Poison hemlock or Conium maculatum is a non-native plant that was brought to the United States as an ornamental in the late 1800s. All parts of the plant are poisonous and contain highly toxic compounds that cause respiratory failure and death in mammals.
Seen on roadsides, in ditches, at edges of farm fields, and now in landscapes and gardens, poison hemlock is easily sneaking through the Miami Valley. Many suspect that this plant is Queen Ann’s lace (Daucus carota) as the blooms and foliage look similar.
However, besides being extremely poisonous, poison hemlock stems are hairless (Queen Ann’s lace has hairs) and have distinct purple blotches on the stems.
Queen Ann’s lace blooms a little later than poison hemlock, but you may also see them both in bloom at the same time. Poison hemlock is in full bloom in central Ohio and likely entering the seed stage in southern Ohio.
Both are biennials, meaning they have a two-year life cycle. In the first year, they grow foliage only. The leaves are arranged in a rosette, all connected to a stem at the ground level.
The year, they send up a flowering stalk and bloom, go to seed, and die. Poison hemlock can grow up to six to seven feet tall, while Queen Anne’s lace is around four feet.
The toxins in poison hemlock must be ingested for poisoning to occur. However, the juices can enter the body through the nasal passages, eyes and any open wounds on the body.
Control with herbicides is best completed when the plant is in its first year of growth or well before it blooms the second year. Spraying while in bloom may kill the foliage, but the seeds continue to develop.
If you are using a weed eater or other mechanical equipment to remove poison hemlock, be sure to wear protective gear to keep the juices from splashing into your eyes, nose, mouth, etc. It can be bagged and put in the trash.
Another similar plant, wild parsnip, can also be quite toxic. Poison hemlock sap does not cause blisters and kills if ingested. The sap of wild parsnip can cause major dermatological problems.
Wild parsnip sap contains a chemical compound that, when exposed to light, results in blisters. This is called phytophotodermatitis. If you get sap on your skin, get out of the sun and wash immediately.
These are not your average “new shoe” blisters, these are blisters are large and can lead to scarring of the skin. The sap in your eyes can lead to blindness.
Again, poison hemlock and wild parsnip should be controlled in the early stages of growth. If you still see the first year’s growth (it will be hugging the ground and not blooming), you can use herbicides to control it.
If you let these go to seed, I guarantee you will see an explosion of this weed in your landscape and fields. Work now to get control.
Pamela Corle-Bennett is the state master gardener volunteer coordinator and horticulture educator for Ohio State University Extension. Contact her by email at bennett.27@osu.edu.
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