Look out for Poison Hemlock - A Toxic Weed Commonly Found Throughout Indiana

Poison hemlock (Conium maculatum) was introduced into North America as an ornamental but escaped cultivation and became naturalized throughout much of the United States. Other common names include deadly hemlock and poison parsley.  This invasive plant typically grows in pastures, roadsides, ditches, waste areas, marshy areas, stream banks, and is found throughout all counties in Indiana.  

Poison hemlock

Poison hemlock is a biennial that produces leaves in a basal rosette during its first year and forms an upright flower stalk when it bolts during the second year of growth. Poison hemlock has dissected leaves, which resemble those of parsley. However, the stems and leaf stalks (petioles) of this plant differ from other family members in that they are hairless and have distinctive purple spots or blotches.

Poison hemlock flower

The umbrella-shaped clusters of small white flowers are similar to flower clusters produced by many other species in this family such as Queen Anne's Lace.  They appear in June and continue to form through August. A single plant may produce over 38,000 seeds. However, few of these seeds will remain viable after 5 years of storage in the soil seed bank. 

Poison hemlock is easily controlled manually or with herbicides such as those containing the active ingredient glyphosate or 2,4-D. For more information on control, see this publication from Purdue University.  

All parts of the plant contain toxic substances that cause respiratory failure in humans and other animals if ingested. Seeds and roots are more toxic than foliage; also, toxins are present in greater amounts in plants grown under sunny compared with moist conditions. The taste of leaves and seeds of poison hemlock is reported to be unpleasant, so toxic quantities are seldom consumed.

Similar species:

Wild carrot or Queen Anne's Lace (Daucus carota) looks similar to poison hemlock except its stems and petioles are hairy while those of poison hemlock are hairless and have purple mottling. Wild carrot has a carrot-like taste and smell while poison-hemlock tastes somewhat like parsnip and has a mouse-like smell. 

Wild parsnip (Pastinaca sativa) lacks purple mottling on its stems and petioles, although it is otherwise very similar in appearance to poison hemlock. 

Water hemlock (Cicuta maculata) shares many characteristics with poison-hemlock including hairless stems that have purple mottling and compound leaves. Unlike poison hemlock, water hemlock usually has a cluster of fleshy taproots at its base. Also, habitat may help to distinguish the two as water hemlock grows in marshes and other very wet sites and is generally confined to the central plains and mountainous areas of North America. 

Experts taken from this publication from Ohio State University.