COLUMBIA, Mo. – Poison hemlock, a plant that can kill livestock, is flowering and setting seed now in much of Missouri. While early control is recommended, it still can be managed with spot treatments or mowing, says Kevin Bradley, University of Missouri Extension weed scientist.

Poison hemlock is one of the first pasture weeds to emerge in the spring. It grows in ditches, roadways, fencerows, occasionally in pastures or hayfields that aren’t mowed regularly, and sometimes in no-till cropping systems.

If you missed earlier control measures, mowing is probably the best weapon against poison hemlock at this time of year, when the plants are naturally senescing and setting seed, says Bradley. “I’ve never seen this plant become established in pastures that were mowed on a regular basis,” he says.

If you can’t mow, and assuming you still see signs of active plant growth, spot-spraying individual plants can still work and may prevent some amount of seed production.

You can use glyphosate products or other effective growth regulator herbicides for spot treatments. “It is important to keep in mind that glyphosate is a nonselective herbicide that will also kill desirable forages,” says Bradley. Other growth regulator herbicides that can be applied as a spot treatment include herbicide products containing the active ingredients metsulfuron, picloram, dicamba, and/or aminopyralid.

Bradley notes that he would expect less-than-ideal control now compared to earlier in the season when plants should be treated in the rosette stage of growth. Early control is important because mature plants with flowers do not respond as well to chemical control.

Bradley urges wearing protective gear to avoid breathing in the pollen of the plant if you cut with a brush hog and open-station tractor.

Nonchemical control options include hand weeding, mowing and tillage.

Every part of this biennial contains gamma-coniceine, coniine and other toxic compounds that are harmful even in very small amounts, even after mowing, so avoiding livestock exposure to this plant is ideal, especially if there has been a lot growing in a pasture, says Timothy Evans, MU Extension state specialist in veterinary toxicology. These toxins affect cattle most, particularly with respect to birth defects, but they also affect sheep, goats, swine, horses and other domestic animals.

Evans says these toxins affect the nervous system like nicotine, initially causing tremors that may progress to total paralysis and suffocation. Symptoms usually appear within an hour of ingestion, and the animal can die in a few hours.

To identify poison hemlock, visit the MU Plant Sciences Weed ID guide.