Invasive Poison Hemlock Is Aggressively Spreading Across Pennsylvania

By Jeffrey S. Graybill, Penn State Extension Educator, Agronomy

Horse and livestock owners should be on the lookout for Poison Hemlock, an invasive weed spreading across the state.

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 which resemble wild carrot (also commonly referred to as Queen Anne's lace). 

Poison hemlock has a musty smell, while the leaves often have a unique parsley smell when they are crushed. 

This weed has been expanding to new regions in the state and its growth has become more aggressive. 

The weed can be found growing in fallow areas, fence rows, pastures, roadsides and creeksides.

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 reportedly killed the 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, and some studies have shown toxicosis with consumption of 0.25 percent fresh weight (of the animal’s weight) for horses and 0.5% for cattle.  This would be 2.5 to 5 lb. of fresh material per 1000 lb. animal. 

Mature seeds are the most poisonous. Significant poisoning can result in muscle paralysis and suffocation.

Being a biannual, seeds typically germinate in late summer through autumn. A small rosette forms which grows vegetatively and may remain green throughout much of the winter. 

In early spring the rosettes quickly green up and begin aggressively growing. Then, in late May the plant will bolt, shooting a single large stalk with small white flowers. 

After flowering the seed plant will die, scattering new seed, which will germinate and produce new plants as fall approaches.

Identification and eradication of this plant wherever livestock could come in contact is important. Also, hayfields and meadows which will be harvested should be closely walked, eradicating hemlock prior to any harvest. 

The toxicity can remain when harvested with dry hay or baleage, therefore, do not feed contaminated hay. Additional help in identifying this poisonous plant can be found on Penn State Extension's video fact sheet on Poison Hemlock Identification.

Finally, care should be taken when eradicating the plant to wear gloves and protective clothing. Contact with the skin has been known to cause irritation for some people. 

You can easily dig individual plants out using a shovel. 

For larger infestations, several herbicides are effective for control. Applications are most effective when made before plants bolt in the spring. Preferred herbicides include 2,4-D + Banvel, Crossbow (2,4-D+triclopyr), or glyphosate as a spot treatment.

[Visit DCNR’s Invasive Plants In Pennsylvania to learn about other invasive plant species.]
(Reprinted from Penn State Extension website.)
NewsClips:

Don Hopey: Poison Hemlock Invades State’s Yards, Fields And Roadsides
Marcus Schneck: Poisonous, Invasive Poison Hemlock Plant Spreading In PA

[Posted: May 6, 2020]  PA Environment Digest