Poison Hemlock, and Those Weeds With Yellow Bloom!

By Christine Gelley

Warm temperatures and steady rainfall have accelerated the maturity of grass, legume, and weedy species in the landscape and on-farm so far this spring. Two main species of concern that have been actively growing quickly around the county include poison hemlock and cressleaf groundsel.

Poison hemlock is a biennial weed that does not flower in the first year of growth but flowers in the second year. It blooms on stalks 6-10 feet tall. All parts of the plant are toxic to all classes of livestock if consumed and it is prevalent along roadsides, ditches, and crop field borders. The earlier you can address poison hemlock with mowing and/or herbicide application, the better your control methods will be.

Poison hemlock is related to Queen Anne’s lace, but is much larger and taller, emerges earlier, and has purple spots on the stems. Another relative that is poisonous is wild parsnip, which looks similar to poison hemlock, but has yellow flowers. Giant hogweed is another relative of poison hemlock that is also toxic. All of these plants have umbel shaped clusters of flowers.

Treatment for poison hemlock includes aggressive mowing to prevent flowering and seed production or herbicide application. Some commonly available herbicides that are generally effective on this weed include glyphosate, 2, 4-D, Remedy Ultra, and Crossbow. All treatment methods are most effective when employed while plants are less than 2 feet tall.

Learn more about poison hemlock identification, concerns, and control by watching the YouTube video by OSU Extension embedded below

Cressleaf groundsel is a member of the aster family and displays yellow daisy-like blooms in the springtime on upright hollow stems that have a purple hue. All parts of the plant are considered toxic in both fresh and dried states. These plants are winter annuals, meaning the seed germinates in the fall producing vegetative growth and then flowers in the springtime. If allowed to set seed, the plants will appear again in greater numbers the year following. The plants typically go unnoticed in the fall, which is the best time for weed control.

Cressleaf is most commonly found in annual crop settings because bare soils in the fall and spring allow the plants to grow without competition from the crops. However, it can be found in pastures and hayfields if seed travels from an infested field nearby. Effective herbicide control can be accomplished in the fall using products that contain 2,4-D. If the plants are already flowering, mowing and removing the plant residue is the best option to prevent animal consumption and seed deposition.