– Stan Smith, PA, OSU Extension, Fairfield County
Poison hemlock plants in Seneca County, Ohio in January of 2017
As we’ve discussed a couple of times in the past, poison hemlock is a biennial member of the carrot family that can be fatal to livestock if ingested in sufficient quantities. That said, while much of the poison hemlock we’re seeing today has been alive but dormant much of the winter, those plants are now in the early stages of bolting across much of Ohio, and also positioned nicely to be controlled at this time.
While the taste of poison hemlock leaves to livestock is unpleasant and toxic quantities are seldom consumed, if grazing animals are turned out in early spring onto pastures with less than adequate forage, the risk of ingesting a lethal dose is greater. Cattle can usually survive poison hemlock if consumed in amounts less than 0.4% of their body weight (4 to 5 pounds for mature cows) although abortions are possible at lower rates. The toxicity of the plant changes little if fermented with silage or dried in hay later in the spring.
Early stages of bolt, in Fairfield County on April 2, 2017.
Crossbow, dicamba and 2,4-D are effective on small poison hemlock plants in rosette or the early stages of bolting. And, if planning to use a selective broad-leaf killing herbicide for control of this plant, there are a number of advantages to using it now. The volatility of products like dicamba presents less risk this time of year since few sensitive crops or landscape plants are up and growing. Herbicide pre-harvest intervals or any grazing restrictions are more easily managed now rather than later in the spring. And perhaps last but not least with Ohio weather presently allowing little if any field work to be done, the use of a backpack sprayer and ATV are ideal for scouting out and spot spraying poison hemlock right now while minimizing field damage.
Taller plants may need to be controlled with glyphosate, however the use of glyphosate can become a long term problem because it also eliminates desirable grass plants that provide competition for newly germinating hemlock and other weed species. This can be particularly problematic in early spring along road ditches and fence rows that border pasture fields by allowing early grazing livestock easy access to hemlock once desirable pasture forages have been consumed.
Mowing after the plants have bolted and before seed set is effective for preventing seed production. Hand pulling or cutting maturing plants is also an option. While all parts of the plant are poisonous including the leaves, stems, seeds and roots, simply handling the plant seldom causes a toxic reaction in humans, however, care should be taken when mowing or hand cutting the plants since ingesting it through the eyes, open wounds, or orally does cause poisoning.
You may recall that perhaps poison hemlockâs most famous claim to fame was when it was used to execute Socrates in 399 B.C.
Photo above taken May 8, 2015. Photo below taken on April 3, 2017. The moral of the story: If poison hemlock is allowed to go to seed each year, it will continue to be found in that same place each year, and is likely to spread!
