I am seeing lots of poison hemlock along the road. I am keeping a watch on...

Asked June 7, 2017, 1:40 PM EDT

I am seeing lots of poison hemlock along the road. I am keeping a watch on our property in case any shows up here. My question is what do you recommend as the best and safest method to remove this nasty weed? I don't want to do the wrong thing.

Hamilton County Ohio

1 Response

The safest approach to controlling poison hemlock is to make herbicide applications. This is a biennial weed meaning that it takes two years for the plants to produce seed, then the plants die. The first year is spent in the "rosette" stage which are small plants that arise from seed. The rosettes will start developing later this season. During the spring of the second year, tall flower stalks arise from the rosettes: that's happening right now in Cincinnati. So, it's very important to spray the flowering plants before they seed: it would be right now! And follow-up with spraying the rosettes early next spring that will develop from previous seed.

Poison hemlock plants can be killed with 2,4-D mixed with dicamba or 2,4-D mixed with triclopyr. These are "selective" herbicides meaning that won't kill everything, but you'll need to read the labels to see what they won't kill. You can also kill poison hemlock with "non-selective" glyphosate (e.g. Roundup), but this herbicide will kill many other plants. However, it's a good choice for the early spring application to kill the rosettes because there's not much else growing at that time.

Some people use mowing to control poison hemlock, but I don't recommend this. The sap is extremely toxic and I although I've never heard of this happening, I have concerns that mowing could atomize the sap making it behave like an aerosol that can be inhaled. I think it's best to not risk it. Rototilling would be safe but the soil disturbance creates a nice "seed bed" for old poison hemlock seed to germinate: the seed remains viable for many years.