Poison hemlock (Conium maculatum) was originally brought to the United States in the 1800s as an ornamental garden plant (another good idea gone awry) and is now found throughout North America. Rogue plants remained relatively rare until around 30 years ago. Since that time, poison hemlock has elevated its profile from an uncommon oddity to a common threat. This non-native is one of the deadliest plants found in North America. It is the plant used to kill Socrates as well as the Greek statemen Theramenes and Phocion. Poison hemlock plants contain highly toxic piperidine alkaloid compounds, including coniine and gamma-coniceine, which cause respiratory failure and death in mammals.
Poison hemlock is a biennial that forms a rosette its first year, often going unnoticed, and then produces white umbel flowers (umbrella-like) on tall stems in the spring-summer of its second year. These types of flowers are common in the carrot family and are similar to wild carrot (also known as Queen Anne’s lace, Daucus carota), which is often mistaken for this plant at younger stages. The second
-year stems of poison hemlock are hairless and have purple spots, which help distinguish it from wild carrot.
Purple spotting on smooth, hollow stems is a tell-tale sign of poison hemlock. Poison hemlock reproduces exclusively by seed. It can produce up to 38,000 seeds per plant that generally fall near the mother plant but can be dispersed by water, birds and other wildlife. Seeds can germinate immediately or lay dormant for up to six years. The plant will die soon after it sheds seed, but the stems will remain standing and slowly release seed into winter.
All parts of the plant are poisonous: leaves, stems, seeds, and roots. However, the toxins must be ingested or enter through the eyes or nasal passages to induce poisoning. The toxins do can cause sensitivity to UV light from the sun, resulting in blisters. Regardless, this plant should not be handled because sap on the skin can be rubbed into the eyes or accidentally ingested while handling food. Immediate emergency medical attention should be sought if an accidental poisoning from this plant is suspected.
Do not burn plants to prevent any accidental inhalation. Mowing or weed whacking will not kill the plant but can reduce seed production in second year plants. If you are mowing or weed whacking in areas infested with poison hemlock, wear protective eyewear and a dust mask to prevent exposing your eyes and lungs to small particles.
Choosing a herbicide will depend on the desirable surrounding vegetation. If poison hemlock is growing amongst grasses, synthetic auxin herbicides such as 2,4-D, dicamba, clopyralid and triclopyr will selectively control the poison hemlock, leaving the grass
unaffected. If the poison hemlock is in a bare ground area, or other area where the surrounding vegetation is not of importance, a broad-spectrum herbicide, such as glyphosate (active ingredient in Roundup Weed and Grass Killer, among others) can be effective. Note that glyphosate will injure or kill other plants contacted during application, so care is needed to avoid green plant material, exposed roots and injured bark of desired plants.
Source: Michigan State University and OSU Buckeye Yard and Garden Line