Poison Hemlock (Conium maculatum, Fig. 1), is a member of the plant family Apiaceae, which contains a few important crops such as carrots, celery, and parsnips. This weed is a tall, invasive, and highly poisonous weed that is sometimes mistaken for one of its crop relatives. It is also commonly mistaken for Queen Anne's Lace (Daucus carota).
Poison hemlock is an erect biennial (flowering plant that takes two years to complete its life cycle) weed that can grow six to ten feet tall. During the first year, growth is limited to a rosette of dark green leaves. More growth is observed in the second year, where the plant develops branching and alternately arranged leaves on erect stems. This erect plant has smooth, hollow stems that are covered with purple spots (Fig 2). The leaves are pinnately compound (each leaf is made up of several pairs of leaflets), multi-stemmed, and fern-like (Fig 3).
During the reproductive stages, the flowers are white umbrella-like clusters that during June and July, turning to fruit that is small, flat, and grayish-green in color from August to September. The root system is a fleshy white, with a long and sometimes branched taproot (Fig 3).
Poison hemlock is an herbaceous (soft stem) plant that reproduces solely by seeds. The seeds separate from the plant when it becomes mature, and despite the plant’s prolific seed production, does not have a mechanism for long-distance seed dispersal. The seeds drop close to the parent plant and may be spread over distance by water, birds, and rodents and remain viable for only two to three years.
Poison hemlock is native to northern Europe, western Asia, and North Africa and it was introduced to North America as an ornamental back in the 1800s. Over time, this weed has spread throughout America, Mexico, and Canada.
Poison hemlock is toxic to humans and to livestock if ingested by affecting the respiratory, central nervous, and reproductive systems. Some humans and animals can experience dermatitis on the skin just by coming into direct contact with this weed 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.
Poison hemlock grows at low elevations bordering pastures and cropland. This weed grows where moisture is adequate and disturbance of its surroundings is relatively frequent, such as streams, ditches, riparian woodlands, and flood plains. Seeds that were carried by flood waters can explain patches of poison hemlock that are found in areas outside the normal water levels.
As with any weed that is spread by seed, your best level of defense is to prevent the weed from producing seeds in the first place (Fig 4).
If you have many plants in an area, you may want to make sure that you contain them by pulling the newly established plants by hand (wear gloves), or by hoeing them out. You could also spray an herbicide around the border of that area to prevent the immediate spread of this weed. When you decide to control the larger plants of this area, plan to control them from the ground up. Keep in mind that sap of this plant is toxic. You do not want your skin to come into contact with the sap, and you do not want to burn these plants in case of accidental inhalation by someone or by a nearby animal.
When pulling the plants by hand, the entire taproot needs to be removed from the soil to prevent regrowth. Plowing or repeated cultivation of newly germinated plants can prevent the weed from becoming re-established in the area. If plowing or cultivation is not an option, repeated mowing once the plants have bolted but before they bloom will reduce seed production and can be key to controlling, and even eradicating, a weed infestation. The act of mowing depletes the energy reserves in the taproot of the poison hemlock, thus weakening its competitive edge, as well as prevents flower and seed formation.
There are several herbicides (e.g. glyphosate, or 2,4-D) on the market to control poison hemlock, so take care which product you select and make sure that you follow the directions on the label for dosage, application, and what personal protection you should wear during application. Most of these herbicides are going to require you to take action on this weed early in the season when this plant is in a seeding or small rosette stage. This may require you to carefully spot spray the rosette at a given height with the appropriate dosage of the herbicide that you are applying for control. If it is too late for your chemical control measures for this year, start planning your control measures for next year/next season today. Keep in mind that the problem is not going to go away with one application of herbicide; this weed is persistent. You may need to combine mechanical and chemical control for best results, and it may take more than one or two years to control this weed.
This article appears on July 2021, Volume 12, Issue 4 of the Agronomy news.
Agronomy News is a statewide newsletter for farmers, consultants, researchers, and educators interested in grain and row crop forage production systems. This newsletter is published once a month during the growing season and will include topics pertinent to agronomic crop production. Subscribers will receive an email with the latest edition.