Is Hemlock Poisonous? Poison Hemlock, Water Hemlock, and the Hemlock Tree

If you think someone just ingested hemlock and you mean the weedy plant, not the tree: call 911 and poison control at 1-800-222-1222 (US) now. Do not wait for symptoms.

There are two entirely different plants called hemlock. Poison hemlock (Conium maculatum) and water hemlock (Cicuta species) are among the most deadly plants in the world. The hemlock tree (Tsuga canadensis) is a non-toxic conifer with no meaningful toxicity. This article covers all three, with the toxic hemlocks as the primary focus.

The Critical Name Confusion

The hemlock tree and the toxic hemlock weeds share a common name but are botanically unrelated. The hemlock tree is a conifer in the pine family, grown as an ornamental and timber tree. It is not poisonous to dogs, cats, horses, or people.

Poison hemlock and water hemlock are flowering weeds in the carrot family (Apiaceae). They are among the plants responsible for the most serious plant poisoning deaths in North America and Europe. The name overlap is a genuine safety hazard, because someone searching “is hemlock poisonous” might be looking at either plant.

If you are not certain which plant you are dealing with, treat it as the toxic one until confirmed otherwise.

Poison Hemlock (Conium maculatum)

Poison hemlock is a tall biennial weed that can reach several feet in height. It grows on roadsides, in disturbed ground, along streams, and at the edges of agricultural fields. It has hollow stems with distinctive purple-red blotches and produces small white flowers in umbrella-shaped clusters. Crushed leaves have a strong, unpleasant musty odor often described as mousy.

The toxic compounds are piperidine alkaloids, primarily coniine and gamma-coniceine. These compounds cause a progressive ascending paralysis: muscle weakness beginning in the extremities and moving upward, eventually paralyzing the muscles used for breathing. The mind may remain clear while the body fails. This is the plant that killed Socrates.

All parts of the plant are toxic. Seeds contain the highest concentration of alkaloids. Young growth in spring is particularly potent.

Water Hemlock (Cicuta Species)

Water hemlock (Cicuta maculata in eastern North America, Cicuta douglasii in the west, Cicuta virosa in Europe) grows in wet habitats: stream banks, marshes, wet meadows, and drainage ditches. It is widely regarded as the most violently toxic plant in North America.

The toxic compound is cicutoxin, a polyacetylene alcohol that acts as a GABA antagonist in the nervous system. This causes extremely violent convulsions. The onset of symptoms after ingestion can be rapid, sometimes within 15 to 60 minutes.

The roots of water hemlock are particularly dangerous because they resemble edible plants such as wild parsnip or water parsley. When cut, they reveal a chambered cross-section and may release a yellowish sap. They have been fatally mistaken for edible roots by adults.

Which Parts Are Toxic?

For both toxic hemlocks, all parts are poisonous. For poison hemlock, seeds and young spring growth have the highest alkaloid levels. For water hemlock, the roots contain the most cicutoxin, but stems and leaves are also toxic.

For the hemlock tree (Tsuga), no part is known to be toxic to dogs, cats, horses, or people.

Symptoms

Dogs and cats: Trembling, incoordination, drooling, slow heart rate, and respiratory distress after poison hemlock ingestion. Water hemlock causes rapid onset of violent seizures. Both can be fatal within hours.

Humans and children: Poison hemlock produces ascending muscle paralysis. The legs weaken first, then the arms, then breathing becomes impossible. Water hemlock causes sudden violent convulsions, vomiting, and in untreated cases, cardiac and respiratory failure. Children are at severe risk because of their smaller body mass.

What to Do

This is a medical emergency.

1. Call 911 immediately if seizures, collapse, or serious breathing difficulty have occurred.
2. Call poison control at 1-800-222-1222 (US) or your vet for immediate guidance even if symptoms have not yet started.
3. Do not induce vomiting.
4. Bring a sample or clear photo of the plant to the emergency room or veterinary clinic.
5. Do not leave the affected person or animal unattended.

There is no specific antidote for either plant’s toxins. Treatment is supportive and must begin as early as possible.

Avoiding Dangerous Confusion

Both toxic hemlocks are commonly mistaken for edible plants in the carrot family, including wild carrot (Queen Anne’s lace), wild parsnip, cow parsley, and fennel. They share the same umbrella-shaped white flower clusters and similar leaf shapes.

Never eat any plant from the carrot family that you have not positively identified with expert guidance. The risk of a fatal mistake is not theoretical.

Frequently Asked Questions

Is the hemlock tree poisonous? No. The hemlock tree (Tsuga canadensis and related species) is a conifer unrelated to poison hemlock or water hemlock. It is not toxic to dogs, cats, horses, or people.

How fast does water hemlock poisoning act? Convulsions can begin within 15 to 60 minutes of ingesting a significant amount. Poison hemlock typically takes longer, with symptoms developing over several hours, depending on the amount consumed.

Can touching these plants cause harm? Significant toxicity from skin contact alone is unlikely, but alkaloids can be absorbed through broken skin. Wash hands thoroughly after handling any unknown plant. Do not rub your eyes or mouth after contact.

What does poison hemlock smell like? Crushed leaves and stems produce a strong, unpleasant musty or mousy odor. This is one useful identifier, but smell alone is not reliable enough to confirm identification. Do not taste-test any unknown plant.