Poison Hemlock: Identification and Control

Poison hemlock (Conium maculatum) was originally brought to the United States in the 1800s as an ornamental garden plant (another good idea gone awry) and is now found throughout North America.

Poison hemlock is a biennial that forms a rosette its first year, often going unnoticed, and then produces white umbel flowers (umbrella-like) on tall stems in the spring-summer of its second year. These types of flowers are common in the carrot family and are similar to wild carrot (also known as Queen Anne’s lace, Daucus carota), which is often mistaken for this plant at younger stages.

Poison hemlock

Biennial
Finely-divided, lacy leaves
Green stems and leaves lack hairs
Hollow stems
Exudes unpleasant odor when crushed
6-10 feet tall at maturity
Flowers June through August
White, delicate, umbrella-like flower

Queen Anne's Lace

Biennial
Finely-divided, lacy leaves
Fine hairs on leaves
Solid green stems
Root smells like carrot
1-2 feet tall at maturity
Flowers June through September
White, delicate, umbrella-like flower

reproduction

Poison hemlock reproduces exclusively by seed, up to 38,000 of them per plant. Can germinate immediately or lay dormant for up to six years. The plant will die soon after it sheds seed, but the stems will remain standing and slowly release seed into winter.

toxicity

While looking at the plants or touching them is generally not hazardous, all parts of poison hemlock are toxic if ingested by humans and it is infamously known as the form of death chosen by Socrates. It is also highly toxic to livestock and wildlife.

Blistering

The phototoxic compounds, furanocoumarins, that cause blisters in those who touch giant hogweed are also present in the sap of all other plants in the carrot family, including poison hemlock. Contact with the sap can cause sensitivity to UV light from the sun, resulting in blisters. The difference is that people are much less likely to encounter the sap on carrot family species other than giant hogweed as it is inside the stem, whereas on giant hogweed it is also present on the exterior.

Getting rid of poison hemlock

Digging up

If you find any poison hemlock in your yard or pasture, you can dig up plants, including the tap roots, for removal. Given the reproductive capacity of this plant, remove it as soon as possible to avoid perpetuating the infestation. Wear gloves, long sleeves, pants, socks and shoes to protect your skin from the sap. Compost or dispose plants with yard waste. Poison hemlock is not considered an invasive species, therefore it should not be disposed with regular trash.

Mowing

Do not burn plants to prevent any accidental inhalation. Mowing or weed whacking will not kill the plant but can reduce seed production in second year plants. If you are mowing or weed whacking in areas infested with poison hemlock, wear protective eyewear and a dust mask to prevent exposing your eyes and lungs to small particles.

Herbicides

Herbicides can also be effective for controlling poison hemlock when sprayed on first year plants and small plants before flowering in the second year. Mature, flowering plants like the ones in the photos will not likely be chemically controlled, and mechanical control measures should be used prior to seed set. Choosing a herbicide will depend on the desirable surrounding vegetation.

Other poison hemlock look-alikes

They can grow in disturbed areas, woodland edges and along road sides include Japanese hedge parsley (Torilis japonica) and false chervil(Anthriscus sylvestris). Japanese hedge parsley has lacey, finely divided leaves like poison hemlock, but has fine hairs along the leaves and stem and does not have the distinct purple spotting (pictured below). There are also hooked hairs on the fruit. False chervil fruit are narrowly egg-shaped. Stems are hairy, ribbed and the leaf sheath wraps around the stem. Both Japanese hedge parsley and false chervil generally do not reach more than 6 feet at maturity.

And also it could be...

Another type of poisonous hemlock is water hemlock. Two species of water hemlock—Cicuta maculata (biennial) and Cicuta bulbifera (perennial)—are native to the state. Both exhibit some of the same toxic properties as the western relative, Cicuta douglasii.

Water hemlock grows in wet areas such as in bogs, ditches and along streams. The toxin, cicutoxin, produced by water hemlock has a carrot-like odor and is a strong convulsant produced almost entirely in the thick taproots and, to a lesser extent, in the stems, leaves and seeds.

This article was published by

Michigan State University Extension