Controlling Poisonous Weeds

Some weeds are worse than others, especially poisonous weeds that are dangerous to humans, livestock, and pets! While attending several summer parties in Northwest Ohio (graduation, July 4 th, picnics), several poisonous noxious weeds were observed this year. Poison hemlock (Conium maculatum L.) and wild parsnip (Pastinaca sativa L.) are invasive non-native weeds often found growing together in Ohio. Both plants are in the carrot family and are similar to Queen Anne’s Lace or wild carrot (Daucus carota), the difference being that both these plants are poisonous and bloom earlier in the growing season. Both weeds are prolific seed producers with seeds remaining viable for 4 – 6 years for poison hemlock and around 4 years for wild parsnip, making these poisonous weeds hard to control.

Poison hemlock can grow up to 6-10 feet tall and looks very similar to Queen Anne’s Lace or wild carrot except that poison hemlock is hairless, with no hairs on the stems or leaves. Poison hemlock has a biennial (two year) life cycle producing white flowers with over 30,000 seeds that mature around mid-July. The first year is spent in the “vegetative stage” as a low-growing basal rosette. Plants “bolt” during the second year “reproductive stage” to produce erect multibranched stems topped with umbrella-like white flowers.

Mature wild parsnip plants grow up to 4 – 5 ft. tall and has hollow, grooved stems that are hairless. The plant's leaves resemble large celery leaves. They are yellow-green, coarsely toothed and compound, with 3-5 leaflets. Wild parsnip has a biennial life cycle but it may occasionally spend more than a year in the vegetative stage before flowering once and then dying. Wild parsnip produces a yellow umbellate flower with hundreds of flowers clusters, producing about 1000 viable seeds per plant.

Poison hemlock plants contain highly toxic compounds which may cause respiratory failure and death in mammals. All parts of the plant are poisonous: leaves, stems, seeds, and roots. However, the toxins must be ingested or enter through the eyes or nasal passages to induce poisoning. The toxins do not cause skin rashes or blistering but this plant should not be handled because sap on the skin can be rubbed into the eyes or accidentally ingested while handling food. Wild parsnip sap, on the other hand, contains toxic organic compounds that causes severe blistering when exposed to sunlight. Sometimes the reaction can be delayed 48-72 hours depending on sunlight exposure. Both these plants and the symptoms are often confused, since they often seen growing together.

Unfortunately, poison hemlock and wild parsnip are becoming more common throughout Ohio, especially along highways, railroads, land in the conservation reserve program (CRP), and ditches. Poison hemlock can be mechanically removed if no wild parsnip is growing with it but wear personal protection equipment (eye protection, gloves, and clothing to cover arms and legs) to prevent sap from entering through the eyes or skin wounds. Mowers will not kill young growing plants and its best to wait to mow until the plants start to “bolt” just before flowering.

Mechanical weed pulling, tilling, or mowing of wild parsnip is not recommended because the sap can splatter causing blisters to arms, legs, and face. The safest approach to controlling both poison hemlock and wild parsnip is to use herbicides. Both poison hemlock and wild parsnip are susceptible to several selective and non-selective post-emergent herbicides. However, nonselective herbicides such as glyphosate (e.g. Roundup) may kill off other plants that compete with these weeds. Non-selective herbicides open up the canopy and provide an opportunity for more wild parsnip and poison hemlock to grow from seed. Thus, it’s important to have a plan for establishing competitive plants such as over-seeding with grasses.

Selective post-emergent herbicides will preserve competitive plants. Herbicides effective against wild parsnip and poison hemlock include clopyralid (e.g. Transline), triclopyr (e.g. Pathfinder II), metsulfuron (e.g. Escort XP), and combination products such as 2,4-D + triclopyr (e.g. Crossbow), or 2,4-D + mecoprop + dichlorprop (e.g. Triamine). Applications before plants start to flower can significantly reduce infestations of both wild parsnip and poison hemlock. Unfortunately, it may be too late to spray this year but if you see you have a problem, get prepared to fight them next year because they are prolific! The source information for this article came from Joe Boggs, OSU Horticulturalist. See photo below: The following photo can be used for educational purposes with credit to Joe Boggs, OSU Extension.