Source: Mark Loux, Curtis Young, OSU Extension
Poison hemlock remains one of the more persistent and prevalent poisonous weeds that we deal with in Ohio. It’s most typically a biennial plant (sometimes perennial), emerging from seed in year one and developing into a low-growing rosette by late fall. The rosette overwinters and then resumes growth in the spring of year two. Stem elongation initiates sooner in spring than many other biennials, and this is followed by continued growth and development into the often very tall plant with substantial overall size. Flowering and seed production occur in summer.
Failure to control poison hemlock occurs partly because, while it often grows in edges and fencerows around crop fields, no one really pays much attention to it until it does reach this large size when it’s less susceptible to herbicides. And everyone is busy getting crops planted in spring anyway so control of hemlock gets low priority. Stages in the poison hemlock life cycle when it is most susceptible to control with herbicides are: 1) fall, when in the low-growing rosette stage; and 2) early spring before stem elongation occurs. It’s most easily controlled in fall, but several products can work well in spring. Herbicide 
effectiveness ratings for poison hemlock can be found in Table 21 of the current Weed Control Guide for Ohio, Indiana, and Illinois. Herbicides rated 8 or 9 on poison hemlock include the following: 9 – Crossbow, Remedy Ultra; 8 – Cimarron Max, Curtail, dicamba, glyphosate. Mixing glyphosate and dicamba can improve control compared with either applied alone.
Several online resources cover poison hemlock more comprehensively than this article does, including this one from the University of Missouri. Information on toxicity can also be found via an internet search or by contacting OSU Extension if help is needed to resolve a specific concern.