CHAMBERSBURG, Pa. — As a county Extension educator, I regularly receive calls and emails about a variety of horticultural topics and this week has been no exception. From yellowing tomato leaves to mildew on greenhouse lettuce, the rainy, cloudy weather is perfect for plant problems. In this week’s column, I’m covering a problem I was reminded about from a county landowner who called me about control options for a highly poisonous weed that is becoming all too common along our county roadsides, ditches, and field edges.
Poison hemlock, Conium maculatum, is part of the Umbellifer family of plants which also includes Queen Anne’s lace and wild parsnip. The plant is native to parts of Europe and was originally brought to the US for use as an ornamental. Poison hemlock has a biennial life cycle which means that in the 1st year, the plant germinates from seed and forms a basal rosette which is the botanical way to say, leaves growing close to the ground radiating from the base of the stem. The basal rosette overwinters and the 2nd year, the plant will shoot out branched stems with white flowers that produce seed. The seed drops to the ground and the process is repeated year after year. This is a good lifecycle for a plant that you want growing on your property. But poison hemlock is not that plant! Poison hemlock can be differentiated from wild parsnip and Queen Anne’s lace by looking at the stems on the 2nd year plant. The stems on poison hemlock are hollow and hairless with purple blotches or spots. Queen Anne’s Lace has a hairy stem with no purple blotches. Wild parsnip has green, hollow, hairless stems, and yellow flowers whereas the poison hemlock has white flowers. Wild parsnip is also toxic and a severe skin irritant. Another common weed often confused for poison hemlock is the native water hemlock which has a purple stem. If you are not sure which species of weed you have, please contact your county Extension office for proper identification.
For people and livestock, every part of the part is extremely poisonous when ingested. Livestock do not have to consume much of it to be poisoned. For example, 100 grams of fresh poison hemlock is enough to cause poisoning in sheep. For reference, 100 grams is about the weight of a medium tomato. Cattle can be poisoned by eating as little as 300 grams of green poison hemlock. Dried poison hemlock is also toxic and must not be allowed to grow and mature in crop or hay fields. The sap of the plant causes skin irritation, and the USDA (https://www.ars.usda.gov/pacific-west-area/logan-ut/poisonous-plant-research/docs/poison-hemlock-conium-maculatum/) reports that whistles made from hollow stems of poison hemlock have caused death in children. When weed-eating or mowing poison hemlock, extreme care must be taken not to come in contact with the fresh cut plant material. Mature seeds are the most poisonous part of this plant, which has special meaning for those who pasture livestock.
Thankfully, controlling poison hemlock is possible and within the reach of landowners. Several herbicides are recommended for controlling the weed. Herbicides work best on the basal rosette stage or before the plant sends out its flower stalk the 2nd year. However, this is not always possible if you encounter a mature stand of poison hemlock ready to set seed. At that point, you must remain diligent in treating and understand that eradication may take 1-2 years. Hand-pulling the weed is permissible but extreme care must be taken to protect your skin by wearing gloves, long sleeves, and pants. The plant has a long taproot which makes pulling it out more difficult especially if the ground is dry. Digging out the plant is also an option if you have a small stand of the weed. Effective herbicides include active ingredients glyphosate; 2,4-D+dicamba; and 2,4-D+triclopyr.
As I write, the flower stalks are not yet bolting from 2nd year plants in Juniata County, so now is a very good time to look around your property especially at the edges of roads, ditches, and wet areas. If you would like to see photos of the weed or talk more about it, please call your county Extension office, or email me at firstname.lastname@example.org.
–Leah Fronk, Penn State Extension