By Bill Kleiman
Poison hemlock, Conium maculatum. The maculatum means spotted and the stem has conspicious red spots. Hemlock is a modestly invasive plant that can form thickets if it gets started in disturbed or fallow ground. I sometimes see it along some local creeks where flooding may kill the sod and leave open spaces. Or recently shaded areas that are opened up can sometimes have a flush of hemlock that had been suppressed somewhat by shade. Hemlock is highly poisonous. A member of the carrot family. These are biennial plants so the best time to spray them is the year one basil leaves or in the spring of year two before it bolts.
On May 26, 2023, we tested a 2% solution of Garlon 3A in water, with methylated seed oil at a half ounce per gallon on hemlock plants that had already started to bolt their flower stalk. The plant can be 4 to 7 feet tall. The plants we sprayed were just starting to bolt. We wondered if the Garlon 3A would kill them.
Below, looking down, is one of dozens of hemlock about a week after being foliar sprayed. The plant is drooped, including the flower stalk. But is the root going to die?
Below, about four weeks after being sprayed. We flagged several plants to make sure we were looking at the right plants.
Below is after six weeks. I also applied a bit of blue tree marking paint so as to keep track of this plant. As you can see it looks dead. I think the root is dead, but I will have to wait until next spring to find out. I conclude 2% Garlon 3A controls hemlock.
Hemlock can become huge, especially if a second year plant re-sprouts after mowing, it can come back a third year and be huge like this one below. Hemlock can be spaded out, like wild parsnip. You don’t need or want to touch the plant to spade it. You don’t need to get all the root, just “cut the carrot” root and it will die. Step on the plant to knock it down so you know it was stabbed.
Various other herbicides will also control hemlock.
We sometimes mow hemlock or wild parsnip when they are tall and late in flower. This seems to set them back, but we have not tracked individuals. At least they don’t make seed that year.
I asked a few colleagues for what works for them:
From Julianne Mason: We have primarily used Transline 0.5% on poison hemlock, and also some treatments using triclopyr 3A 3% or Vastlan 2.5%. All those herbicides seem to work well in the spring, before the plants bolt and flower.
We have done a couple of instances of base clipping the hemlock at late flower, primarily in areas of widely scattered hemlock not dense patches. The hemlock hasn’t seemed to come back in those locations the following year, but we haven’t tracked or documented it too closely. Haven’t tried mowing bigger patches at late flower.
Josh Clark uses Garlon 3A on hemlock and dames rocket.
From Bryon Walters: Below wilted hemlock Bryon sprayed with Buckshot, a mix of glyphosate and 2-4-D on a ruderal edge. He was able to spray some reed canary grass at the same time. He likes to spray before the plant bolts. But if bolted he mentions that 3% Garlon 4 should work. The G4 mix also works at the same time for mullein (Verbascum thapsus), and some woodies that are leaved out early in the year.
May 16, 2024: I tested 2% Garlon 3A with MSO added. I sprayed the hemlocks April 6 and about 5 weeks later they were dead and wilted to nothing as in below photo. The blue paint in on a dead hemlock.
grassland restoration network 
Thank you as these posts are great!
I don’t think its seed bank is particularly long-lived either. In a past life when I was managing pastured sheep and cattle in eastern Iowa I had a pasture that was densely infested with it, but preventing flowering through spading, pulling bolting plants (with gloves), and mowing the densest patches short in the early flowering stage for a few years while concurrently sowing grasses (in this case pasture species) led to a drastic drop from thousands of new plants each year to single digits.
I will add that this plant has been highly invasive in eastern Iowa in agricultural settings for at least a few decades but not so in settings without degradatory disturbance, and it spreading from foci elsewhere. It is one of the broad class of short-lived invasive species that rely on land use and land management activities that create soil disturbance, weaken the perennial-dominated prairie sods, and/or increase nutrient availability/speed up nutrient cycling (part of why we see it under white mulberry and boxelder, whose litter is basically fertilizer). As the post suggests it is mostly a problem where prairie is being reconstructed where prior land use has been intense (margins of cultivated fields, areas of present or former livestock concentration, disturbed riparian areas, sparse/bare areas where brush has been removed). It’s not a problem for intact, established native grasslands. Most of the “tools” we use can either be stabilizing/neutral or destabilizing, and on prairies the new occurrence of this species is a symptom of the latter and the undoing of ecological integrity. In such cases the presence of this species is symptomatic of other underlying problems that stewards should give thought, because it is preventable.
We’re still learning, but so far it looks like spading them out like musk thistles, or just chopping them off right at the ground kills them dead. Mowing with a tractor seems to leave too much plant behind and they regrow (though repeated mowing in a season can at least reduce or prevent seed set. I don’t understand why this species has suddenly decided to become invasive after 150 years or so of just persisting in old homestead areas or under weedy trees, but I guess we need to add it to the list of weeds to fight now. Thanks for drawing attention to it!
If they’ve bolted the lower leaves are senesced, and much like Parnsip, it dramatically reduces subsequent flower in and seed set. Mowing while leaves are primarily basal does nothing.
I like Milestone (aminopyralid) if I have to use herbicide I try my best to use it earlier in the season before hemlock starts bolting. Later in the season I will use a spade or predator to slash the root.
Aminopyralid stays active in the soil for about 6 months so new seedlings don’t come up later in the summer.
We are just starting to see this in DeKalb County Illinois. I will be spraying with Garlon 3A this week.
Josh, Maybe report back how that works. I saw hemlock basal leaves are up. When do you think they will bolt?