Poison Hemlock is Blooming

By: Christine Gelley, Agriculture and Natural Resources Educator, Noble County, OSU Extension

Poison hemlock is up and actively growing right this minute. It is already prevalent on roadsides in Noble County. If you stand next to poison hemlock it will feel like you are in that scene from “Alice in Wonderland” where the flowers are giant, and she is tiny. It looks like Queen Anne’s Lace, but much larger. It blooms earlier and it is has distinct purple spots on the stem.

All parts of the poison hemlock plant are poisonous to people and livestock, wet or dry. This can be an extremely concerning weed in hay fields. You won’t have to look hard to find it. If you come across it in bloom, you can mow it down to prevent seed production, but it will come back to haunt you later. A similar look alike is wild parsnip, which is in the same family, causes additional concerns for skin rash, and has yellow flowers. We have yet to see giant hogweed in Noble County, but it is another look alike that can be found in other parts of Ohio with similar concerns.

Control on poison hemlock is most effective when the plants are small (less than six inches tall), but some are already 6 feet tall and flowering. Most people do not notice they have it until it is flowering, which happens in year two of growth, making control challenging. If you are wondering if 2020 is the year for action on poison hemlock, the answer is “yes.” For the longer we wait to take action to control this weed, the greater impact it will have on overall production and the more difficult they will be to treat in both hayfields and grazed pastures.

Poison hemlock in short pastures pose the greatest risk to grazing livestock as they are more likely to consume the plant in the absence of more desirable forage.

A recent article written by Joe Boggs of OSU says that:

“Wild parsnip and poison hemlock are both susceptible to non-selective post-emergent herbicides such as glyphosate (e.g. Roundup).  However, “non-selective” means all plants – both good and bad – may be killed and there is a considerable downside to killing the competition as well as the targeted weeds.

Post-emergent herbicides do not affect seeds.  Thus, “herbicidal openings” that occur when all plants are killed provide the perfect opportunity for more wild parsnip and/or poison hemlock to spring forth from previously deposited seed.  Thus, it’s important to have a plan for establishing competitive plants after the wild parsnip dies off such as over-seeding with grasses.”

The decision of how and when to wage war on damaging weeds is one based on many factors. Extension always recommends utilizing an integrated pest management program to control pests and weeds. The most effective programs are a combination of cultural, mechanical, and chemical control.

Weeds are a symptom of site weaknesses. These could be related to soil moisture, pH, fertility, erosion, compaction, or poor harvest methods. In order to make progress on weed treatment, we have to strengthen the health of our soils and desired species.

Step one is always to address soil fertility. Step two is species I.D. Step three is deciding on a treatment method. Some weeds are simply annoying, while others can be seriously hazardous for animals and people. Before attempting a treatment, get a confirmed I.D.

Contact your Extension office to set up arrangements for I.D. and to conduct a site management plan specific to your situation if you have poison hemlock, wild parsnip, or any other concerning weed.

The recent article by Joe Boggs can be read online at: https://bygl.osu.edu/index.php/node/1598.


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