Controlling Poison Hemlock

Published 3:00 pm Saturday, October 8, 2022

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By Levi Berg

Clark County Extension Office

I love when fall finally arrives, but I hate the arrival of certain weeds. One of those weeds is poison hemlock, and it is already in the rosette stage. Poison hemlock is originally a native of Europe, and was introduced to North America as a garden/ornamental plant. Poison hemlock is famously known for being the poison that killed Socrates in Athens in 329 B.C. The easiest way to identify young poison hemlock is to look for low-lying rosettes with a purple spotting on the stems, and mature plants will be between 3ft-10ft tall with stout, smooth stems with purple spotting. The leaves have a fern-like appearance with alternating arrangements. Mature poison hemlock can be easily confused with wild carrot (Queen Anne’s Lace) or wild cow parsnip because of the small umbrella-shaped flower clusters, but neither wild carrot nor wild cow parsnip have purple spotting on the stems.

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Poison hemlock causes reason for concern because of two reasons; contains highly poisonous alkaloid compounds and is extremely prolific. If ingested, poison hemlock can be deadly to livestock, humans, and other animals. All parts of the plant are extremely poisonous, and the stem and roots are particularly deadly. Grabbing the stem with your bare hand can cause extreme irritation. On many occasions, cattle, horses, and other livestock have been found dead within 30 minutes to two hours after ingesting parts of poison hemlock.

This weed is not only extremely poisonous, but will quickly run wild in pastures, gardens, row crops, property lines, yards, and many more places. This weed is a biennial weed that produces seeds that are easily spread by mowing, road maintenance or agricultural equipment during its second year of life.

Management and prevention of poison hemlock can be tricky because of timing. The key to controlling poison hemlock is to prevent the production of seed. If the plant cannot produce seed, the plant cannot reproduce and spread. So persistent mowing throughout the early spring and summer will keep poison hemlock from producing seeds and spreading. Single plants can be dug with a spade, placed in a trash bad, and disposed, but make sure to always wear gloves and eye protection.

If mowing or digging doesn’t work, you can use certain herbicides to control growth. However, herbicides will only control young, rosettes or very small second-year plants which means herbicides need to be sprayed now in the fall or early in the spring as soon as possible. I personally like using 2,4-D because it will kill the weed without killing grass. However, glyphosate will also work, but glyphosate is a non-selective herbicides that will damage or kill any plant it contacts. Other herbicides that work well are chlorsulfuron, clopyralid, dicamba, and imazapic. For any herbicide, follow the label because THE LABEL IS THE LAW.

Poison hemlock can become a hassle to control, but if you start now you can prevent its spread. Just remember that poison hemlock is extremely poisonous if ingested. Information was obtained from Purdue University Extension publication FNR-437-W and Montana State University Extension publication MT200013AG.