Poison hemlock (Conium maculatum) is among the most deadly plants in North America. This non-native invasive weed contains highly toxic piperidine alkaloid compounds, including coniine and gamma-coniceine, which cause respiratory failure and death when ingested by mammals. The roots are more toxic than the leaves and stems; however, all parts of the plant including the seeds should be considered dangerous. It is a common misconception that poison hemlock sap will cause skin rashes and blisters. In fact, poison hemlock toxins must be ingested or enter through the eyes, cuts, or other openings to cause poisoning.
Poison hemlock is a biennial weed. It spends the first year as a basal rosette and the second year as an erect, towering flowering plant that can measure 6-10' tall. Despite its common name, poison hemlock is not a tree; it is a member of the carrot family, Apiaceae (formerly Umbelliferae). Thus, it shares many characteristics with other members of the carrot family found growing in Ohio including native plants such as QUEEN ANNE'S LACE (Daucus carota) and other non-natives such as WILD PARSNIP (Pastinaca sativa).
Wild parsnip is notorious for containing sap loaded with furanocoumarins that produce a light-linked condition called phytophotodermatitis in humans and livestock. If plant juices contact skin and the skin is then exposed to sunlight (specifically ultraviolet light), severe blistering can occur, as well as skin discoloration that may last several months.
All stages of the poison hemlock plant have bluish-green leaves that are 3-4 times pinnately compound, and the deeply cut parsley-like leaflets have sharp points. Flowering plants have hairless, light-green to bluish-green stems that are covered with obvious purplish blotches. Clusters of tiny white flowers are borne on structures called umbels that look like upside-down umbrellas.
While poison hemlock can be partially managed by mowing and tilling, the most effective control approach involves properly timed applications of selective or non-selective post-emergent herbicides including glyphosate (e.g. Roundup). It is a prolific seed producer, so applications of herbicides made now will control both the first season rosette stage and the second season flowering stage, before seeds are produced.