POISON HEMLOCK (Conium maculatum) Part 2 of 2: Control Methods
So, you’ve found a patch of poison hemlock on your property. Now what?
As with all unwanted or invasive plants, early detection and persistence are the keys to effective control, not to mention doing one’s homework. In the case of poison hemlock, targeting the first-year growth when the plants are in the basal rosette stage is best. Unfortunately, this is also when they are hardest to find and are most likely to escape notice. If you do find adult plants, take heart. With repeated measures, you can eventually deplete the seed bank and regain control. Regardless of whether you are treating the rosette or adult plant, work from the ground up and do your best to prevent the plants from going to seed.
There are multiple ways to control poison hemlock. Preferred treatment methods will depend on several factors, including the level of infestation (Are you looking at a few plants or an entire field?) and location (Is it in your garden or backyard where children and pets frolic or a pasture where livestock graze?). This article touches on some of these, but please refer to the sources provided at the end for more information. Remember, in the words of Schoolhouse Rock! “knowledge is power.”
(from left to right: Poison hemlock, Image by John Cardina, The Ohio State University. Poison hemlock leaf, Image by Robert Vidéki, Doronicum Kftt. Poison hemlock stem, Image by Eric Coombs, Oregon Department of Agriculture. All images via Bugwood.org)
Manual or Mechanical
Hopefully, it goes without saying that one should wear protective gear to avoid contact with toxic sap. For small patches, the plants can be removed manually by hand pulling or digging them out, but be sure to remove the entire taproot to prevent regrowth. This can be difficult as the taproots are long and fibrous, and any remaining pieces left behind will simply grow back. Another complication with digging is that soil disturbance can encourage germination. Turn your back on this and you’ll have a whole field of poison hemlock before you know it. As with most things in life, persistence is the key to success.
Dispose of the plants in plastic garbage bags; do not compost. In the case of large infestations, some sources suggest plowing and repeatedly cultivating newly germinated plants to weaken the root system. While routine mowing before the plants have flowered may also do this, it may also spread seeds from previous growing seasons, as well as the chopped plant parts around. Burning the plant is also not recommended, as the fumes can irritate the airways.
Targeted spraying with herbicides is an effective approach if applied during the rosette stage or before the plant starts to bolt and repeating applications when needed. Do not use on fully mature plants. You’ll essentially be wasting your time. As these plants are often found in wet places, be sure to use aquatic-safe alternatives if needed. Refer to the sources below for more information about what to use.
Poison hemlock is the only host plant for the European palearctic moth (Agonopterix alstroemeriana). As both plant and moth are non-natives, it was most likely accidentally introduced but is now considered widespread in northeastern U.S. Unfortunately, it is not considered to be an effective control, so don’t hang all your hopes on the moth showing up and taking care of the problem for you.