Learn to Recognize Poison Hemlock

Poison Hemlock is aggressively spreading in many regions of Pennsylvania. Poison hemlock is toxic and can be fatal to humans, pets, and all classes of livestock. All parts of the plant are known to be poisonous, even after the plant has died. Learn this invasive weed’s key identification features to avoid exposure and livestock poisoning. Consider making others who use your property aware of the dangers of poison hemlock and teach them how to identify it as well. A number of links are provided at the bottom of this post which can help with identification.

photo by Jeff Stachler

Poison hemlock is typically seen along roadsides, fallow areas, fence rows, pastures, and creeks. Native to Europe, this weed is a biennial, completing its life cycle in two years. In its first year, it will produce a rosette of leaves close to the ground. In the second year, it will bolt; this means that it will send up a stem, producing more leaves, flowers, and many seeds.

Poison hemlock is closely related to wild carrot (also called Queen Anne’s lace). Poison hemlock has white flowers and lacy leaves similar to wild carrot. However, it is a larger plant, growing 4 to 6 feet tall when mature. The stems of poison hemlock have purple spots and are hollow and hairless. The whole plant has a musty smell, and the leaves produce a parsley-like odor when crushed.

As its name suggests, it is a poisonous plant. Touching this plant has caused skin irritation for some

photo by Pedro Tenorio-Lezama

people. If ingested, it is toxic to both humans and livestock. It can take as little as 0.25 percent and 0.5 percent of a horse and cow’s weight, respectively, to cause poisoning and severe damage to the nervous system. If too much is ingested, it can cause death. Therefore, it is important to eradicate this weed in areas where livestock could come into contact with it. Mature seeds are the most poisonous. Ingesting significant amounts can result in muscle paralysis and suffocation.

When the plant is in late flower mowing should set it back and prevent seed production, and possibly control it. According to Timothy Abbey, Extension Educator, there are no pre-emergent herbicides to use against poison hemlock in ornamental settings. Post-emergent herbicides include: diquat, pelargonic acid, glyphosate (all non-selective), and 2,4-D (selective to broadleafs). The most effective approach is to treat the 1st year rosettes and not the larger, mature plant. When using an herbicide to control and eradicate poison hemlock use an approved herbicide and always follow the label and safety instructions.

photo by John Cardina

To remove the weeds by pulling be sure to wear rubber gloves and protective clothing. Hand-pulling of poison hemlock works best with young plants or small infestations in moist soils. Mature plants should be dug up and removed. Once plants (and roots) are extracted, place them in a plastic garbage bag and dispose of in a trash receptacle. Wash all clothing and tools afterwards. Do not attempt to compost poison hemlock, the poisons are persistent. Using weed trimmers needs to be conducted with precautions so that plant material doesn’t come into contact with the body.

Identification and eradication of this plant wherever livestock and people could come in contact is important. Be sure to wear gloves and protective clothing. Contact with the skin has been known to cause irritation for some people.

Additional Poison Hemlock information:

Weed of the Month: Poison Hemlock

PA DCNR Poison Hemlock fact sheet

USDA Poison Hemlock fact sheet

Invasive Plant Atlas – Poison Hemlock