Poison hemlock invades state’s yards, fields, roadsides, more
If Socrates were alive and living in Pennsylvania today he would find it increasingly easy to find poisonous hemlock, the source of his deadly drug of choice, along rivers and roadsides, in wooded lots and around farmers’ fields.
Some 2,420 years after the Greek philosopher mixed himself a fatal hemlock cocktail, the invasive plant is growing wild and flourishing in more areas of the state than ever before.
And it’s spreading rapidly, according to the Penn State Extension service, which issued an advisory last Friday warning farmers and horse and livestock owners about the plant’s toxicity — sometimes fatal when ingested — and noted it has been “expanding to new regions in the state and its growth has become more aggressive.”
Rose Reilly, a biologist with the U.S. Army Corps of Engineers Pittsburgh District, said she sees it all over southwestern Pennsylvania and said several have recently popped up in the landscaped area next to the drive-thru lane of the Starbucks coffee shop off the I-79 Wexford Exit.
“It’s all along I-79 and I-279, but most people don’t notice it,” she said. “It comes up in the spring and its leaves look like carrots or parsley. Right now it’s about 1½ to 2 feet high but it will grow taller and begin to flower at the end of May or beginning of June.”
According to Penn State Extension and state Department of Conservation and Natural Resources fact sheets, poisonous hemlock, Conium maculatum, is a biennial herb, meaning it has a two-year life cycle.
Its first year is spent as low ground cover, but in its second year it displays umbrella-like clusters of small white flowers, similar to Queen Anne’s lace, atop hollow, purple spotted stems that can grow to be from 2 to 8 feet tall.
Not to be confused with the Eastern hemlock, a native conifer and the Pennsylvania state tree, poisonous hemlock was intentionally brought to North America for ornamental landscaping from Europe, western Asia and North Africa in the 1800s, and has also spread wild through South America and Australia.
All parts of the plant are very toxic to animals and extremely poisonous to humans. Mature seeds, thousands of which are dispersed by a single plant, are the most poisonous.
The extension service fact sheet states that poisoning of a 1,000pound horse or cow can occur if the animal eats 2.5 to 5 pounds of the plant, which can result in muscle paralysis and suffocation. For humans, just touching the plant without gloves can produce a rash similar to poison ivy.
“Rivers are among the most naturally disturbed areas, carrying sediment and seeds, and provide lots of space for non-native species to get a foothold,” Ms. Reilly said. “It’s all around now and I personally see it spreading everywhere in the upper Ohio River drainage.”
An example is Sheraden Park where the Army Corps, in partnership with the Allegheny County Sanitary Authority, the city of Pittsburgh and the Pittsburgh Water and Sewer Authority, completed an aquatic habitat restoration project along Chartiers Creek in 2019.
In the floodplain near where Sheraden Run flows into the creek,
poison hemlock and a number of other non-native plant species have taken root, Ms. Reilly said.
Poisonous hemlock also finds fertile ground along roadways, in pipeline rightof-ways and in the open spaces around shale gas drilling pads and access roads.
The state Department of Transportation controls poisonous hemlock within state road right-of-ways “for the safety of the traveling public,” Joseph Demko, roadside manager for PennDOT’s Bureau of Maintenance and Operations, said in an email response to questions.
“The department controls the spread of invasive weeds like poison hemlock with herbicide applications to control the roots and stems while mowing to prevent seed formation,” Mr. Demko wrote. “PennDOT needs to control many weeds at the same time and poison hemlock is included in the broadleaf weed programs in each county across the state.”
He wrote that COVID-19related work suspensions and delays will impact the timing and budgets of the state’s broadleaf herbicide and mowing programs this spring and summer.
The Pennsylvania Department of Agriculture classifies hemlock as a Class B noxious weed, meaning it’s widespread and the agency works to “control injurious infestation” and provide technical consultation.
“Poison hemlock is a spreading and growing problem,” said Ryan Reed, a natural resource program specialist with the state Department of Conservation and Natural Resources Bureau of Forestry. “I would classify it as an emerging threat, but the rate of the emerging threat is alarming.”
Mr. Reed said he first noticed hemlock starting 10 years ago along roads in Lancaster County.
Now, he said, poison hemlock is ubiquitous along all the major northsouth and east-west road corridors. In most of the state’s forests, he said, poison hemlock has rapidly joined a growing listing of tree and plant invasives that includes the Norway maple, the Korean Bee Bee tree, cork tree, Tree of Heaven, multiflora rose, giant knotweed, Japanese barberry, Japanese stiltgrass and garlic mustard.
Mr. Reed said poisonous hemlock and the other successful invasives have received a helping hand from the state’s changing climate.
“It may be inaccurate to say climate change is the primary reason, but it’s reasonable to say it’s a contributing factor. When conditions change rapidly, native plants have more trouble adapting,” he said. “Climate change can’t help.”