Poison Hemlock or Poison Parsley (Conium maculatum)

Description

A biennial, or sometimes a perennial, that can grow to 3 to 10 feet in height. A large rosette appears in the first year and tall stems and flowers appear in the second year. Poison hemlock stems are stout, hairless, erect and distinctly ridged. The stems branch extensively and are hollow except at the nodes. Reddish-purple mottling is common on the lower stem. Dried stems persist throughout the winter. The alternate, pinnately compound leaves are shiny, green, hairless and dissected. The leaves, when crushed, emit a musty, unpleasant odor. Flowers are white and arranged in broad umbrella-shaped clusters. The umbels appear on individual stems that extend from a common stalk. The umbel diameter ranges between 4 to 6 inches.

Look-alikes

Misidentification of poison hemlock with edible varieties in the carrot family can result in deadly consequences. A distinguishing characteristic is the lack of hairs on the leaves and stems of poison hemlock. Queen Anne’s lace or wild carrot (Daucus carota) has finely divided leaves but its leaves and stems are quite hairy. Cow parsnip (Heracleum lanatum) can be identified by its palmately compound leaves which branch out in a semicircle from the stalk end. Poison hemlock has pinnately compound leaves which are arranged in rows along the stalk. The stem of cow parsnip lacks purple mottling and its surface is downy or fuzzy. Water hemlock can be distinguished by its taproot, which is branching and tuberous.

Life History & Invasive Behavior

Poison hemlock can quickly colonize disturbed sites and prefers moist pastures, meadows and riparian areas. Poison hemlock is a large threat to livestock when grazed upon or unknowingly harvested with hay. Although not as common, poison hemlock can invade perennial crops like alfalfa. It is a threat during the first cutting but not the second because the regrowth of alfalfa can suppress poison hemlock growth.

All parts of the plant are toxic although the lower parts of the plant are extremely poisonous. Environmental factors can affect the concentration of poisonous alkaloids. The principle known poisons are piperidine, coniine and coniceine. Coniceine is eight times more toxic than coniine. Both affect the reproductive and nervous system of the animal.

Control Methods

Mechanical: Hand pulling or grubbing is most effective with small infestations prior to flowering. Follow-up monitoring is necessary to ensure seeds in the soil don’t germinate and become established. Because the plant can remain toxic for several years after being pulled, the dead plant should be bagged and landfilled. Several mowings close to the ground, prior to flowering may be effective.

Chemical: For large infestations chemical controls are most effective. The application of 2,4-D mixed with diesel oil can be applied to leaves and stems. To apply to a 1-acre area, mix 2 quarts of diesel oil with 1.5 lbs of 2,4-D ester and 100 gallons of water.

Metsulfuron, 0.6 oz. of active ingredient mixed with a nonionic surfactant, can be applied to actively growing plants. It is recommended to only apply this on pastures and rangeland and areas not used for growing crops. Glyphosate can be used when the plant in its first year, in its rosette life-stage. It is recommended that 1.0 lb. of active ingredient is used per acre. Repeated applications of herbicide most likely will be necessary to deplete the seed bank.

Additional Information (Links exit DNR)

UW-Madison Herbarium
USDA
Pacific Northwest Weed-Control Guide
Images by Steven J. Baskauf