Poison Hemlock

Poison Hemlock is pictured in spring. You may see tall, dried stalks nearby from last year.



Editor’s Note: This is a column on area watersheds by Blyden Potts and guest columnists to spread awareness of the area’s tributaries and the efforts of area volunteers to keep them clean.


There are multiple plants called Hemlock. When I hear “Hemlock,” my first thought is of a shady stand of conifers, Eastern Hemlocks, the official state tree of Pennsylvania. I read recently these trees are called Hemlocks because their needles smell a bit like a very different plant, Poison Hemlock.

Until the past year, I paid very little attention to and knew little about Poison Hemlock, other than knowing it was used in ancient Greece to kill condemned persons, most famously Socrates. Recently, my attention has been called to Poison Hemlock in our area, and I have read up on it.

Poison Hemlock (Conium maculatum) is a flowering plant native to Europe and North Africa. In North America, it is an invasive species and a noxious weed. All parts of the plant are toxic to mammals. The main danger, and overwhelming cause of fatalities, is ingestion, but the toxins can also be absorbed via skin contact or even inhalation. Some people develop an allergic skin rash in reaction to it.

This nasty plant has cousins by the name of Water Hemlock, genus Cicuta. The most common species is Cicuta maculata, sometimes also known as Cowbane or Poison Parsnip. Water Hemlock is usually regarded as the most toxic plant native to temperate North America. Like its invasive cousin, the main danger of Water Hemlock is ingestion, but again toxins can be absorbed by skin contact.

Poison Hemlock and Water Hemlock are very similar in appearance. Each grows 6-8’ feet tall. Each is in the carrot and parsley family, Apiaceae. Like other species in that family, each has lacy, umbel flower clusters, like upturned floral umbrellas, which give the family its alternate name Umbelliferae. The two Hemlocks have similar leaf and stem structure, and similar spots on their stems, referenced in their species names, maculata and maculatum. Each also has taproots, like carrots and parsnips, but this is one of their few clear differences.

Poison Hemlock has a single taproot. Water Hemlock has a cluster of several fleshy taproots.

Each tend to be found near streams. As you might expect from the name, Water Hemlock favors more obviously wet areas, like marshes. Poison Hemlock can grow in drier areas, like drainage ditches, that are not riparian.

Other members of the Apiaceae family include Wild Parsnip and Wild Carrot. These plants pose a different danger: phototoxicity. Chemicals from these plants can react with sunlight to cause severe skin rashes, burns and blisters. This is especially true of Giant Hogweed, another invasive species. Giant Hogweed looks a bit like Poison Hemlock, but with larger, frond-like leaves, and grows even taller, up to 14 feet or so.

Poison Hemlock is spreading aggressively in Pennsylvania. Keep an eye out for it. I have found bunches of it growing along streams in the Shippensburg area. If it has been there more than a season, there may be dried stalks of last year’s plants standing among the new plants. See: https://extension.psu.edu/poison-hemlock-is-aggressively-spreading-across-pennsylvania and https://extension.psu.edu/poison-hemlock-identification.

May is a good month to find and remove Hemlock plants. The plants are large enough to identify, but still manageably small and not yet gone to seed. If there are just a few plants, they can be dug out with a shovel.

Where there are clusters of plants, properly applied herbicide is a better and safer approach. If you remove them yourself, wear gloves and protective gear to avoid direct skin contact. If you use herbicide, choose one well-suited to Hemlock, and follow the directions carefully. Misuse of herbicides can cause serious environmental damage and is a violation of law.

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