Poison hemlock (Conium maculatum) is a non-native biennial weed that spends its first year as a low-growing basal rosette; the stage that is currently very apparent. During its second year, plants produce erect, towering stalks and multi-branched stems topped with umbrella-like flowers. Mature plants can measure 6-10' tall and are prolific seed producers.
Despite its common name, poison hemlock is not a tree; it is a member of the carrot family, Apiaceae (formerly Umbelliferae). It shares floral characteristics with other non-native members of the carrot family such as Queen Anne's lace (Daucus carota) and the wild parsnip (Pastinaca sativa) which is notorious for producing sap that causes skin blisters.
All stages of the poison hemlock plant have bluish-green leaves that are 3-4 times pinnately compound. 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 is one of the deadliest plants in North America. This invasive plant was imported as an ornamental in the late 1800s from Europe, West Asia, and North Africa. The plant contains highly toxic piperidine alkaloid compounds, including coniine and gamma-coniceine, which cause respiratory failure and death in mammals.
The toxins must be ingested or enter through the eyes or nasal passages to induce poisoning; they do not cause skin rashes or blistering. Regardless, this plant should not be handled because sap on the skin can be rubbed into the eyes or accidentally ingested while handling food.
The roots are more toxic than the leaves and stems; however, all parts of the plant including the seeds should be considered dangerous. Unfortunately, this dangerously toxic plant is becoming more common throughout Ohio including growth in landscape plantings where the close proximity to people increases poisoning risks.
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). Each plant produces hundreds of seeds, so applications of herbicides made now will control both the first season rosette stage and the second season flowering stage before seeds are produced.