Poison Hemlock (Conium maculatum), also known as poison parsley or spotted parsley, is an erect biennial weed that can grow 6 to 10 feet tall. Originally imported from Europe as an ornamental plant, its spread across the North America has been rampant. All parts of this plant are poisonous.
The leaves and flowers of Poison Hemlock are similar to those of parsnips and carrots. In fact, many deaths have occurred as a result of people mistaking it for the edible species of the carrot family. The use of hemlock as a poison goes back many years in history. It is said that the ancient Greeks used it to poison their enemies and political prisoners. Socrates, condemned to die as a political prisoner in 329 B.C., drank the juice of the hemlock plant and committed suicide. Native Americans were known to dip their arrows in hemlock.
Poison Hemlock is an erect plant with smooth, hollow stems that are covered with purple spots. Its shiny green leaves are pinnately compound, multi-stemmed and fern-like in appearance. Flowers are showy, white umbrella-like clusters that occur during June to July. The fruit is small, flat, grayish-green, and matures in August to September.
The large, fleshy white taproot can be easily pulled from moist ground when plants are small. Several herbicides are effective in controlling Poison Hemlock. Glyphosate (Roundup) can be used on newly emerged sprouts. Other post-emergents that are most effective in early spring are 2,4-D, 2,4-DB and MCPA. 2,4-D should be combined with a wetting agent when applied. Because of the large number of seeds that may have been produced, repeated herbicide applications may be necessary. The frequency of application varies with the herbicide. Read labels carefully and follow directions as given.
Biological control has been effective using the Hemlock moth (Agonopterix alstroemericana). The larvae of this moth feed on the leaves, young stem tissue, flowers and seeds of Hemlock plants causing severe defoliation and death of the plant.
Photos of Poison Hemlock, stem, foliage, & flowers: Judy Sedbrook
Photo of taproot: Mason Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010