One of North America’s most toxic plants, poison hemlock, is currently blooming in areas across the United States where the plant has taken up residency. The weed is part of the parsley family and is most easily recognizable by its finely divided leaves that look very similar to wild carrot, aka Queen Anne’s lace. Unlike wild carrot, however, which is mainly just a nuisance, poison hemlock can literally kill both humans and animals.
Poison hemlock is native to Europe, northern Africa, and western Asia, it is now naturalized in almost every state in the United States. Infestations occur along roadsides, field margins, ditches, marshes, meadows, and low-lying areas, but poison hemlock prefers shaded areas with moist soil. It’s commonly referred to as simply hemlock, but is also known as poison parsley, carrot fern, spotted hemlock, and devil’s bread. Historically poison hemlock was used in ancient Greece to poison condemned prisoners, the most famous victim being Socrates. The famous philosopher chose execution by hemlock tea after being sentenced to death for corrupting the youth of Athens.
Despite poison hemlock’s known toxicity, it was introduced to North America as an ornamental garden plant. The plant has white flowers that grow in small erect clusters. Each flower develops into a green, deeply ridged fruit that contains several seed. After maturity, the fruit turns grayish brown. Poison-hemlock starts growing in the early spring. It usually grows for 2 years, but in favorable locations it may be a perennial. It can also easily reach heights of up to 6 feet or more. The hollow stem usually is marked with small purple spots.
The plant contains a number of closely related pyridine alkaloids with the main one being coniine, a colorless, volatile, and strongly alkaline oil. Mature seeds are the most poisonous but all parts of the plant are toxic. Leaves are especially poisonous in the spring, up to the time the plant flowers. Fresh leaves are unpalatable, so livestock seldom eat hemlock when other feed is available. Still, identification and eradication of this plant wherever livestock could come in contact is important. Also, hayfields and meadows which will be harvested should be closely walked, eradicating hemlock prior to any harvest. The hemlock’s poison affects the nervous system and even a small amount can lead to respiratory collapse and death.
Some studies have shown toxicosis with consumption of 0.25% fresh weight (of the animal’s weight) for horses and 0.5% for cattle. This would be 2.5 to 5 lb. of fresh material per 1000 lb. animal. Signs usually appear within an hour after an animal eats the plant. Animals die from respiratory paralysis in 2 to 3 hours. Convulsions, which are common in western water hemlock poisoning, seldom occur with poison-hemlock. Skeletal deformities or cleft palate may be induced in offspring of cows, sheep, goats, and pigs that eat poison-hemlock during gestation.
More common in humans is a severe skin rash that occurs from contact with the plant. Poison hemlock’s sap is loaded with compounds that produce a light-linked condition called phytophotodermatitis in humans and livestock. If plant juices contact skin and the skin is then exposed to sunlight (specifically ultraviolet light), severe blistering can occur, as well as skin discoloration that may last several months.
While poison hemlock can be partially managed by mowing and tilling, the most effective control approach involves properly timed applications of selective or non-selective post-emergent herbicides including glyphosate. It is a prolific seed producer, so applications of herbicides made now will control both the first season rosette stage and the second season flowering stage, before seeds are produced. Experts advise standing downwind from poison hemlock if you choose to burn it as its toxins could be transferred to the smoke, which in turn can get into your lungs and eyes. You can learn more about poison hemlock at the USDA’s website HERE. Joe Boggs of OSU Extension posted some great pics to help identify the poison plant HERE.(Sources: USDA, Invasive Plant Atlas, Buckeye Yard & Garden onLine)