Poison Hemlocks

Aug 18, 2011

By Lorraine Julien

There are two species of highly poisonous Hemlock weeds that grow throughout Ontario: Spotted Water Hemlock (Cicuta maculata) and Poison Hemlock (Conium maculatum). Both are members of the Carrot family, as is Wild Parsnip, and all look similar to Queen Anne’s Lace, at first glance. To add to the confusion, there is also the non-toxic Common Cow-Parsnip and Common Water-Parsnip which look very similar to their very dangerous, poisonous relatives.

Just last week, I was surprised to see that there are a couple of plants near our dock that look very similar to Queen Anne’s Lace except that the overall flower cluster is not flat but instead is shaped like a convex group of little umbrellas (see my photo). The leaves are long and narrow (about 3-10 cm). Probably these plants are the Common Water-Parsnip but I don’t know for sure. We’ll wait until this fall and, once we are covered with long sleeves and work gloves, will carefully remove and burn them.

Spotted Water Hemlock

Spotted Water Hemlock is considered to be our most toxic wild plant – it is native to most of North America from northern Canada to southern Mexico. Its tuberous white roots are often mistaken for parsnips. It’s shocking to think but, according to my research, a single root stock can kill a horse. Children have been poisoned by using peashooters made from water hemlock stems. Though a few people have died from Water Hemlock poisoning, livestock have fared much worse. Symptoms such as nausea, vomiting and convulsions can begin quickly, within 30-60 minutes, followed by severe cramps.

Though Spotted Water Hemlock contains a number of toxins, the main poison is Cicutoxin and it’s mainly concentrated in the roots. This is a scary weed and ingestion of Spotted Water Hemlock in any quantity can result in death or permanent neurological damage of the central nervous system.

Both Spotted Water Hemlock and Water Parsnip have similar clusters of small white flowers shaped like little umbrellas and both like the same habitat near the shore line of lakes and rivers. Water Parsnip, however, is not poisonous but it so closely resembles the poisonous varieties that it’s not worth taking a chance.

Poison Hemlock

Native to Europe and the Mediterranean region, this weed was spread into Ontario in the early 1980s in a seed lot of alfalfa. It’s often found on poorly drained soils, particularly near streams, ditches and other surface water. It also appears on roadsides, edges of cultivated fields and waste areas.

This weed looks very similar to the other nasty members of the Carrot family. At first glance the flowers also look like Queen Anne’s Lace, except the small, white flowers are clustered in little umbrella shapes rather than flat. It is a biennial plant that can grow up to 8 feet (2.5 meters) tall with long, hollow stems. The stems are smooth, green and usually spotted or streaked with red or purple on the lower half. The leaves appear to be lacy and fern-like. It produces a large number of seeds that allow the plant to form very thick stands, sometimes encroaching on cultivated fields, and to the extent that it stops the spread of any other type of vegetation inside its area of growth.

All Poison Hemlock parts are poisonous; however, the seeds contain the greatest concentration of poison. Once the plant is dead, the poison is greatly reduced, although not gone completely. This plant contains a number of poisonous toxins, the worst of which is Coniine, a neurotoxin that has a chemical structure similar to nicotine. It disrupts the workings of the central nervous system and is toxic to humans and all classes of livestock. Coniine causes death by blocking the neuromuscular junction – this results in muscular paralysis with eventual paralysis of the respiratory muscles leading to death due to lack of oxygen to the heart and brain. Death can be prevented by artificial ventilation until the effects have worn off some 48-72 hours later.

This is the poisonous plant that was used to poison condemned prisoners in ancient Greece. Socrates was the most famous victim to be killed in this way.

According to what I’ve read, Poison Hemlocks have been used medicinally, in ancient times as a sedative, to treat symptoms of arthritis and for its antispasmodic properties. The problem was that it wasn’t always effective - there was a fine line between the exact amount needed to treat the ailment and the amount that was toxic enough to kill the patient!

Please feel free to report any observations to Lorraine Julien at This email address is being protected from spambots. You need JavaScript enabled to view it.  or Steve Blight at This email address is being protected from spambots. You need JavaScript enabled to view it.