This Weed Kills

August 29, 2009 02:26 PM

When the roadside exploded in a show of white lacy flowers this past spring, it made more than a few weed watchers scratch their heads. It was too early in the season for Queen


Anne's lace. As the weeds began to stretch to 6' and 8' heights, it became apparent that this was no dainty wild carrot. Then came some reports of livestock deaths, and the die was cast.

The toxicity of poison hemlock has been well documented throughout history. In ancient Greece, it was used to poison condemned prisoners, the most famous of which was the philosopher Socrates. Now the killer has become an invasive troublemaker.

"We've always had a little poison hemlock around,” says J. D. Green, University of Kentucky weed specialist. "It's ordinarily seen along roadways, abandoned lots, fence rows and other noncropland sites.
"However, in more recent years, it's begun to expand beyond the roadsides and into pasturelands and hay fields. It's become much more prominent across the entire landscape in this state and that's a concern, since it is one of the most toxic plants in the world,” he notes. The plant produces the volatile alkaloids coniine (an alkaloid similar in effect to nicotine) and gamma-coniceine. All parts of the plant can be toxic. Young leaves in the spring are the most dangerous, while the fruit is most lethal in the fall.

Flower or foe? Purdue University weed specialist Bill Johnson says he often gets questions about wild carrot, only to find the problem is actually poison hemlock. "The easiest way to tell the two plants apart is that poison hemlock has purple spots or blotches on the lower part of the stems,” he says. The stems are also hollow and hairless, and the root has a long taproot.

Johnson adds that in Indiana, poison hemlock is beginning to invade no-till corn and soybean fields. Native to Europe, the weed was introduced into the U.S. as an ornamental plant in the 1800s and is now widely naturalized and common in most U.S. states and Canadian provinces. Although associated with moist soil conditions, it is known to survive in dry conditions.

It is classified as a biennial, which means its life can stretch over two years. The first year is spent in vegetative growth. A relative of parsley, the plant's leaves are fernlike in appearance. During the second year, it sends up flower stalks. When they mature, the flowers are small and white and form an umbrella-shaped cluster at the end of each terminal stalk. A fully mature plant is capable of producing 35,000 to 40,000 seeds.

Flowers and seed are typically produced in late May and June. In some southern states, such as Kentucky, poison hemlock is capable of completing its life cycle as a winter annual if it ger-minates during the fall, Green explains.

"Control is a matter of preventing seed production,” Green says. "Mechanical controls, such as mowing or cutting down individual plants, should be started just before peak flower production to avoid or reduce the amount of new seed being produced,” he adds.

Green notes that most animals tend to avoid grazing poison hemlock if other forage is available. "It's when the plant gets cut and mixed into hay that there is concern because the potential for toxicity still exists in dried hay.”

Literature from Purdue's veterinary medicine department pegs lethal doses of poison hemlock at 4 lb. to 5 lb. of leaves for horses; 1 lb. to 2 lb. for cattle; and 4 oz. to 8 oz. for sheep. Young animals are more susceptible. Symptoms can occur within one hour of ingestion. What starts with nervous stimulation progresses in a few hours to respiratory paralysis. Although it is less likely that humans would accidently ingest such a weed, precautions should be taken when handling poison hemlock.

"We're trying to spread the word to control this weed in the fall,” Johnson says. In fields returning to corn or soybeans, he says, glyphosate and dicamba have shown slightly better efficacy than 2,4-D. 

Poison hemlock is found in nearly every U.S. state and Canadian province and is officially considered a noxious weed in many states.

When using herbicides in pastures or along roadsides, Green says, farmers should be mindful of the surrounding vegetation. "Spot treatments are usually sufficient, and most broadleaf herbicides will provide control when applied to small actively growing plants,” he says. "It's when plants begin to push toward the flowering stage that control is more difficult.”

Johnson recommends waiting until after harvest to attempt any roadside spraying of poison hemlock because of potential drift issues, especially when using nonselective products.


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