If Socrates were alive today, he would do well to consume only food and drink which he himself prepared, as the plant which was used to kill him is alive and well throughout the contiguous United States.
In fact, poison hemlock (Conium maculatum L.) is spreading like wildfire, anywhere where vegetation is unmanaged, particularly along roadways and alleys. Historical records indicate that Socrates was killed when he consumed food or drink laced with poison hemlock, causing respiratory failure and rapid death.
One of the deadliest plants in North America
Poison hemlock is one of the deadliest plants found in North America, containing highly toxic piperidine alkaloid compounds, which cause respiratory failure and death in mammals.
While all parts of the plant are toxic, the toxins must be swallowed or enter the body through the eyes, nasal passages or cuts in the skin to induce poisoning.
This plant should never come into contact with bare skin because sap from the plant transferred to skin can be accidentally rubbed into the eyes or ingested while subsequently handling food. Immediate medical attention is warranted if accidental poisoning from this plant is suspected.
The toxins found in poison hemlock do not cause skin rashes or blistering. Many individuals mistake poison hemlock for wild parsnip (Pastinaca sativa L.), which contains a phytochemical called psoralen, which can cause severe skin blistering, burn-like symptoms and skin discoloration.
Many times, wild parsnip can be found growing in the same location as poison hemlock, leading someone to believe that it was exposure to poison hemlock that caused a skin rash or blistering, when in fact it was contact with wild parsnip.
Herbicide needed for safe control
The safest approach to controlling this dangerous weed is to use a contact herbicide that will minimize the risk of you coming into direct contact with the plant. There simply is no safe, organic or cultural control method for this poisonous weed.
As with any pesticide, it is important to read and follow all label directions, paying close attention to recommended rates and any cautionary statements. Never attempt to control poison hemlock with the use of a lawn mower or string trimmer.
Controlling poison hemlock now, when it is relatively small, will be more effective than when the plant is much larger later this summer. Desirable plants growing around poison hemlock will be killed by the application of a contact herbicide, but you can easily reseed treated areas.
Contact herbicides will not prevent germination of any type of seed, so eliminating poison hemlock will likely require another herbicide application in subsequent years as seeds will continue to germinate.
The most effective type of herbicide to use on these plants are nonselective post-emergent herbicides. These types of chemicals will kill all green plants when they come in contact with foliage, after the plant has germinated and emerged from the soil.
Glyphosate, the active ingredient in many nonselective post-emergent herbicides found at your local garden center, will be very effective in controlling poison hemlock in its current stage of growth. Apply herbicides on a warm, sunny day when the air temperature is at least 60 degrees Fahrenheit and rain is not in the forecast.
Managing vegetation is critical after control
Keep in mind that when you kill poison hemlock, it will open up the area for increased germination of other weed seeds, some of which may also be invasive, non-native and undesirable substitutions for the vegetation which you are controlling, including new seedlings of poison hemlock!
With the canopy removed and bare soil exposed, seeds of various undesirable plants such as poison hemlock, honeysuckle and others can germinate.
Depending upon the type of landscape setting where poison hemlock is being controlled, you may want to overseed these areas with desirable grasses, or even wildflowers.
You will also need to inspect these areas in early spring in subsequent years for any seedlings which germinate from poison hemlock seeds in the soil. Poison hemlock seedlings which germinate in subsequent years can be spot-treated again with a contact herbicide.
Mike Hogan is Extension Educator, Agriculture and Natural Resources and associate professor with Ohio State University Extension.
This article originally appeared on The Columbus Dispatch: Poison hemlock identification and prevention in your garden, yard
