Joe Boggs reported that poison hemlock plants are now becoming very evident in southern Ohio and are ripe for control. This non-native invasive weed is among the most toxic plants in North America. The plant 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.
Poison hemlock is a biennial weed and spends the first year as a basal rosette, and the second year as an erect, towering flowering plant that can measure 6-10' tall. It is a member of the carrot family, so it shares many characteristics with other weeds found in Ohio including native plants such as QUEEN ANNE'S LACE (Daucus carota) and other notorious non-natives such as WILD PARSNIP (Pastinaca sativa). All stages of the 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.
Poison hemlock can be controlled by mowing, tilling, or by using selective or non-selective post-emergent herbicides including glyphosate (e.g. Roundup) or herbicide mixtures containing 2,4-D, 2,4-DP and MCPP or 2,4-D, dicamba and MCPP (e.g. Trimec). Applications of herbicides made now control both the first season rosette stage and the second season flowering stage, before seeds are produced.