Water Hemlock vs Poison Hemlock: The Leaf-Vein Test

Safety first. Poison hemlock and water hemlock are both deadly. Do not touch, taste or crush either plant to identify it. If you think someone has eaten any part of either plant, call Poison Control at 1-800-222-1222 or go to an emergency department now. If the person has collapsed, is having a seizure, has trouble breathing or cannot be woken, call 911.

Do not try to settle the identification by pulling a plant. Photograph it from a safe distance and ask your extension office or state agriculture department to confirm it.

Water hemlock vs poison hemlock identification: the leaf-vein test

The clearest difference is on the leaf. Poison hemlock veins run to the tips of the leaf teeth. Water hemlock veins run into the notches between the teeth. Wisconsin DNR states both halves of that rule on its poison hemlock page, and NC State Extension describes the water hemlock veins the same way.

You need a close, clear view of the underside of a leaflet. Use a zoom lens from a safe distance, or ask someone who knows the plant to point it out. Do not pick a leaf from either plant.

That one difference is useful because the two plants share so much: both are in the carrot family, both have hollow stems and white umbrella-shaped flowers, and both can grow in wet ground.

Stem: both are hollow

Both stems are smooth and hollow, so “hollow stem” does not decide anything.

• Poison hemlock has red or purple spots and streaks on a stem that is otherwise green and hairless. King County, Wisconsin DNR and Penn State all describe the same marking.
• Water hemlock has a hollow stem that is purple-striped or mottled, with cross-walls at the nodes. NC State Extension notes that these partitions are common near the base of the plant.

A purple stem is a reason to stop, not a reason to choose a name. The spots and the cross-walls are different patterns, but they can be hard to see in poor light.

Roots: taproot or tubers?

The roots are the next check, but only on a plant that is already out of the ground or on a photograph of an excavated plant.

• Poison hemlock has a thick, white taproot that looks like a carrot. Penn State gives a length of about 10 inches, with many fibrous roots.
• Water hemlock has short, thickened tuberous roots. NC State Extension describes them at the base of the plant, and notes that the roots are more poisonous than the rest of the plant.

Never dig either plant to look at the root. The root is the most toxic part of water hemlock, and poison hemlock’s roots and seeds carry the highest concentration of coniine.

Habitat: wet ground or dry ground?

Habitat is a strong clue, but not a guarantee.

• Water hemlock grows in wet meadows, thickets, fresh-water swamps, roadside ditches and floodplains. NC State lists pond, stream and ditch banks.
• Poison hemlock grows in roadsides, fields, stream banks, disturbed areas, riparian woodlands and floodplains. Wisconsin DNR lists all of those, and King County adds trails, yards and vacant lots.

The overlap is real: both can grow along a ditch or a stream bank. Wet ground points toward water hemlock, but it does not rule out poison hemlock.

Why both plants are deadly

Poison hemlock contains alkaloids, mainly coniine. Penn State says all parts are toxic to people and other animals, with the roots and seeds holding the highest concentration. Poison Control describes the early symptoms as nausea and vomiting, excessive salivation, sweating and muscle weakness. As the poison affects the nervous system, it can cause tremors, seizures, paralysis and trouble breathing. Muscle damage and kidney failure can occur in severe cases. Poison Control says it is not safe to treat a poison hemlock exposure at home.

Water hemlock contains cicutoxin and cicutol. NC State Extension rates it as highly toxic and potentially fatal if eaten. Its listed symptoms include muscle spasms, dilated pupils, dizziness, diarrhea, stomach pain, convulsions, seizures, tremors, fever and respiratory depression. The roots are the most dangerous part.

Both plants are also dangerous to livestock and pets. A patch in a pasture is not a landscaping problem; it is a poisoning risk.

If someone is exposed

1. Call Poison Control at 1-800-222-1222. The service is free, confidential and available 24 hours a day. You can also use the online tool at poison.org.
2. If the person is seriously ill, call 911.
3. If sap gets on skin, wash it off with soap and warm water. Poison hemlock sap can irritate skin, and King County says a serious reaction is possible in some people.
4. Do not wait for symptoms to decide whether to get help. Poison Control says hemlock symptoms can progress quickly.

Do not try to treat the exposure at home. Poison Control says that is not safe.

What to do this week

• Mark the location and keep children, pets and livestock away from the patch.
• Photograph the whole plant, the stem, a leaf and the flower head from a safe distance.
• Send the photos and location to your extension office or state agriculture department.
• Do not mow, weed-whack or pull a stand near water without advice. Cut material and sap can reach your skin, and water-adjacent sites have different rules.
• If the plant is in a pasture or hayfield, tell the landowner or farmer before doing anything else.

For the poison hemlock profile, see poison hemlock. For the Queen Anne’s lace comparison, see poison hemlock vs Queen Anne’s lace. For safe removal, see how to get rid of poison hemlock. For other pairs, see look-alikes, and for plant profiles by category, see plants.