White flowers in Missouri pastures, fencerows and along roadsides in June may look harmless, but some can be dangerous, said Todd Lorenz, University of Missouri Extension field specialist in agronomy.
Poison hemlock is a prolific, highly toxic biennial weed that poses threats to people and livestock. Each plant can produce up to 38,000 seeds, increasing its spread, especially in no-till crop fields.
Every part of the plant is poisonous, whether green or dried, Lorenz said.
Where and when it grows
Poison hemlock came to the U.S. as a garden plant. It now grows along roadsides, field edges, creek beds and ditches.
It is one of the earliest weeds to green up in spring and can grow as tall as 6 feet, with flowers measuring 7 to 15 inches across.
In its first year, poison hemlock grows as a low-lying basal rosette. In the second year, it produces erect stems topped with large, umbrella-shaped clusters of white flowers.
Early control needed
“The key to managing poison hemlock is early identification and control,” Lorenz said. “Once plants mature, they become much harder to control.”
Nonchemical control options include hand removal, mowing and tillage. Lorenz urges extreme care to avoid poisoning through touching the plant or breathing residue while removing it.
In crop and pasture settings, growth-regulator herbicides and products containing 2,4-D, dicamba, metsulfuron or triclopyr can be effective when applied at the rosette stage, said MU Extension weed scientist Kevin Bradley.
Plants that are flowering and more noticeable at this time of year are well past that stage, Bradley said. Mature plants do not respond well to broadcast sprays, and plants that are blooming or past the bloom stage require spot treatments at much higher concentrations.
Bradley noted that research shows tank mixes of dicamba plus glyphosate may provide more consistent control than glyphosate combined with 2,4-D.
Even small amounts harm livestock
Poison hemlock is especially dangerous to livestock; even small amounts can cause death. Its alkaloids can affect the nervous system and lead to symptoms such as trembling, dilated pupils, abnormal heart rate and respiratory failure.
Poison hemlock’s young leaves are not palatable, and livestock will avoid it if another feed source is available. Dried leaves can still end up in hay and be deadly.
If poisoning is suspected, immediately remove animals from infested pastures and contact a veterinarian. Pregnant livestock exposed to poison hemlock during the first trimester may give birth to offspring with limb deformities, a condition often referred to as crooked calf syndrome.
Take care when handling
When handling poison hemlock, wear protective clothing, including gloves, long sleeves, long pants and eye protection. Contact with plant sap can cause skin irritation, blisters and welts.
Poison hemlock is often mistaken for Queen Anne’s lace, also known as wild carrot. Its roots can be confused with wild carrots or parsnips. Both plants have hollow stems and white, lacy flowers, but poison hemlock has smooth, hairless stems with distinctive purple blotches. Queen Anne’s lace typically grows no taller than 2 feet and is not toxic.
Poison hemlock can grow 6 feet tall or more when mature. Its stems are smooth, hollow and hairless, with distinctive purple blotches or streaks. It has finely divided, fern-like leaves with a strong, unpleasant odor when crushed. Its small white flowers are arranged in umbrella-shaped clusters. The plant forms a low basal rosette the first year, then bolts and flowers in the second year.
If a plant’s identity is uncertain, avoid handling it, Bradley said.
MU Integrated Pest Management offers several tools to identify weeds. More information is available at weedid.missouri.edu, through the MU Extension guide “Plants Poisonous to Livestock” or in the MU Extension article “Poison Hemlock.”
