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Controlling Poison Hemlock — Spring 2010 | Out Here Magazine

Invasive plant poses growing hazard to grazing livestock

By Amber Stephens

Photograph courtesy of
Benton County (AR) Cooperative Extension Service

Each spring, rosettes of ferny foliage provide some of the first glimpses of greenery to emerge after winter's slumber. While the inviting leaves of poison hemlock may be a welcome sight, the plant is an unwelcome invasive species that is deadly to livestock.

"It has typically been a border plant," says Glenn Nice, Purdue University Extension Weed Science Professional. "It's almost everywhere now."

Poison hemlock once was found primarily along creek banks and waste areas. In the past few decades, however, this cousin to Queen Anne's lace, or wild carrot, has spread to roadsides, back yards, and, most dangerous to livestock, in pastures and hayfields.

Cattle, horses, and other grazers normally would bypass poison hemlock because of its odor and taste. But hemlock spreading into hayfields can inadvertently be cut and mixed into hay bales and fed to livestock.

The biennial's early spring tender leaves in the rosette form are the most toxic, but the entire plant — stems, ferny foliage, and seed-head clusters — contains alkaloids that affect the nervous system and respiratory muscles, eventually leading to death.

The plant can remain poisonous even when it's cured and harvested with hay.

As little as 1 or 2 pounds of poison hemlock can kill cattle, Nice says. For sheep, 4 to 8 ounces of the plant can be fatal.

Poison hemlock has been aggressively spreading for about two decades now, says Robert Seay, University of Arkansas county extension agent staff chair. "It's gone from a non-noticeable plant to a dominant weed," he says.

Poison hemlock, as other invasive species, grows in large clusters, some as large as 5 acres, which choke out native plants. Like Queen Anne's lace, the plants are tall — up to 8 feet — with ferny foliage and small seed-head clusters.

It can be differentiated from its non-toxic cousin, however, by purple streaks and blotches on the stems that are sometimes known as "the blood of Socrates," which refers to the ancient philosopher's death by hemlock poisoning.

Seay recommends spraying "user-friendly" herbicides to control poison hemlock while the plant is still in the first-year rosette stage for greatest effectiveness.

Dicamba and glyphosate are other common options for controlling the invasive weed in corn and soy fields. Herbicide treatment can begin as soon as daytime temperatures are at least 60 degrees for three consecutive days or more.

Homeowners, though, can prevent the plant's spread by digging or pulling out any unwanted patches. Although poison hemlock is unlikely to be toxic from skin contact alone, wear gloves, just to be safe, Nice recommends.

Whether pulling plants or treating them with herbicides, the experts suggest removing poison hemlock long before the low-lying ferny fronds of spring become the 7-foot seed stalks of summer.

Amber Stephens is a freelance writer and editor in Amanda, OH.