Poison Hemlock

Poison hemlock (Conium maculatum) is an invasive plant native to Europe and North Africa. It is highly toxic (i.e., fatal) to humans and livestock. Poison hemlock requires considerable sunlight to flourish and is found often near railways, rivers, ditches, field edges, farms and bike paths. It is a biennial plant (having a two year life cycle), and is unlikely to grow in very shady areas or places that are frequently mowed.

Leaves, flower, seeds, and stem of poison hemlock
Poison hemlock. Clockwise from left: lacy, triangular leaves; white flowers in umbel; green stem with purple spots and blotches; seeds.

Identification

Leaves are triangular, dark green, very lacy, and fern-like. Stems are hollow between nodes, ridged, and hairless with purple spots and blotches. Flowers are white with five notched petals arranged in an umbel (umbrella shape) approximately 2 to 3 inches across. 

White flowers will likely get your attention May through August, but many look-alikes (described below) have similar white flowers. Poison hemlock plants in their first year are attractive, shaggy mounds of lacy, dark-green leaves. Poison hemlock plants in their second year are usually 4-6 feet tall (extremes are 1-9 feet) with a green but purple-spotted stem and multiple white flowers. Over the summer months the flowers appear high up on the plant, and very airy and lacy. This white lacy look is a distinguishing characteristic when viewed from a distance. 

All parts of the plant are highly poisonous (including composted plant parts) to humans and animals and may result in death if ingested. Do not eat or compost any part of the plant. Do not let children near the plants. Livestock poisoning usually occurs from the presence of poison hemlock in hay or when pastures are overgrazed and other sources of food have been depleted. Extra care should be taken to wear protective clothing before working with or exposure to poison hemlock.

Common Look-alikes

Native Look-alikes
These plants are natives and and critical for healthy wildlife ecosystems. No management necessary.

  • Angelica (Angelica atropurpurea) has purple stems and leaves are not lacy. A native plant.
  • Common elderberry (Sambucus canadensis) main stems are woody without purple spots. A native shrub.
  • Common ragweed (Ambrosia artemisiifolia) leaves are very lacy but the stems are hairy. A native plant.
  • Tall meadow rue (Thalictrum dasycarpum) may have purple spots on green stems, but does not have lacy leaves. A native plant.
  • Yarrow (Achillea millefolium) leaves are very lacy but are not triangular. A native plant.

Toxic Native Look-alikes
Native, non-invasive plants. Part of a healthy wildlife ecosystem; should only be managed if their location poses a risk to human or livestock health. USE CAUTION and wear protective clothing if walking through or handling.

  • Cow parsnip (Heracleum lanatumdoes not have purple spots on the stem and it has very large lobed leaves. This plant’s sap causes blisters on skin when exposed to sunlight.
  • Water hemlock (Cicuta maculata) stems may have purple spots, but leaves are not lacy. Highly toxic to humans and livestock.

Noxious Look-alikes
These species are regulated noxious invasive plants, and several are toxic.

  • Giant hogweed (Heracleum mantegazzianum) stems may have purple spots, but leaves are not lacy. Giant hogweed is a prohibited noxious invasive plant that causes painful burns and severe blistering. USE CAUTION and wear protective clothing if walking through or handling.
  • Queen Anne’s Lace (Daucus carota; also known as wild carrot) has lacy leaves, but stem has hairs and does not have purple blotches. It has line-like lacy bracts under the flowers. A noxious invasive plant descended from domesticated carrots with an edible root.
  • Wild parsnip (Pastinaca sativadoes not have purple spots on the stem. Another noxious invasive plant, this one descended from domesticated parsnip with an edible root. Like giant hogweed, wild parsnip can cause severe blistering and swelling when the sap contacts human or animal skin and is exposed to sunlight. USE CAUTION and wear protective clothing if walking through or handling wild parsnip to prevent these chemical burns.

Reporting Poison Hemlock

If you believe you have found poison hemlock, contact Arrest the Pest and send a picture of the purple spots on the stem, a leaf, and the specific location of the plant. We ask that you report the plant so that we can track the extent of the invasion and need for future control across the state. Natural resource and other land management professionals should use the GLEDN app to report poison hemlock plants.

Once you have determined it is poison hemlock decide if you or a professional will treat the plants on your property.

Management

It is highly recommended that you call a professional to handle this plant. However, if you choose to manage poison hemlock yourself, be sure to wear gloves and other protective clothing including eye wear. Shower afterwards to be sure you have no toxic sap on your skin, and launder your clothes.

  • First year plants

First year plants (those that don’t have flowers) can be treated at any time of year. They should be sprayed with herbicide, frequently mowed (ensure only non-flowering plants are mowed) or dug out and bagged in plastic. Herbicide is most effective on smaller first year plants early in the year. If left alone these plants will produce many seeds in their second year; as an invasive plant the seeds will spread and grow quickly.

  • April – May (before flowering)

April and May are perfect times to ID and spray plants of any age with an herbicide appropriate to the area (e.g., glyphosate or triclopyr). If using herbicide, know that it is your legal responsibility to read the label and follow the directions. April through May is a perfect time because the plants have not flowered yet and, good news, you have caught them early avoiding a successful invasion.

If you choose a non-herbicidal route during April and May you can pull or dig out the root, being sure to take protective measures outlined above. Once it is dug out, place the whole plant in a plastic bag.

  • May – August (during flowering)

During flowering you may spray green plants with herbicide or dig the whole plant out, ensuring the flower and/or seed heads are not left on the plant or the ground. With gloves on, cut the flower or seed heads off and place in a plastic bag. Do not mow unless flower and seed heads have been removed. Be sure to take protective measures outlined above.

  • September – December (after flowering)

In the fall, the plants that have already flowered (second year plants) die so there is no need to dig the plant out – they will be dead by December and that plant will not return in the spring. Dead plants remain toxic for three years. Seeds on the dead or dying plants will be viable and will germinate over subsequent years. To stop the seeds from germinating, while wearing protective clothing and eyewear, simply cut the seed heads off, and place the seed heads in a plastic bag. Do not mow unless flower and seed heads have been removed. The dead second year plants can be left in place as long as all the seeds are gone.

Surveillance

Due to seeds persisting in the soil, check the same area the next spring for dark, green lacy-leaved plants with purple spots on the stems. If these are found please report them to Arrest the Pest and send a picture of the purple spots on the stem, a leaf, and the specific location of the plant.

Disposal

We recommend you call a professional to safely dispose of the poison plants. Do not compost the plants as the seed and poison will stay viable into the future and more plants will reappear later. Do not burn the plants as this may release the toxin.

If your garbage services accept poisonous plants (as does Dodge, Fillmore, Olmsted and Winona counties) then place the plants in plastic bags and put them in the garbage. The plastic bags will protect the health of the people who handle the garbage even after leaving your property.

If your county does not accept poisonous plants, then bury the plants, flowers and seed heads in an area that will remain undisturbed.

Human health impacts

All parts of poison hemlock (roots, stems, leaves, and seeds) are extremely toxic to humans and livestock when ingested; small amounts can be deadly. This toxin impacts the nervous system causing trembling, salivation, lack of coordination, dilated pupils, weak pulse, respiratory paralysis, coma and death. Extra care should be taken to wear protective clothing and eyewear before working with or exposure to poison hemlock.

Emergency, Livestock, and Other Resources

For suspected ingestion call the Minnesota Poison Control Center 1-800-222-1222