Poison hemlock (Conium maculatum)

White flowers of a large clump of poison hemlock plants.

Poison hemlock (Conium maculatum) was introduced to North America in the 1800s as a garden plant. It can be found along streams and roadsides, and in wet areas, fields, and disturbed habitats. This plant is highly poisonous. Do not ingest any parts of the plant as it is poisonous to humans and livestock.

Warning: This plant is highly poisonous. Do not eat any parts of the plant as it is poisonous to humans and livestock. Can be fatal if ingested.

Description

Appearance

This herbaceous plant has clusters of white flowers and fern-like leaves. It can grow three to eight feet tall and has purple spots or splotches along the stem.

Leaves and Stem

Leaves are triangular, dark green, very lacy and fern-like. They can be 8-16 inches long. Leaf stalks clasp the stem, which appears slightly swollen where they attach. The hairless stems are hollow, have ridges, and are marked with purple spots or a mottled pattern. Crushed leaves smell like parsnip or fennel (use gloves when handling this plant).

Flowers

Small white flowers with five petals are found in umbrella-shaped clusters that are 3-6 inches across. Typically three to four of these flower clusters are found at the tips of branches. These airy and lacey-looking blooms can be seen from May-August.

Seeds

Seeds are flat with ridges. An individual plant can produce over 30,000 seeds.

Roots

Poison hemlock has a thick white taproot that can be confused with wild carrots or wild parsnips. The roots are highly toxic (can be fatal) to humans and livestock. Crushed roots and leaves smell like parsnip or fennel (use gloves when handling this plant).

Biology

This plant requires considerable sunlight and can be found along streams and roadsides, and in wet areas, fields, and disturbed habitats. It is a biennial, meaning that it grows only leaves in its first year and flowers in its second year. All parts of this plant are poisonous, including composted plant parts.

Origin and Spread

Poison hemlock is native to Europe, northern Africa and western Asia. It was introduced to North America in the 1800s as a garden plant. This plant spreads by seed and is present in most states in the continental United States. Visit EDDMapS for the current distribution of this species.

Don't be fooled by these look-alikes

  • Poison hemlock look-alike chart (University of Minnesota Extension)
  • Spotted water hemlock, Cicuta maculata (native) – Leaves are not lacy or fern-like. Spotted water hemlock is also poisonous to humans and livestock.
  • Queen Anne's Lace, Daucus carota (invasive) – Often has a purple flower in the center of each white flower cluster. Typically, each stem holds only a single flower cluster and stems are frequently hairy and do not have purple spots.
  • Sweet cicely, Osmorhiza claytonia (native) – Smaller plants, seldom more than 2 feet tall, usually with some hair on the stems and small flower clusters.
  • Erect and spreading hedge parseley, Torilis japonica and Torilis arvensis (invasive) – Sparser leaves with more branching than poison hemlock; small bracts at the base of the flower clusters are very narrow, almost threadlike, while those of poison hemlock are broader and more leaf-like.
 
Regulatory Classification

This species is a Minnesota Department of Agriculture Prohibited Noxious Weed on the Eradicate List meaning that the above and belowground parts of the plant must be destroyed. Additionally no transportation, propagation, or sale is allowed.

Threat to Minnesota
  • Poison hemlock is highly poisonous. Do not ingest any parts of the plant as it is poisonous to humans and livestock. Wear gloves when handling the plant.
  • This plant can grow in dense patches and displace native species along streams, wet areas, fields and disturbed habitats, such as roadsides.
What you should do

One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.

PlayCleanGo: Stop Invasive Species in Your Tracks

  • REMOVE plants, animals & mud from boots, gear, pets & vehicle.
  • CLEAN your gear before entering & leaving the recreation site.
  • STAY on designated roads & trails.
  • PLANT non-invasive species.
Native Substitutes
 
Control Methods

Special precautions for this plant are noted by the University of Minnesota. They state "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."

Mechanical control can be done by hand pulling of small infestations while wearing gloves. Cutting the taproot 1-2 inches below the ground using a shovel can also be effective. Mowing after flowers emerge, but before seeds are formed, can reduce the population if repeated in future years. First-year plants may be too short to be impacted by mowing, but the purpose of mowing is to reduce seeds by removing the flowering stalks of second-year plants.

Kill but do not remove weeds from an infested area when possible. If removal of plant materials is necessary, properly contain and transport the materials to a disposal site that will accept and properly dispose of noxious weed materials. Visit the Minnesota Department of Agriculture website for additional information on disposal. Special precautions for this plant are noted by the University of Minnesota. They state "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." Contact your local county agricultural inspector to determine if your county has special disposal options for poison hemlock.

Herbicide control can be done using a foliar spray of triclopyr, glyphosate, or 2 4-D.These are systemic herbicides that are taken up by plants and move within the plant, which can kill leaves, stems, and roots.

Reporting

Report new occurrences by submitting a report through EDDMapS Midwest, emailing Arrest The Pest, calling Arrest the Pest (1-888-545-6684), or contacting your local county agricultural inspector.

Resources