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Home > Plants, Pests & Pest Control > Pest Management > Noxious & Invasive Weed Program > Weed of the Month > August 2016 - Poison Hemlock

August Weed of the Month: Poison Hemlock

Poison hemlock stemAugust’s Weed of the Month is poison hemlock (Conium maculatum), a toxic member of the carrot family. It is native to Eurasia and has naturalized throughout North America. In Minnesota, it has been recorded in isolated pockets, most recently in southeastern Minnesota. Unfortunately, it appears to be spreading quickly in the St. Charles and Lanesboro areas.

Poison hemlock is a biennial plant. It forms a basal rosette in its first year, then produces a flowering stalk in its second year. The leaves are finely dissected and fern-like. It forms a long, white taproot that can be mistaken for wild carrot or wild parsnip. The flowers are small and white with umbrella shaped clusters on the tops of stems that can range in height from two to eight feet. The stems are hairless and have purple blotches. The plants emit a disagreeable odor. The flowers of poison hemlock look similar to wild carrot (Daucus carota) and water hemlock (Circuta maculata). The fern-like leaves and purple blotches on the stems of poison hemlock distinguish it from these related species. Wild carrot is invasive but much less toxic. Water hemlock is native, very toxic and can be found growing in wet areas such as meadows.

All parts of poison hemlock (leaves, stem, flowers, fruit, roots) are poisonous. If a person may have ingested poison hemlock or any other poisonous plant, Minnesota Poison Control should be called immediately at 1-800-222-1222. If the individual is unresponsive or having trouble breathing, call 911. For more poison prevention and safety information, visit Minnesota Poison Control System. If a livestock animal may have ingested poison hemlock, contact your local veterinarian.

Poison hemlock prefers rich, moist soils and frequently grows in low or poorly drained areas. It has been found in pastures, roadsides, ditches, streambanks, and marshy areas. Management strategies are aimed at preventing seed formation and spread, and include the following:

  • Clean equipment, boots, clothes, after being in an infested area.
  • Regular site monitoring for several years will be required to ensure that new seedlings are destroyed and the seedbank is depleted.
  • Herbicide treatments to rosettes before they begin to produce a flowering stalk have been effective. If using herbicide treatments, check with your local University of Minnesota Extension agent, co-op, or certified landscape care expert for assistance and recommendations.