Invasive Watch: Poison Hemlock (Conium maculatum)

Poison Hemlock

We regularly highlight noxious and invasive species, sharing defining characteristics to help you identify them in the field as well as providing treatment recommendations to help you get rid of it. In this Invasive Watch, we feature poison hemlock.  

What does poison hemlock look like?

Poison hemlock is an invasive biennial in the Apiaceae family that can grow between 6 and 10 feet tall. Its five-petaled umbrella-shaped clusters are white or yellowish in color and appear on hollow green stems that commonly feature purple blotches. Unlike the wild carrot (Daucus carota), which it closely resembles, the poison hemlock’s stems lack hair and its fernlike leaves are finely divided and give off a strong, unpleasant odor when mowed.

Where is poison hemlock found?

Indigenous to Europe, Western Asia and North America, poison hemlock is most often found in commonly disturbed areas such as roadsides, edges of cultivated fields, creek beds, waste areas and irrigation ditches. Now naturalized in nearly all states in the United States, this herbaceous plant thrives at lower elevations and areas where moisture is abundant. As the plant’s sole source of reproduction, matured seeds separate from the plant and primarily disperse between July and late February.

How to treat poison hemlock

For small infestations of fully matured plants, hand-pulling can offer effective control. Due to the toxicity of the poison hemlock’s sap, gloves should be worn to minimize direct contact with the plant during removal. As for larger infestations, the application of herbicide can enhance control for seedlings and small rosettes.

At only 2.85 ounces per acre, testing of foliar applications of TerraVue herbicide has delivered 99% visual control of poison hemlock on roadsides and other natural areas. Optimum results have been achieved when the herbicide has been applied during the rosette stage of growth.

As seed remains viable in soil for extended periods of time, routine monitoring is necessary to ensure complete control on application sites for up to five years post-treatment. As TerraVue offers selective control of poison hemlock and many other broadleaf weeds, most desirable vegetation is able to thrive throughout initial treatment and subsequent applications. By restoring desirable vegetation in previously infested areas, the propensity for reinvasion is significantly reduced.

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