Poison Hemlock - U of I Extension

News Release

Poison Hemlock

This article was originally published on June 20, 2011 and expired on June 27, 2011. It is provided here for archival purposes and may contain dated information.

Have you noticed all the tall white flowers growing along roadsides throughout West Central Illinois? According to Rhonda Ferree, University of Illinois Extension horticulture educator, "This weed is poison hemlock and is increasing in numbers."

Poison hemlock grows two to six feet high and is closely related to wild carrot (Queen Anne's Lace). Both plants have lacy, fern-like leaves. Hemlock, however, grows much taller and has purplish spots and blotches on its hollow stems. Both plants have white flat flowers. Although usually found growing along streams and roadsides, poison hemlock has been spreading into gardens, pastures, and waste areas the past few years.

As the name implies, poison hemlock is a very poisonous plant. In fact, it was the juice of this plant that the ancient Greeks used to poison Socrates. Many cases of human poisoning occur because the hemlock roots are mistaken for parsnips; the leaves, for parsley; and the roots and seeds, for anise. For livestock, poisoning is most likely to occur in early spring, when tender new leaves come from the root. Later in the year all parts of the plant are poisonous.

Poison hemlock has a biennial lifecycle. This means it grows close to the ground with all leaves the first year. The second year, it grows much taller and produces a flower, releases seeds, and then dies. New seeds then germinate each year to repeat the lifecycle. As with most plants, poison hemlock is easiest to control when it is young in its first year.

Other plants are seen, as well, that look similar to poison hemlock and Queen Anne's Lace. Wild parsnip has a comparable flower head, but with yellow flowers. Wild parsnip grows two to five feet tall with celery-like leaves. Some people are very sensitive to parsnips, developing large blisters when touching the plant.

Most of the other similar plants have white flowers. Water hemlock looks similar to poison hemlock but is a perennial plant that grows from tubers. Water hemlock is perhaps the most poisonous plant that grows in Illinois. Although the leaves are not toxic, a relatively small amount of the tuber will kill a cow in a short time. Also prevalent this spring is the large cow parsnip. Not a toxic plant to eat, but very impressive in size at nine feet tall. Cow parsnip also causes blisters on some people.

For more information on this or other horticultural issues, contact your local Extension office by visiting www.extension.illinois.edu. You can also post questions on Rhonda's new facebook page at www.facebook.com/ferree.horticulture.

Source: Rhonda J. Ferree, Extension Educator, Horticulture, ferreer@illinois.edu

Pull date: June 27, 2011