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This story was first published on July 13, 2022

This story is part of our series, Wild Pennsylvania. Check out all of our stories here

An invasive plant that’s now common in Pennsylvania can cause a rash, and if accidentally ingested, it can be fatal.

To avoid poison hemlock (Conium maculatum) you have to know what to look for

“It’s got a parsley, almost fern-like foliage to it,” said Chris Kubiak, director of education at the Audubon Society of Western Pennsylvania. “It will get a series of white flowers in its second year, that kind of rosette off of the top stem.”

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Kubiak said poison hemlock was brought to North America as an ornamental plant. It’s not our native eastern hemlock tree, but it is confused with another harmless plant in the same family: wild carrot or Queen Anne’s lace.

Here’s how to spot the differences

  • Poison hemlock displays multiple flowers, while Queen Anne’s lace has one.
  • Queen Anne’s lace has a hairy stem, and poison hemlock’s stem is smooth.
  • Queen Anne’s lace has a tiny purple flower in its center, and poison hemlock does not.

Poison hemlock is biennial. In its first year plants are just ankle high. In the second, they can shoot up to eight feet tall. It starts to flower in May and June, and by now, in mid-July, begins to set its seeds. That’s another key difference from Queen Anne’s Lace, which begins flowering later.

There will be an overlap here in July between the two,” Kubiak said.” But I have found Queen Anne’s lace flowering all the way into October, which you are definitely not going to get with your poison hemlock.”

Poisonous, even fatal

It’s poisonous at any stage of its lifecycle, as are all parts of the plant. But he said the root and the seeds contain most of the poison alkaloid. Ingesting even small amounts of it can be fatal.

People who forage for wild parsnip root should be especially careful, since the roots look a lot alike. Kids are also at risk since they tend to put things in their mouths. 

When it comes to casual contact with the plant, people should treat poison hemlock plants like poison ivy. If your skin comes into contact with it, wash thoroughly because it may cause a rash for some, which can be severe. 

“It does often get confused with a close relative, giant hogweed,” Kubiak said. “When you get some of the sap from giant hogweed on [you] you get a type of burn from the sun which would concern me more.”

Where poison hemlock grows

Poison hemlock plants are most commonly found in urban and suburban areas on roadsides, mowed edges and along fences, according to Kubiak. In the country, the plant grows along power line right of ways, and can be a danger to cattle along fields.

“One of the keys to their success is the fact that they’re able to exploit areas that are disturbed and they’re known as pioneer species,” he said. “They’re some of the first that will emerge and they outcompete pretty much everything else.” 

That includes milkweed species which are key to the success of monarch butterflies. 

Poison hemlock seeds aren’t scattered by the wind, but can be spread by water running downhill after a heavy rain, by tractor tires, or even by shoes. 

One poison hemlock plant can produce 30,000 seeds, and they have a high germination rate. It doesn’t have any natural enemies in the wild, either, Kubiak said. Even deer won’t touch it.

“It is a fascinating plant in how it is produce chemical warfare to stop things from eating it,” Kubiak said. 

Its smell should keep people away, too. It’s been described as smelling similar to mouse urine.

How to get rid of poison hemlock

According to Kubiak, Audubon is managing poison hemlock at two of their properties where the public comes to enjoy nature. If you have it in your own yard, they recommend you:

  • Wear gloves and a long-sleeved shirt 
  • Pull it out in its second year before its seeds fall
  • Be sure to get the entire root
  • Place the whole plant into a trash bag for disposal. 
  • Don’t burn the plants because you could breathe in the toxins

Spot application of herbicides can also be used to take care of some larger infestations. 

“We don’t recommend mowing, but you can certainly [do that] before it flowers,” Kubiak said. In that case, a mask should be worn because some of the alkaloids can be dispersed through the air.

Seek immediate medical attention if you suspect poison hemlock has been ingested. According to the University of Minnesota Extension, the toxins in the plant affect the nervous system, and symptoms can include trembling, salivation, lack of coordination, dilated pupils, weak pulse, and respiratory paralysis.

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