What to know about poison hemlock and how and when to deal with it
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Poison hemlock (Conium maculatum L.) has been spreading like wildfire throughout Greater Columbus (and much of Ohio) in recent years, and if it has taken root on your property, now is 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. Too often, landowners attempt to control this deadly weed later in the growing season when it reaches several feet tall.
If this weed has been present on your property in previous years, now is the time to get out and scout for the its presence. I monitor several patches on our Columbus campus farm and this year the weed seems to have germinated and started to push out new growth a couple of weeks earlier than usual. This past week I found hundreds of these plants more than one foot tall before April 1!
Know thy enemy: What to know about poison hemlock
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 6foot-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 germinate in early spring and also in late summer and early fall. It is very common to find both stages of this plant growing at the same time in the
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 that 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 direct contact with the plant. There simply is no safe, organic 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.
From a plant-growth stage perspective, now is the perfect time to apply an herbicide to poison hemlock 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
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Applying a contact herbicide at this time will not negatively affect surrounding vegetation that has not yet begun to grow. Cool-season grasses growing around poison hemlock will be killed by the application of a contact herbicide, but you can easily re-seed treated areas. Contact herbicides will not prevent germination of any type of seeds, including grasses.
The most effective type of herbicide to use on these plants are non-selective 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 non-selective post emergent herbicides found at your local garden center will be very effective in controlling poison hemlock in its current stage of growth. Choose a warm sunny day when the air temperature is at least 60 degrees and rain is not forecast to apply these herbicides.
Have a plan to manage weeds after controlling poison hemlock
Keep in mind that when you kill the rosettes of poison hemlock, 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. Including new seedlings of poison hemlock!
Depending on the type of landscape setting where poison hemlock is being controlled, you may want to overseed these areas with desirable grasses, or even wildflowers depending upon the landscape setting. 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 contact 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 an 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 problem on your property.