A member of the Apiaceae family of plants, poison hemlock is related to many familiar crops and herbs including carrots, celery, parsley, cilantro, and dill. However, this plant is definitely one to keep away from your next dinner recipe. Poison hemlock (Conium maculatum), is a poisonous herbaceous plant that is native to Europe. Introduced in the 1800s as an ornamental, it has since spread across most of the continental United States and is becoming more common in the Alabama landscape.

Threats

All parts of poison hemlock (roots, leaves, flowers, seeds, and stem) are poisonous to people and other mammals. Plants contain several strongly alkaline oils that are highly toxic. The most severe reactions related to ingestion involve muscle paralysis followed by suffocation and death. Do not eat any part of this plant.

Livestock typically avoid eating these plants as the leaves are unpalatable, but if other feed is not available, the plants may be ingested.

Skin contact with the plant or plant sap can cause dermatitis or skin irritation. Like with poison ivy, the degree of reaction can vary greatly depending on the amount of contact with the plant and amongst individuals. More severe reactions may occur if sap enters cuts on the skin or gets into the eyes, nose, or mouth.

Habitat and Distribution

Populations of this plant are slowly moving south through the state. Most reports are from north Alabama including Colbert, Limestone, and Madison counties, but it has also been reported further south. It grows best in shaded areas with moist soil but can tolerate a range of conditions. Infestations often occur in disturbed areas such as rights-of-ways, roadsides, and ditches but also can be found in pastures, field margins, and low-lying areas.

Identification

Poison hemlock is a biennial, meaning that it has a two-year life cycle. In the first year, plants form a rosette or a cluster of leaves spreading outward from a short stem and not gaining much height. In the second year, with the help of a well-established taproot, the plants bolt, reaching heights of 3 to 10 feet tall and producing many flower heads. After seeds have set, the plants die but remain standing, and seeds are dispersed to begin the process over again.

Leaves of poison hemlock are pinnately compound and broadly triangular in outline, with finely divided leaflets that taper to a point (figure 1). Stems are hollow, glabrous (not hairy) and slightly ribbed, with distinctive purple stripes and splotches (figure 2). Plants bloom in the early summer, producing umbrella-shaped flower heads (figure 3) that contain distinct clusters of small, five-petaled, white flowers (figure 4).

Control

Tillage or cultivation can effectively prevent seedlings from establishing. Small plants can be pulled (wearing gloves, long sleeves, long pants, and protective eyewear), with all plant material bagged and placed in the trash as dead plants remain toxic. Mowing is effective only if repeated throughout the growing season as plants can regrow and reflower after mowing. Avoid contact with plant debris when mowing.

The active herbicide ingredient glyphosate can offer good control when applied in the spring. Treatment is most effective if applied to first-year rosettes but can also be applied to second year stalks prior to flowering. Because seeds can remain viable in the soil for several years, several years of monitoring and treatment of emerging plants may be required for complete control.