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Poison Hemlock Information for Landowners

Posted: August 22, 2016

Poison Hemlock is aggressively spreading in many regions of Pennsylvania.
Identification and eradication of poison hemlock wherever livestock and people could come in contact is important. Photo by George Hurd.

Identification and eradication of poison hemlock wherever livestock and people could come in contact is important. Photo by George Hurd.

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. You should review this invasive weed’s key identification features to avoid human exposure and livestock poisoning. You should also consider making others who use your property aware of the increase of poison hemlock in the area.

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.

Just as its name suggests, it is a poisonous plant. Touching this plant has caused skin irritation for some people. But it is also toxic if ingested by livestock and humans. 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. Significant poisoning can result in muscle paralysis and suffocation.

Using an herbicide to control poison hemlock is best done in its first year as a rosette, rather than when it is flowering and close to the end of its lifecycle. When the plant is in late flower, mowing should set it back, 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-emergents include: diquat, pelargonic acid, glyphosate (all are non-selective), and 2,4-D. 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 on that label.

To remove the weeds by pulling, wear rubber gloves and protective clothing and follow up with an herbicide to prevent future growth. Hand-pulling of poison hemlock works best with young plants or small infestations in wet soils. Mature plants should be dug up and removed. Once the plant and its roots are extracted from the ground, place the plant into a plastic garbage bag and dispose of it into a trash receptacle. Wash all clothing and tools afterwards. Do not attempt to compost poison hemlock as the poisons are persistent. Even the use of weed trimmers needs to be conducted using 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. Also, care should be taken when eradicating the plant to wear gloves and protective clothing. Contact with the skin has been known to cause irritation for some people.

Additional Poison Hemlock information: