Poison hemlock | The Morton Arboretum

Poison hemlock is an exotic weed in Illinois. The Illinois Exotic Weed Act prohibits the sale or distribution of this plant in Illinois. This plant spreads by seed. It commonly grows in disturbed sites such as roadside edges, but can also be found along the borders of natural areas. It is found throughout the continental United States and Canada.

All parts of poison hemlock are toxic to humans and other animals. One or more Midwestern states legislate against poison hemlock. As an alternative, great angelica (Angelica atropurpurea) is a native species to most of the Eastern United States that would be a suitable alternative. There are a number of native and nonnative species that could be used in place of poison hemlock. More desirable species may be found using the Search Trees and Plants page of the Arboretum website. Before purchasing or planting, be sure to check for any local or state guidelines on this species, and make sure that this plant is suitable for its habitat by checking its attributes at mortonarb.org or plants.usda.gov.

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Size and Method of Spreading

Poison hemlock is a large biennial plant. In its first year, it will form a rosette of leaves. During its second year, it can grow from 5 to 10 feet tall. The hollow stems are mottled, hairless, and ribbed.

Leaf Description

The leaves of poison hemlock are alternate and may be double- or triple-pinnately compound, giving them a delicate, fernlike appearance.

Flower Description

The flower cluster (inflorescence) is a loose, open compound umbel that is typical of the carrot family. Each compound umbel consists of multiple umbels (a cluster with small flowers on stalks arising from a central point). The entire structure is 5 inches wide with individual white, five-petaled flowers. Flowering time is late spring into early summer.