Above: purple blotching on stem of poison hemlock
Besides the ominous name, poison hemlock has become infamous as a problematic invasive weed in recent years. It’s essential to learn how to identify poison hemlock accurately and to implement control measures safely and effectively.
Poison hemlock looks a lot like wild carrot, a.k.a. Queen Anne’s Lace. Both poison hemlock and wild carrot belong to the parsley family (Apiaceae). Both have the characteristic umbel inflorescence (umbrella-shaped flowers) of small white flowers and leaves that expand at the bases sheathing the stems. You can tell poison hemlock apart from its benign cousin, wild carrot, by the hairless hollow stems with random purple blotches. As a fellow educator described it, “It looks as though somebody dipped a paint brush in purple paint and flicked it on the stem.” The leaves of poison hemlock are also sharper in detail compared to wild carrot. Poison hemlock is a biennial plant, forming a basal rosette of fern-like growth the first year, then bolting a 3- to 10-foot flowering stalk the second year.
Ron Rathfon, Regional Extension Forester for Purdue, recently wrote about management and control measures for poison hemlock and some precautions you should take.
“The biggest risk with poison hemlock is ingestion,” he said. “Lethal doses are fairly small, so it is important for animal owners or parents of young children to identify it in their area and remove it if possible.” He said the toxins can also be absorbed through the skin and lungs, so be sure to wear gloves and a mask when handling these plants.
“Symptoms of hemlock poisoning include dilation of the pupils, weakening or slowing pulse, blue coloration around the mouth, and eventually paralysis of the central nervous system and muscles leading to death,” he said. “Quick treatment can reverse the effects, so act quickly.”
Poison hemlock is considered highly invasive and is listed on both Indiana’s noxious weed list and the terrestrial invasive plant rule. “It primarily invades disturbed and early succession sites such as roadsides, stream and ditch banks, vacant lots, and field borders,” he said. “However, it can spread into pastures, hay fields, and other grassland habitats.” Additionally, I have seen it bordering homeowner lawns in Whitley County.
“Its sap contains alkaloid toxins which are highly toxic to humans and animals,” he said. “Although humans would not normally purposely or accidentally ingest poison hemlock, care should be taken when mowing or cutting to avoid dermal [contact] and respiratory inhalation exposure.” He urged livestock farmers to control poison hemlock in pastures, feedlots, and holding pens and ensure it is not baled in hay.
“Poison hemlock seed is often inadvertently spread by mowing, road maintenance, or agricultural equipment,” he said. “Mow infested areas along roadsides, ditch banks, and field edges before seed development.” A single mowing may delay but not stop seed production, so be prepared for a second or multiple mowings.
“Herbicide control should focus on the first-year basal rosette,” he said. “Herbicide control can still be achieved in the early stages of flower stem development.”
For specific herbicide recommendations, search for the free publication on poison hemlock at https://edustore.purdue.edu/. Find Rathfon’s original article at https://www.purdue.edu/fnr/extension/control-management-of-poison-hemlock/.