Control Poison Hemlock Now with Mowing or Spot Sprays

By Blake Jackson

Poison hemlock, a toxic plant dangerous to livestock, is currently in bloom and setting seed across much of Missouri. While early spring is the best time for control, there are still management options available, according to Kevin Bradley, a weed scientist with University of Missouri Extension.

One of the first weeds to emerge in the spring, poison hemlock typically grows along ditches, roadsides, fencerows, and occasionally in under-maintained pastures or hayfields. It may also appear in no-till crop fields. If spring control methods were missed, Bradley advises mowing as the best current option.

“I’ve never seen this plant become established in pastures that were mowed on a regular basis,” says Bradley. Mowing now, as the plant is naturally dying back and producing seed, can help reduce its spread.

For those unable to mow, spot-spraying may still be effective assuming the plant shows signs of continued growth. Herbicides such as glyphosate can be used, but caution is necessary.

“It is important to keep in mind that glyphosate is a nonselective herbicide that will also kill desirable forages,” Bradley warns. Alternative spot treatments include products with metsulfuron, picloram, dicamba, or aminopyralid.

However, Bradley notes that chemical control at this stage is not as successful as treating the plant earlier, during its rosette growth phase. “Mature plants with flowers do not respond as well to chemical control,” he adds.

If mowing, Bradley recommends wearing protective gear to avoid inhaling plant pollen, particularly when using an open-station tractor and brush hog.

Nonchemical strategies such as hand pulling, mowing, and tillage also offer potential control, though caution is necessary due to the plant’s toxicity.

Every part of poison hemlock contains dangerous compounds like gamma-coniceine and coniine, which remain harmful even after cutting. Timothy Evans, MU Extension specialist in veterinary toxicology, emphasizes keeping livestock away from infested areas.

“These toxins affect the nervous system like nicotine, initially causing tremors that may progress to total paralysis and suffocation,” Evans explains. Symptoms appear within an hour of ingestion, often leading to death in just a few hours.

Photo Credit:university-of-missouri