Published April, 2011

Controlling poison hemlock growth in early spring could help keep pastures and livestock healthy, according to J.D. Green, PhD, extension weeds specialist with the University of Kentucky College of Agriculture.

"Poison hemlock is potentially poisonous to livestock, particularly when animals graze poison hemlock plants when other forages are limited, or if large quantities of hay containing poison hemlock are consumed by animals," Green says. "In addition, poison hemlock can crowd out desirable plants in areas where it becomes established."

Introduced into the United States as an ornamental in the 1800s, poison hemlock is widespread throughout much of North America. In the past it was typically found along roadways, abandoned lots, fencerows, and other noncropland sites. But in recent years its population has exploded, and it now grows in many pastures and hayfields.

Poison hemlock can be toxic if ingested by livestock or humans. Cattle, goats, and horses are considered to be the most susceptible animals but others can also consume it. If ingested, clinical signs of poisoning appear within 30 minutes to two hours, depending on factors such as animal species and quantity consumed. These signs include nervousness, trembling, muscle weakness, loss of coordination, pupil dilation, coma, and eventually death from respiratory failure. If ingested by a pregnant animal it can cause fetal deformities.

The best time of the year to effectively control poison hemlock using herbicides is in early spring when plants are smaller and in the rosette growth stage. However, poison hemlock can be more difficult to locate while in its rosette growth stage, since it grows close to the ground during this period. It can be recognized in fields due to its parsleylike leaves, which are shiny green and triangular. When trying to locate poison hemlock, search field areas where the plant was present the previous year; larger plants might be 12-18 inches tall at this point. When full grown, this invasive, noxious weed can reach six to eight feet tall.

Poison hemlock is often confused with the nontoxic weed Queen Anne's lace (also called wild carrot). Both plants produce similar leaves and clusters of small white flowers. However, poison hemlock has smooth stems with purple spots throughout while Queen Anne's lace has hair along its stem and leaf bases. During poison hemlock's peak bloom period in late May and early June Queen Anne's lace is just beginning active growth for the season. If farm managers find poison hemlock growing later in the season, they should mow before the plant flowers to prevent further seed production. If it is discovered while making hay, Green recommends mowing around the plant to keep it out of the animals' food supply.

For additional information, see March's "Weed of the Month" feature on poison hemlock.

Katie Pratt is an agriculture communications specialist.