Poison Hemlock and its discontents

Poison Hemlock (Conium maculatum) is considered an invasive species in N. America. That is, it is both non-native and causes ecological and/or economic harm. Mums, garden tulips, Bleeding Hearts and the like are non-native, but since they don’t bother anyone and just sit there looking pretty, they aren’t invasive. Only a fraction of the many non-native species are considered invasive.

I’ve seen Poison Hemlock become more and more common along Indiana roadsides. It “is a biennial weed that exists as a low growing herb in the first year of growth and bolts to three to eight feet tall in the second year, when it produces flowers and seed” (see more from Purdue extension).

By the name, you should have enough common sense not to consume it. It’s what they made Socrates drink on death row. But don’t touch it either… exposure to bare skin can cause serious reactions in some people. Many plants in the Carrot family, Apiaceae, are like this, with Queen Anne’s Lace / Wild Carrot (Daucus carota) a noteable exception.

In Northern Indiana, the plants are coming into full flower. The window for spraying the plant with herbicide is closed, as there isn’t time enough to stop the plant from producing viable seed. Remember, as a biennial, all the flowering plants you see will be shortly dead, the species living on through seed. Cutting is the only option left, and even that isn’t foolproof. Even when cut, a few robust plants will have enough moisture available in the stem to take the flower all the way to a viable seed. Talk about tenacious! If you want to be absolutely sure at this point, you have to cut and landfill the flowers.

Poor photo of a Poison Hemlock plant about to flower. Hmmm… why does it look so scraggly?

But a few weeks ago I was out spraying, and I saw a few plants with scraggly and curled leaves. I was very excited! It sure looked like insect impacts to me.

One reason that we think some plants become “invasive” is that they are out of place from the web of life in which they evolved over a long period of time. Any given plant species has numerous rusts, viruses, fungus, insects, and animals that are adapted to feeding on it. Most plants have evolved a specific chemical resistance that works against most insects, except for a small suite of species or group of species who evolved a way around it (think of milkweed and monarch butterflies).

This complex and biodiverse web ensures that any one species isn’t permanently dominant. There is an ebb and flow with the cycles of each species, the variation in climate, and stochastic events. When liberated from this web of checks and balances, sometimes plants go berserk and can dominate native communities.

Back to the insect-impacted plant! I didn’t know what it was right away, but after some Googling, I suspect it’s the European Hemlock moth (Agonopterix alstroemeriana), which was accidentally introduced to N. America in 1973. I didn’t actually observe any larva or moths, so I’m basing that just off a quick observation of the leaves. To my knowledge, the moth has not been documented feeding on native plants, so that’s good!

So… problem solved, right! Well, unfortunately, it doesn’t appear that the moth provides any serious control of the spread of this plant. Nonetheless, I decided not to spray the plants that had insects present (since I didn’t know anything about it upon first observation).

It turns out that there is a native wasp (Euodynerus foraminatus) that parasitizes moth larva, and has learned to also do so to the European Hemlock Moth. This, in turn, may limit the moth’s ability to impact Poison Hemlock. What a tangled web!

It’s a good reminder that our terms native/non-native, invasive, naturalized, etc. are just human constructs that attempt to make sense of the world around us. Nonetheless, I still think they are important concepts. Lots of things are “merely” constructs! But it’s good not to be overly-ideological about it, and remember to continue to test the concepts with observation.

So… learning to identify Poison Hemlock (especially in the year 1 rosette stage, long before final flourish of flowering) and control it will still be important tasks. Should you choose to go into battle, just be sure you wear proper PPE for skin protection!

As a reminder of why we choose to spend so much time on invasive species, I’ll share one more carroty photo.

Poison Hemlock (left) right next to it’s native cousin, Great Angelica (Angelica atropurpurea).

On the left is Poison Hemlock, growing adjacent its native cousin, Great Angelica, aka Purple-Stemmed Angelica (Angelica atropurpurea), seen towering on the right. Angelica is one of our tallest herbaceous (non-woody) plants. It’s usually indicative of high-quality natural wetlands. It sure looks like it was designed by Dr. Seuss. Like the hemlock, it’s growth is biennial. It contains furanocoumarins, which can cause phytophotodermatitis, which happens if you touch the plant and then expose the area to UV light from the sun. Which is why I’m very careful while harvesting the seeds from this plant. Like I said, don’t mess with the carrots!

So: if we can keep the invasive species at bay, we can let the native plant community (and insects, etc) do their thing. Doing so isn’t rocket science, but it does require attention, diligence/commitment, trial-and-error, and strategizing/triage.

5 Replies to “Poison Hemlock and its discontents”

    • Adam Thada Post author

      Great! That’s the goal. I have to be reminded that equal to the technical aspects of my work is the usability (education) of the information for the intended audience. I can forget that.


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