Experts warn against ‘highly toxic’ plant as it becomes more widespread in Eastern Iowa

The toxic plant that killed Socrates thousands of years ago is becoming more prevalent in Iowa.

Poison hemlock is an invasive biennial plant that has tall, smooth stems with fern-like leaves and clustered small white flowers that can grow anywhere between 2 and 10 feet tall.

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Meaghan Anderson, an Iowa State University Extension and Outreach field agronomist in central Iowa, said the plant isn’t necessarily spreading faster in the state, but it is becoming more widespread due to several environmental factors.

Those factors include mowing, birds’ unintentional movement of seeds from one place to another, and hikers inadvertently transporting seeds on their shoes or clothing.

She said tree loss in parts of Eastern Iowa from the 2020 derecho also could be a factor. Cedar Rapids estimates it lost roughly 720,000 trees, or 65 percent of the overall tree canopy that existed before the derecho flattened trees with hurricane-force winds.

“The loss of so many trees and opening of canopies has likely allowed for many weedy species to gain a foothold in areas they were not in the past,” Anderson said.

Poison hemlock can grow throughout the United States, but the plant prefers moist soils.

Mark Leoschke, a botanist with the Iowa Department of Natural Resources’ Wildlife Bureau, said poison hemlock is “pretty widespread” across the Midwest and benefits from “disturbed areas.”

Examples of disturbed areas include roadside ditches, flood plains, and creeks or rivers, where running water can carry seeds downstream.

“It just benefits from periodic disturbance, and it is the way it can grow and maintain itself,” Leoschke said.

Anderson said the plant also favors areas along fences, margins between fields and woodlands.

“It would make sense that poison hemlock would survive and spread better in areas with more of these areas between fields and near woodlands, which are likely more prevalent in Eastern Iowa,” Anderson said.

Anderson said she also has noticed poison hemlock to be more commonplace in central Iowa in recent years.

Generally, the plant isn’t a threat to lawns and residential yards, Leoschke said, because lawns are typically mowed regularly, which keeps the plant from maturing.

A ‘highly toxic’ plant

Poison hemlock — which is known by its scientific name conium maculatum and is native to Europe and Western Asia — starts growing in the springtime and is highly toxic, posing a health risk when consumed.

“The most serious risk with poison hemlock is ingesting it,” Anderson said. “The plant is highly toxic and could be fatal to human and livestock if consumed.”

According to the U.S. Department of Agriculture’s Agricultural Research Service, every part of the plant — from its stem to its leaves, as well as the fruit and root — is poisonous.

The leaves are especially poisonous in the spring, up to the time the plant flowers.

The toxic compounds found in the plant include coniine, g-coniceine and piperidine alkaloids, which can cause respiratory failure and disrupt the body’s nervous and motor systems.

Anderson said it is possible for the toxins in poison hemlock to be absorbed through the skin.

“Some of the population could also experience dermatitis from coming in contact with the plant, so covering your skin and wearing eye protection when removing the plant is important,” she said.

Jean Wiedenheft, director of land stewardship for the Indian Creek Nature Center in Cedar Rapids, said no one should eat anything from the wild unless they know exactly what they are ingesting.

The Nature Center offers classes on wild edibles and foraging to teach people about edible plants and how to identify them.

Managing the plant

With poison hemlock being a biennial plant — meaning it takes two years for its biological life cycle to be complete — it can be difficult to eradicate.

Anderson said removal strategies vary depending on where the plants are located, how abundant they are, what time of year it is and the ability of the person trying to manage the plant.

For example, Anderson said flowering plants generally need to be cut out and disposed of as trash. However, Anderson said that using herbicides on the hemlock’s rosette — when the plant is growing close to the ground in its first year — to eradicate the plant is often more efficient and more effective.

In some situations, mowing can be an effective option to manage isolated infestations of poison hemlock as well, she said.

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“Since (they’re) a biennial species, if we remove plants prior to producing seed, we can eliminate the possibility of new plants or increasing populations of these plants,” Anderson said. “Any location with poison hemlock will need to be monitored for several years to make sure new plants don’t continue to emerge from seed in the soil.”

Wiedenheft said there are several patches of poison hemlock on the Indian Creek Nature Center property but she said it doesn’t seem to be expanding.

“We spray it, and poison ivy, if it pops up in our outdoor classroom area, but don't take specific action elsewhere on the property,” she said.

Anderson said it is not uncommon for poison hemlock to co-occur with other species that are more likely to cause skin reactions, like poison ivy and wild parsnip. But successful hemlock management comes back to prevention.

“We often talk about the species this time of year because the white flowers atop the tall stems are very obvious on the landscape, but the species exists for the rest of the year as a relatively unassuming rosette of leaves on the ground that people don’t think of until they see the flowers, when it is too late for most effective management strategies,” she said. “Every time a plant is allowed to produce seed, it adds to the soil seed bank and creates more future management challenges. We need to be paying attention to infestations this time of year when they are obvious, but we also need to remember these infestations in the fall and early spring when it is much easier to manage.”

Olivia Cohen covers energy and environment for The Gazette and is a corps member with Report for America, a national service program that places journalists in local newsrooms to report on under-covered issues.

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Comments: olivia.cohen@thegazette.com