Poison hemlock poses growing concern in Ohio

Poison hemlock (not to be confused with the eastern hemlock tree, a native evergreen) is in the same family as carrots, celery and parsnips. This weed can be mistaken for Queen Anne’s lace, which also is a member of the carrot family. However, poisonous hemlock is easily distinguished by its fern-like leaves and unpleasant odor when crushed.

The stems are hairless and hollow with purplish blotches, but Queen Anne’s lace has stems that are hairy, green and lack purple blotches. Also, the flower heads of Queen Anne’s lace have small, leaf-like bracts directly beneath the flower head, and the stems are green and hairy.

Unfortunately, highly poisonous hemlock has quietly taken over our roadsides, fields, stream banks, flood plains, pastures and wood edges. It spreads rapidly almost anywhere where the ground is frequently disturbed.

According to the Ohio Department of Natural Resources website, it is likely found in all 88 counties in Ohio and is most common in the central and western parts of the state. It was first reported in Ohio in 1860 as an escape from plantings.

Hemlock has spread throughout the United States, Canada and Mexico. It is native to Europe, Northern Africa and Western Asia and was introduced as a decorative garden plant. If left unchecked, a single plant will produce as many as 30,000 seeds, which can be viable for two to three years or more. The seeds drop near the plant and also are dispersed by wildlife and water.

According to Google, poison hemlock quickly outcompetes native plants by monopolizing nutrients, water and sunlight. If left unchecked, it forms dense, monotypic (only one species) stands, crowding out native wildflowers, shrubs and grasses birds and wildlife need for survival.

Doris Behnke at the University of Maryland Extension writes that poison hemlock is highly toxic to humans and livestock if ingested by affecting the respiratory, central nervous and reproductive systems. Some humans and animals can experience blistering on the skin just by merely touching it or brushing accidentally against it due to the potency of the eight alkaloids that contribute to its toxicity. All parts of this plant are poisonous and can be fatal to livestock and humans if ingested.

The University of Maryland Extension website has good instructions on both mechanical and chemical control. The ideal time to remove hemlock is in early spring before they bolt and the ground is moist. The flowering period is late April to early July, depending on your location and weather conditions. The seeds begin to form in June through September.

In the first-year rosette stage, the plants can be contained by hand-pulling the roots (always wear gloves) or hoeing them out. Keep in mind the sap is highly toxic, and even touching the roots can cause contact dermatitis. If you have a large infestation, you also can spray a broad-spectrum herbicide around the border of an area to prevent the immediate spread. Do not compost or burn the plants in case of accidental inhalation by people or animals.

If controlling hemlock by mowing, it must be done in early spring before the plant has seeded; otherwise, you will disperse the seeds even more. Repeated mowing will help weaken the plant and help prevent seed production. Do not mow after the plant has gone to seed, as it can be a risk to your health.

When working with hemlock, be very careful not to come in contact with the sap from the plant, which can result in painful blistering and cause serious complications such as allowing infections to enter the skin. Cover all exposed skin and use protective eyewear. In addition, prep for the possibility of ticks and spray your shoes and clothes with the insecticide permethrin. After working outside, be sure to wash your clothes, then transfer to a dryer with the setting on high, and shower to remove any loose ticks and do a thorough body check for ticks.

Unfortunately, this is not an easy weed to get rid of. It will take a lot of time and plenty of patience, and it will not instantly disappear with just one application of an herbicide. Hemlock is persistent, and you may need to combine mechanical and chemical control for best results, and it may take more than one or two years to control the weed.

Keep in mind if you see one plant, there are usually more lurking nearby. But by taking care of the problem now before it gets worse, it will help prevent the spread. If you have questions about hemlock control, contact the staff at your county extension office or your Soil and Water Conservation District office. Or go to the University of Maryland Extension website for more information at extension.umd.edu/resource/poison-hemlock-identification-and-management/.