Poison hemlock, a toxic, invasive plant, is popping up more and more in Pennsylvania

By some standards, poison hemlock is a rather pretty plant, identifiable by its tall, dotted stalk and delicate, white, umbrella-shaped blooms.

“If you look over there, it’s primarily dominated by poison hemlock, which is a non-native species,” said Rose Reilly, a biologist with the U.S. Army Corps of Engineers in Pittsburgh, pointing out thickets of poison hemlock just behind Sheraden Park.

It’s also toxic, as the name suggests, and is popping up more and more in Pennsylvania, according to horticulture experts.

In fact, the Army Corps recently removed poison hemlock, and other invasive plant species, as part of a stormwater management project near Chartiers Creek this spring.

“Poison hemlock is pretty comfortable in wet areas and you often will find it down along streams and wetland areas, but it seems to be just as happy in poorer soil,” said Sandy Feather, a commercial horticulture educator with Penn State Extension based in Allegheny County.

According to an April article published by Penn State researcher Jeffrey Graybill, the plant has been aggressively spreading across the state. Graybill describes the characteristics of the plant:

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. 

Feather says poison hemlock is native to parts of Europe and Asia, likely brought here as an ornamental plant. Like other invasive species, such as Japanese knotweed, it can spread rapidly and steal resources from native plants.

“I don’t know if animals are moving the seed around perhaps. I really can’t explain why it seems like it’s exploded the way it has but there’s definitely a lot more of it,” Feather said.

The plant is poisonous and ingesting it can be fatal; the toxins in the plant’s seeds and sap can lead to respiratory failure.

“[It’s] toxic to people, pets,  horses, cows,” Feather said. “It doesn’t matter. You don’t want to really have to have it in pastures and things like that.”

If you do find it in your yard and want to remove it, simply touching the plant usually isn’t dangerous, though some people can have an allergic reaction from touching it, resulting in a rash. Regardless, if you’re cutting it or using a weed whacker it’s a good idea to wear gloves and protect your eyes to avoid contamination, said Feather. Additionally, be sure to wash your hands before eating or handling food.