Got Nature? Blog

Photo by: Richard Old, XID Services, Inc., Bugwood.org

Made infamous through the trial of Socrates, Shakespeare’s Hamlet and Macbeth, and several other works of classic literature, poison hemlock is an extremely toxic plant that will pose a risk this summer and should be handled with caution.

Poison hemlock is native to Europe. It is a biennial plant, meaning that it has a two-year lifespan. Last summer, it went through vegetative growth and largely stayed out of sight as a basal rosette. This summer, a long stem (up to 3 to 10 feet tall) will bolt and produce small white clusters of flowers. Poison hemlock is a member of the parsley family and can sometimes be confused with wild carrot. However, its distinguishing feature is its hairless hollow stalks with purple blotches.

The biggest risk with poison hemlock is ingestion. 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. 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. Quick treatment can reverse the effects, so act quickly.

It is considered highly invasive and is listed on both Indiana’s noxious weed list and the terrestrial invasive plant rule list, Indiana General Assembly IC 14-24-12, IC 15-15, IC 15-16-7 to 10, Indiana Department of Natural Resources, Terrestrial Invasive Species – Plants. Thus, its occurrence may have legal implications for landowners. Some Indiana counties aggressively monitor and spray poison hemlock, particularly along county roads. It primarily invades disturbed and early succession sites such as roadsides, stream and ditch banks, vacant lots, and field borders. However, it can spread into pastures, hay fields, and other grassland habitats.

Its sap contains alkaloid toxins which are highly toxic to humans and animals. Although humans would not normally purposely or accidentally ingest poison hemlock (Socrates was a notable exception), care should be taken when mowing or cutting to avoid dermal and respiratory inhalation exposure. Livestock farmers should control poison hemlock in pastures, feed lots and holding pens and ensure it is not baled in hay.

Poison hemlock spreads via seed, so effective management must prevent new seed production, prevent spread of existing seed, and exhaust the existing seed supply in the soil seed bank.

Poison hemlock seed is often inadvertently spread by mowing, road maintenance or agricultural equipment. Mow infested areas along roadsides, ditch banks and field edges before seed develops. A single mowing may delay but  not stop poison hemlock from growing another shoot and ultimately flowering and producing seed. Multiple mowings are likely required to prevent new seed production.

Herbicide control should focus on the first-year basal rosette. Herbicide control can still be achieved in the early stages of flower stem development. However, as the flower stem grows so grows the amount of herbicide required to provide full coverage and thus full control.  A 2% solution of glyphosate (41% active ingredient) in water with a non-ionic surfactant will provide good control of poison hemlock basal rosettes. If the poison hemlock is growing among desirable grasses, broadleaf-specific herbicides such as triclopyr and 2,4-D are also very effective on poison hemlock and will not harm the grasses. AS always, read the pesticide label for proper, safe handling and use.

For more information see Purdue Cooperative Extension Invasive Plant Fact Sheet Series FNR-437-W, Poison Hemlock.  For further guidance on poison hemlock control, contact your county Purdue Cooperative Extension office.

Resources:
Noxious and Invasive Weeds and the Weed Laws in Indiana, Purdue Extension – Botany & Plant Pathology
Be Careful Around Highly Toxic Poison Hemlock Plant This Summer, Purdue Extension – Forestry and Natural Resources (FNR)
Poison Hemlock, Pest & Crop Newsletter, Purdue Extension – Entomology
Pest & Crop Newsletter, Purdue Extension
Recognizing and Managing Poison Hemlock, Purdue Landscape Report
Hemlock Woolly Adelgid: Distribution Update, Purdue Landscape Report
Poison Hemlock (Conium maculatum), United States Department of Agriculture (USDA)
Invasive plants: impact on environment and people, The Education Store, Purdue Extension’s resource center
Woodland Management Moment: Invasive Species Control Process, Video, Purdue Extension – Forestry and Natural Resources (FNR) YouTube Channel
What are Invasive Species and Why Should I Care?, Purdue Extension – Forestry and Natural Resources (FNR)
Indiana Invasive Species Council
Report INvasive, Purdue Extension & Indiana Invasive Species Council
Purdue Pest and Plant Diagnostic Lab

Ron Rathfon, Regional Extension Forester, Southern Indiana Purdue Agriculture Center (SIPAC)
Purdue Forestry and Natural Resources