Jun 23, 2016 3 min read

Poison hemlock has exploded over the last several years, particularly along roadsides. But it also can be found in pastures, along field edges and creeks.

Much of it is now blooming, so everyone is noticing the large plants with white blossoms.

Poison hemlock is a biannual weed of the Umbelliferae family (parsley), which can grow up to 6 feet tall and has a smooth, hollow stem. The weed can easily be identified by the purple spots on its stems, and by its finely divided leaves that resemble wild carrot (also commonly referred to as Queen Anne’s lace).

Poison hemlock has a musty smell, and the leaves often have a parsley smell when they are crushed. This weed has been expanding to new regions in the state, and its growth has become more aggressive.

It is native to Europe, northern Africa and western Asia, and was introduced to North America as an ornamental garden plant. It is infamous as a poisonous plant and hemlock tea that reportedly killed Greek philosopher Socrates in 399 BC.

The plant contains a number of closely related pyridine alkaloids with the main one being coniine, a colorless, volatile and strongly alkaline oil. All parts of the plant are poisonous, with mature seeds being the most poisonous. Significant poisoning can result in muscle paralysis and suffocation.

In the early spring, it can be found growing vegetatively as a rosette, but in late May it bolts, shooting a single large stalk with small white flowers. After flowering, the seed plant will die, scattering new seeds that will germinate and produce new plants as fall approaches.

Identification and eradication of this plant wherever livestock or pets could come in contact is important. Care should be taken when eradicating the plant by wearing gloves and protective clothing. Contact with the skin has been known to cause irritation for some people. The alkaloids reportedly are lost by slow drying or boiling, but nobody should count on dry hay being OK.

Hand removal is difficult because of the tough root system and the fact that the plant sap is, along with being toxic, a skin irritant. Even the use of weed trimmers needs to be conducted using precautions so that plant material doesn’t come into contact with the body.

There are no pre-emergent herbicides to use against poison hemlock in ornamental settings. Post-emergents include diquat, pelargonic acid, glyphosate and 2,4-D. The most effective approach is to treat the first-year rosettes and not the larger, mature plant.

Mowing the plant in late flower should set it back and may even control it and prevent seed production in areas where mowing is possible.