Gardening: How to control poison hemlock, one of North America's 'deadliest' plants
Poison hemlock (Conium maculatum L.) has been spreading like wildfire throughout Greater Columbus in recent years, and if it has taken root on your property, now may be the best time of the year to control this non-native invasive weed. Targeting this dangerous plant with herbicides applied now will prevent flowering and seed production later this summer.
Poison hemlock is a biennial, which means that it spends its first growing season in a vegetative stage, without flowering or reproducing. During this first season of growth these low-growing “rosettes” use carbohydrates acquired through photosynthesis to produce a vast root system. Then in the second growing season, the plant “bolts” in its reproductive stage to produce 6-foot-tall multi-branched stems topped with umbrella-shaped white flowers. The mature plants then die after producing seeds.
Seeds remain viable in the soil for up to 6 years and both new and old seeds in the soil can germinate in late summer, early fall and early spring. It is common to find both stages of this plant growing at the same time in the same location. During a quick inspection this week at a location on the OSU campus where I spotted poison hemlock growing last summer, I found hundreds of small rosettes growing and thriving!
Poison hemlock: Why the plant is so toxic
Poison hemlock is one of the deadliest plants found in North America, containing highly toxic piperidine alkaloid compounds that cause respiratory failure and death in mammals.
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Although 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 in 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 that 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 of poison hemlock
The safest approach to controlling this dangerous weed is to use an herbicide that will minimize the risk of direct contact with the plant. As with any pesticide, it is important to read and follow all label directions paying close attention to recommended rates and any cautionary statements.
Right now is the perfect time to apply an herbicide to both poison hemlock and wild parsnip in Greater Columbus. In many landscapes where these plants are currently growing, surrounding vegetation has not yet begun to grow, making these plants easy to see. Applying a contact herbicide at this time will not negatively affect surrounding vegetation that has not yet begun to grow.
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The most effective type of herbicide to use on these plants are non-selective post-emergent (between seedlings and maturity) 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 non-selective post-emergent herbicides found at your local garden center, will be effective in controlling both poison hemlock and wild parsnip in their current stages of growth. Choose a warm sunny day when rain is not forecast to apply these herbicides.
Do not try to pull the plants by hand, even with gloves.
Have a plan to manage weeds after controlling poison hemlock
Keep in mind that when you kill the rosettes of poison hemlock and sild parsnip, it will open up these areas for increased germination of other weed seeds, some of which may also be invasive, non-native and undesirable substitutions for the weeds that you are controlling.
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.
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You will also need to inspect these areas in early spring in subsequent years for any rosettes that germinate from poison hemlock seeds in the soil. Remember, post-emergent herbicides only kill plants when the chemical comes in contact with the foliage and will not have an effect on seeds in the soil. Rosettes that germinate in subsequent years can be spot-treated with a herbicide.
Poison hemlock has become a summer scourge here in Greater Columbus and controlling it in early spring before it has a chance to flower and reproduce later this summer can help end this menace on your property.
Mike Hogan is an associate professor at Ohio State University and an educator at the OSU Extension.
This article originally appeared on The Columbus Dispatch: Poison hemlock: What you need to know to identify, control it
