hemlock closeup

Poison hemlock in full bloom along the Conestoga River in Lancaster Township.

Lancaster County residents driving along main arteries such as Route 30, Route 283 and Route 222 may notice walls of a lacy, white-flowering plant dominating the roadsides, taller than a person.

The plants may be pretty but they are weeds, invasive, unwanted and poisonous to both humans and livestock.

Poison hemlock continues to explode across the local landscape and much of Pennsylvania.

Yet many are not even aware of what they are seeing.

“It’s kind of ferocious. It’s been an issue in our district, particularly in Lancaster,” says Mike Crochunis, a PennDOT spokesman.

PennDOT, in fact, paid a contractor $27,000 to spray hemlock along major highways in Lancaster County this spring. The work is just now finishing up.

“It definitely seems to be taking over,” says Dwight Lingenfelter, an Extension associate at Penn State University. “It’s one of these weeds they call an explosion in slow motion because it just doesn’t appear on the scene, and people don’t really notice it until it’s really there.”

Poison hemlock  is really here in Lancaster County.

Jeffrey Graybill, an agronomy educator in Penn State Extension’s Lancaster office, first noticed poison hemlock in the county about 10 to 15 years ago. It became really aggressive about five years ago and is becoming more widespread yet.

“I think it’s worse,” he said, comparing it to numbers even two years ago. “I think it’s continuing to spread.”

Poison hemlock is native to Europe, western Asia and North Africa. It was brought to the United States from Europe in the 19th century as a pretty ornamental garden plant. Indeed, it has pretty white flowers, similar to Queen Anne’s lace.

Yet, like many non-native plants introduced to the United States, it has turned out to be highly invasive, crowding out native plants.

Poison hemlock is now established in all states except Florida and Mississippi.

There’s another problem: all parts of the plant, including seeds, are poisonous to humans and livestock.

Poison hemlock is believed to have been used to poison Socrates, the famous Greek philosopher.

And its oil can cause a nasty rash and blisters on some people — something to consider if you try to pull it from your property.

Though it is poisonous to livestock, local agricultural officials do not know of any recent cases of poisoning.

Fortunately, the weed likes disturbed areas, observed Lingenfelter, who considers himself a “weed scientist.”

Well-managed and cultivated fields seem to keep it at bay, he said, reducing the chances it will be mixed in with hay cuttings and reach livestock in feed.

Nor does the plant seem to be tasty to cattle and horses. “If there is no grass or other forage species, then they become curious for various reasons,” Lingenfelter said.

PennDOT has been aware of poison hemlock for about 16 years but it has become decidedly more prevalent in recent years,  Crochunis said.

“We just sort of beat it back but it keeps coming,”  he said.

One problem in keeping the invasive at bay along roadsides is that PennDOT can only  spray to the edge of its right of way. Also, heavy rains carry the seeds back onto roadsides.

The plant can swiftly and easily reach heights of 8 feet, and stands of hemlock can obscure road signs and block views for motorists at intersections.

Part of the reason it appears poison hemlock seems to just explode on the scene is that it is a biennial plant. One year it just grows leaves, stems and roots and is hardly noticed. Then, the next year, it “bolts” upward and flowers.

Why is poison hemlock not on Pennsylvania’s official list of noxious weeds, which currently stands at 13?

“It is too widely spread for consideration to be added to the noxious weed list at this time,” replies Logan Hall, spokesman for the state Department of Agriculture.