Poison hemlock (Conium maculatum), an invasive, exotic plant species from Europe and Asia, has been creeping into our midst and exploding in population over the past several years. It was intentionally imported into the U.S. as an ornamental plant. Initially, it was just a few plants here and there, but now large stands of poison hemlock can be spotted in many NW Ohio areas. Poison hemlock is found along roads and highways, railroad right-of-ways, river, stream and creek banks as well as in drainage ditches, in disturbed sites, along hiking trails, in fence lines, at the edges of agricultural fields, and into pastures and hay fields.
Poison hemlock is a member of the plant family Apiaceae (old name Umbelliferae) which in laymen’s terms would be called the Parsley or Carrot Family. Red flags should be going up at this point, since many of the members of this plant family are edible plants, and the potential for misidentification of poisonous plants for edible plants is a great possibility.
Poison hemlock is typically a biennial plant, meaning that it takes 2 growing seasons to complete its life cycle. However, under optimal conditions, poison hemlock can grow as a perennial surviving for several years. In its first year of growth, the year the seed germinates, the plant grows as a ground-hugging, mounded rosette. In the second year of growth, the plant produces an erect flower-producing stem that can reach heights of 3 to 10 feet. The stem is hollow and green in color with purple spots. The leaves are shiny green in color, are pinnately compound, multi-stemmed, triangular in shape and are fern-like or parsley-like in appearance. When crushed, the leaves have a musty odor. If one walks through a stand of poison hemlock on a hot, humid day, the musty odor from the plants can be sickening. Flowers are white in color, 5-petalled and displayed in an umbrella-like cluster called an umbel. Plants that are very similar in appearance include fennel, parsley and wild carrot (a.k.a. Queen Anne’s lace). Each flower develops into a green, deeply ridged fruit that contains several seeds. After maturity, the fruit turns grayish brown. The dried fruit is easily moved by erosion, animals, wind, and human activity to new locations where they break open releasing their seeds.
Poison hemlock is very toxic to sheep, cattle, swine, horses, and other domestic animals. They can be poisoned by eating either green or dried plant material. The alkaloid toxins of poison hemlock are relatively stable and can remain toxic for three or more years in dried plant parts. This is significant if poison hemlock is accidentally incorporated into hay baled in fields infested with the plant. To avoid these accidental poisonings, do not cut portions of hay fields for baling that are known to be infested with poison hemlock.
Poison hemlock is also extremely poisonous to humans if ingested. Poison hemlock was used to execute political prisoners in ancient Greece. The Greek philosopher Socrates presumably committed suicide by drinking a tea made with poison hemlock.
All parts of the poison hemlock plant (leaves, stems, roots, and fruits) are poisonous and can kill animals and humans if enough is eaten. Poison hemlock contains the neurotoxins coniine, g-coniceine, and related piperidine alkaloids which can cause the central nervous system to shut down. Leaves are especially poisonous in the spring, up to the time the plant flowers. Fresh leaves are unpalatable, so livestock seldom eat poison hemlock when other feed is available in a pasture. If ingested, sheep may be poisoned by eating as little as 3 1/2 oz. to 1 lb. 2 oz. of green leaves. Cattle that eat 10 1/2 oz. to 1 lb. 2 oz. may be poisoned. Signs of poisoning usually appear within an hour after an animal eats the plant. Animals die from respiratory paralysis in 2 to 3 hours.
People may be poisoned by eating any part of a poison hemlock plant. Often, poisonings occur when a victim confuses poison hemlock root with wild parsnip root, poison hemlock leaves with parsley, or poison hemlock seed with anise seed. Whistles made from hollow stems of poison hemlock have caused death in children. The toxins can also be absorbed through the skin and respiratory system so always wear protective clothing (i.e., gloves, goggles, mask) when handling this plant.
Options for management of poison hemlock include manual removal, mechanical destruction, and chemical control.
For sites with a few plants, pull or dig up the plants. Be sure to remove the entire root. All mature plants need to be removed so no new seeds are produced. Do not leave flower heads on the ground as the seeds can remain viable. Composting the removed plants is not recommended; instead place in a plastic trash bag and dispose of the bag in an appropriate trash container for removal.
Mechanical destruction of the plants by mowing or cutting with a weed-trimmer before poison hemlock flowers can be effective. Personal protection is a must when dealing with poison hemlock in this manner. One should wear a dust mask to avoid inhaling toxins while mowing or cutting.
Management of poison hemlock might most easily be accomplished with herbicides. Plants should be treated in the spring before they begin to flower with triclopyr, 2,4-D plus dicamba, or metsulfuron. These products will work well for lawn or pasture areas as it won’t harm grasses. Glyphosate products (e.g. Roundup) work also but they kill grass as well as broadleaf plants. Repeat applications may be needed. Follow all precautions for handling herbicides.
Read and follow labels exactly as written and only use products appropriate and legal for the site. Herbicides should only be applied at the rates specified on the label. Foliar herbicides are most effective if applied to actively growing plants in the spring, followed by another application later in the summer for late sprouting seeds. Apply the herbicide to the entire leaf and stem surface and do not cut down the treated plants until they have died. This may take two weeks or more.
For more information on poison hemlock and other problematic weeds, contact your local OSU Extension office. To find your local office visit http://extension.osu.edu. To contact the Van Wert County OSU Extension office, call 419-238-1214 or stop by at 1055 South Washington Street, Van Wert, Ohio on the fairgrounds.
