Now is a Fine Time to Scout for Poison Hemlock

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Winter of 2024 has been somewhat of a no-show for Ohio. Mild to almost hot temperatures have been moving things along for spring reactivation of growth and development. Here in NW Ohio, this reactivation or reawakening of biological activity can be easily observed in our winter wheat crop where it has gone from a drab yellow-brown color to a vibrant carpet of deep green color across the fields. But winter wheat isn’t the only plant to brighten up with a fresh deep green coloration, the overwintering rosettes of poison hemlock (Conium maculatum) are also coming back to life.


Leaves of a poison hemlock rosette.
Leaves of a poison hemlock rosette in NW Ohio as of March 8, 2024.


Poison hemlock is a non-native, invasive species that is spreading across Ohio at an alarming rate. It has a biennial life cycle wherein it spends the winter between its first and second year of its life cycle as a low growing rosette. It is in this rosette stage that the plant can be easily controlled before it bolts to produce the flowering stalk and eventual seeds in its second season.


As of today, March 8, 2024, these rosettes have already begun growing again in NW Ohio. The newly greened rosettes are fairly easy to spot scattered across the landscape mixed in with the tans and browns of last year’s dead herbaceous plant material. Poison hemlock thrives in disturbed habitats and will be easily found along right of ways, ditch banks, fence lines, field edges, and soil pills.



Poison hemlock rosettes growing under a scrubby tree where the shadow of the tree prevents other plant growth.
Poison hemlock rosettes growing under the shadow of a scrubby tree. Rosettes showing new growth and darker foliage color from renewed growth.


Now is the time to search for established poison hemlock individual plants as well as stands of plants. Once they are found, it is a great time to control them. If there is but a few plants, they can be easily dug out of the ground to disposed of them. Large numbers and stands of poison hemlock plants are more efficiently managed with herbicides.



Large stand of poison hemlock rosettes along a fence line where the boom of the spray rig could not safely reach.
Poison hemlock overwintering rosettes from an established population. This area was slightly out of reach of the spray rig's boom allowing the plants to escape treatment. Dead plants from the previous year lie over top of the rosettes.


It is best to control poison hemlock with an herbicide now before it gets a chance to begin growing more rapidly as the temperatures continue to increase and remain higher for longer periods. Another benefit to treating soon rather than waiting is that few other plants and especially desirable plants are currently not actively growing around the poison hemlocks. There will be less danger of harming the desirable plants.


Poison hemlock rosettes growing in dormant grasses.
Poison hemlock rosettes growing in desired dormant grasses. 


A non-selective herbicide such as glyphosate (e.g., Roundup) is very effective against the poison hemlock rosette, although one may have to wait until temperatures once again return to 50 degrees Fahrenheit and above for better results.


If the poison hemlock is mixed in with grasses, it would be better to use a more selective herbicide that does not kill grasses such as 2,4-D, clopyralid (e.g., Transline), metsulfuron (e.g., Escort XP), or a combination product.


Unfortunately, it is not a one and done management project. More than likely at any site where poison hemlock has become established, there is already a build up of a seed bank. Seeds from one growing season remain viable for 4-6 years. Thus, one would have to keep checking for new poison hemlock plants for several years to be sure that the removal was complete.


Poison hemlock rosettes growing in a broken tile line blow out hole. These plants had been treated the year before. They most likely reestablished from the seed bank.
Poison hemlock rosettes growing in a broken tile line blow out hole. These plants had been treated the year before. They most likely reestablished from the seed bank. 


If at all possible, reseed areas where poison hemlock was removed with a desirable plant that is competitive for the space. Otherwise, the open space where the poison hemlock was removed is an invitation for poison hemlock to reestablish.


For additional information on poison hemlock as well as wild parsnip, this these articles by Joe Boggs at the following links: