnip, cow parsnip, and giant hogweed.
All these species share characteristics with poison hemlock to varying degrees and inhabit both disturbed and undisturbed sites such as roadsides, ditches, pastures, forest edges, and fence lines. Key identifying characteristics of poison hemlock include finely divided leaves (though lesser so than wild carrot, for which it is often confused), hollow stem, and purple spots located on the stem (Figure 2).
All parts of the poison hemlock plant are poisonous to humans and livestock, but only when ingested. Poisoning is relatively rare, though, since livestock avoids eating poison hemlock because of its poor smell and taste. Contact with poison hemlock can also cause skin and eye problems, which are more likely than internal poisoning. The severity of this response varies depending on the sensitivity of the individual and the degree of contact. This does not happen from a distance, though – only with direct contact with plant parts or fluids from the plants. Anyone mowing or removing poison hemlock by hand should keep this in mind and protect themselves from skin and eye contact. Mowing large populations with open-station tractors is not recommended.
Poison hemlock is typically a biennial with its life cycle taking place over two years. In the first year, poison hemlock emerges from seed and develops into a rosette. After overwintering, the plant then bolts and produces a seed head in the second year, completing its life cycle via seed production. At this time of the year, when poison hemlock plants are flowering, producing seeds, and dying, it is not always possible to use chemicals to control them. The goal should be getting rid of existing plants through cutting, mowing, or hand removal, and limiting the production and spread of seed. The most effective timing for the application of herbicides is the fall when plants are low-growing rosettes in their first year of growth, or early the following spring when plants are still small. Herbicide effectiveness ratings in Table 21 of the “Weed Control Guide for Ohio, Indiana, and Illinois”, which lists pasture and CRP herbicides. Additional products labeled for roadsides, industrial areas, etc., but not shown in this guide, are also effective.