Which plants are most toxic to horses? In this series of posts, I continue to describe the most common plants that are toxic to horses. Here we look at the Hemlock.


Hemlock (Conium Maculatum) is an erect, branched biennial plant. It can reach 2m in height. It is common in waste ground, in ditches, and by roadsides. It is common in Europe, but rare in West and Central Scotland. In the US, Hemlock is also known as Musquash Root and Spotted Cowbane. These plants are considered amongst the most toxic of plants, and a deadly poison. One mouthful can apparently kill an animal in 15 minutes.

What the herbalist says:

The herbalist Culpeper tells us that: the common great Hemlock grows up with a green stalk, four or five feet high, or more, full of red spots sometimes, and at the joints very large winged leaves one set against the other, dented about the edges, of a sad green colour branched towards the top, where it is full of umbels of white flowers, and afterwards with whitish flat seed; the root is long, white, and sometimes crooked, and hollow within. The whole plant, and every part, has a 1trong, heady, and ill-favoured acent. It grows by walls and hedges throughout all parts of this country. It flowers and seeds in July, or thereabout. Saturn claims dominion over this herb. Hemlock is exceedingly cold, and very dan­gorous, especially to be taken inwardly. It may safely be applied to inflammations, tumults, and swellings in any part of the body, as also to St, Anthony’s fire, wheals, pushes, and creeping ulcers that arise of hot sharp hu­mours, by cooling and repelling the heat; the leaves bruised and laid to the brow or forehead, are good for red and 1wollen eyes; as also to take away a pin and web grow­ing there; take a small handful of this herb, and half aa much bay-salt, beat together, and applied to the contrary wrist of the hand, removes it in two applications. The root roasted and applied to the hands, helps the gout.

The active agents:

The poisonous agents in Hemlock are alkaloids, and these can be extremely toxic. They are more concentrated in the stem and leaves than the root, apparently. Paralysis and convulsions may result from ingesting Hemlock. Hemlock is best known in human history as the poison that was handed to Socrates in 399 BC.

Herbal Medicine

In herbal medicine, Hemlock has been used as a sedative and for cough relief in bronchitis.

Poisoning with Hemlock?

Horse DVM, offers some interesting insights. Hemlock was native to Europe and western Asia. It was however introduced into North America in the 1800s as an ornamental. It then escaped cultivation. It is now widely distributed across much of the United States. It is present in Canada too. As a biennial, poison hemlock produces leaves as a basal rosette in year one. This develops into an upright flower stalk during year two.

Poison hemlock contains eight piperidine alkaloids. Y-coniceine and coniine are the most abundant. These two are the predominant cause of acute and chronic toxicity in horses. Coniine is similar in structure and function to nicotine. These toxins have a direct effect on the horse’s nervous system.

Whilst all parts of the plant are toxic, the most toxic parts are the fruits when they are still green, and stems.

The best way is to remove Hemlock is before seed production. If you are hand-pulling, wear gloves as the plant is highly poisonous. If Hemlock is growing freely on your land, remove it, safely. Please ensure that your horses cannot come into contact with this deadly herb.


I hope this article has been helpful. Other articles about plants toxic to horses appear on my blog and are linked to my Professional Facebook Page. If there is a plant that you would like me to write about, please get in touch via Contacts.

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Keith Allison: A guide to Plants Poisonous to Horses. J.A. Allen. Revised edition, 2011.
John Gerarde: The Herball or General Historie of Plantes, 1636.
Culpeper: Index Of Herbs By Nich. Culpeper, Gent. Student in Physick and Astrologie, 1616, 1654
Sandra McQuinn: Horse owner’s Guide to Toxic Plants. Skyehorse Publishing. 1996, 2020.

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