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

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

Introduced to the United States as an ornamental in the 1800s, poison hemlock is widespread throughout most of the state and much of North America. In the past, it was typically found along roadways, abandoned lots, fencerows and other non-cropland sites. But in recent years, its population has exploded across Kentucky, and it is now 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 other animals can consume it. If ingested, poisoning symptoms appear within 30 minutes to two hours, depending on several factors including the animal species and quantity consumed. Lethal doses for cattle range between 0.2 and 0.5 percent of the animal’s weight. Poisoning symptoms 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 the early spring when plants are smaller and in the rosette growth stage, particularly when applying herbicides that contain 2,4-D. In the rosette growth stage, plants can be more difficult to find since poison hemlock is growing close to the ground, but producers can easily recognize it in fields due to its parsley-like leaves that are shiny green and triangular. Producers should look in field areas where the plant was present last year; larger plants may be up to 12 to 18 inches tall. When full grown, this invasive, noxious weed can reach 6 to 8 feet tall. 

Poison hemlock is often confused with Queen Anne's lace, which also is called wild carrot and is a non-toxic weed. Both plants produce leaves and clusters of small, white flowers that look similar. 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 producers find poison hemlock later in the season, they should mow it over before it flowers to prevent further seed production. If it is found while making hay, Green recommends mowing around the plant to keep it out of the animals’ food supply.