Don’t wait to control poison hemlock - Dubois County Free Press, Inc.

Over the past few years, everyone seems to be talking about Poison Hemlock. This invasive and noxious weed is so problematic that in 2022 the Dubois County Soil & Water Conservation District (SWCD) and the Dubois County Weed Board even partnered together on a Poison Hemlock outreach campaign that included educational videos, ads, public service announcements, and mailings to Dubois County Landowners. And they plan to continue this outreach with the hope that landowners don’t let up when it comes to getting Poison Hemlock under control.

*Dense Poison Hemlock stand in full bloom; photo was taken in Dubois County. Photos provided by Emily Finch*

You’ve probably seen Poison Hemlock (Conium maculatum) on the sides of roads, field edges, and creeks. It is classified as an invasive plant and a noxious weed, partly because it is highly toxic to humans and livestock if ingested. The rapid spread of this noxious weed in the area has shown that more landowners need to be aware of this threat and the right way to deal with it.

According to Emily Finch, Invasive Species Specialist with the Dubois County SWCD, while several landowners try to manage Poison Hemlock, not all of these efforts are effective in controlling it. That’s because Hemlock needs to be controlled earlier in the year, before plants even flower. To understand why, you have to understand the weed.

Poison Hemlock is a biennial, meaning it only lives for two years before producing seed and dying. The only way to eliminate infestations is to prevent that seed production. Hemlock plants start as short basal rosettes with fern-like leaves. Then, in their second year of life, plants will send up a flowering stem, also called bolting, in spring/early summer (around May/June in southern Indiana). These flowering plants can be quite large, anywhere from 3 to 10 feet tall. Because Hemlock is in the carrot family, it produces abundant umbrella-shaped clusters of white flowers, often compared to the flowers of Queen Anne’s Lace, aka wild carrot. To help distinguish the plants, remember that Hemlock blooms before Queen Anne’s lace, lacks any hairs on its stems/leaves (a common feature of Queen Anne’s Lace), and has distinctive purple splotches on its stems and often the base of large leaves as well.

*Poison Hemlock leaves resemble fern leaves. Hemlock can be distinguished from Queen Anne’s Lace by its larger size and lack of hair on the leaves or stems.*

There are a few options to control Poison Hemlock, so long as plants are not allowed to flower and produce seed. In fact, Hemlock seeds have a very short duration in the soil, and many populations can be eradicated with just a few years of effective treatments (a short timeframe for most weed control). Small populations can be removed by hand, either digging the plants up or severing the root with a sharp shovel (wear gloves to avoid contact with plant toxins). Cutting/mowing the plants is NOT effective unless repeated regularly through the summer as plants will resprout readily. In fact, mowing after plants have produced seed is one way infestations have spread through the area. Herbicides are the most efficient way to control large areas, and several chemicals are effective, including glyphosate, or triclopyr and 2,4-D products (which unlike glyphosate have the benefit of not harming grasses). All of these options also are available in aquatic safe formulations for use near open water creeks and ditches. Whatever product you choose, make sure to read and follow all label directions.

According to Finch, the earlier you can spray Hemlock, the better.

“Spring planting is a busy time for everyone, so why not avoid that and spray your hemlock plants in February or March instead? The Hemlock rosettes are green that time of year, plus the plants are smaller, so you’ll use less herbicide,” said Finch.

For best results, she suggests only spraying on days above 50 degrees.

Wait longer and you may run into problems. As Hemlock plants start bolting and preparing to flower, they also get harder to kill. If herbicide rates aren’t increased when spraying these plants, they may only be knocked back or stunted temporarily. And according to Kenny Eck with Purdue Extension Dubois County, once they flower, there is no point in spraying them.

“Spraying a flowering Hemlock plant is just a revenge killing. It makes you feel good, but it does nothing to control it as it will still go to seed,” said Eck.

He recommends checking out options in the Purdue Extension/SICWMA publication “Poison Hemlock.” This publication, along with more resources on identifying and controlling Hemlock, is available at www.duboisswcd.org. Looking for support or technical assistance in controlling Poison Hemlock? Try contacting your local Soil & Water Conservation District (https://wordpress.iaswcd.org/contact-your-local-swcd/). Several area SWCDs have an Invasive Species or Resource Specialist that can provide free site visits and technical assistance about Poison Hemlock Control. This year Dubois County landowners involved in livestock production may also be eligible for funding to control of Poison Hemlock and other noxious weeds. For details call 812-482-1171 x3, or email duboisswcd@gmail.com.