Fig. 1: Rosette Poison Hemlock
- One of the first weeds to green up each spring along fence lines, ditches and tree lines is poison hemlock.
- Poison hemlock is a biennial weed that produces alkaloids that are toxic to humans and livestock.
- Poison hemlock is plant in the wild carrot family, often mistaken for wild carrot.
- Control measures should be considered before the plant bolts due to lack of consistent control after the plants flower.
Every spring poison hemlock is one of the first weeds that greens up, usually in early March, and is observed along fence lines, waterways and tree lines and no-till corn and soybean fields. The largest concerns with poison hemlock is the natural toxicity of the plant to livestock and humans. It also is rather unappealing to have in large clumps in the landscape.
Poison hemlock is a biennial weed that resides in the same family as wild carrot (Apiaceae). It is a biennial weed with a low-to-the ground habit the first year of its life cycle, and then bolts to 3 to 8-feet in height the second year of its life (Fig. 1 & 2). Flowering and seed production occurs in the second year. Once the plants bolt, control is more difficult and less consistent. Poison hemlock has pinnate leaflets that are alternately arranged along the stem. The stem has distinct purple blotches that help to distinguish poison hemlock from wild carrot (Fig. 3). The flowers are white and flat to slightly convex like an umbrella.
Control of poison hemlock is greatest during its first year of its life cycle prior to the plants bolting. The closer the plants are to the reproductive stages; the less consistent and effective chemical control measures will be. In roadside ditches and pasture fence lines, products that are labeled and contain triclopyr or a mixture of triclopyr and 2,4-D have shown to be most effective. Other more common herbicides like dicamba in combination with another growth regulator like 2,4-D or clopyralid are also effective for controlling vegetative poison hemlock. For row crops like corn and soybeans, a combination of products may be essential for complete control. Tank mixing 2,4-D and dicamba at labeled rates is a viable option when treating ahead of soybeans; however, be cognizant of the plant back intervals to avoid crop injury. Tank mixes of mesotrione with 2,4-D or dicamba have shown to be effective in reducing edge infestations in corn fields. Field observations of tank mixing rimsulfuron with either dicamba or 2,4-D ahead of corn may also be an option for control; however, these observations have not been validated by replicated research trials. Burndown applications followed by in-crop applications containing glyphosate in combination with either 2,4-D, dicamba, glufosinate or mesotrione will reduce the competitiveness of poison hemlock. Be sure to always read the label and always follow the plant back interval for each product.
In recent years, poison hemlock has started to creep into no-till fields and has become more problematic to control ahead of a cash crop. The lack of control along fence lines, tree lines, roadside ditches and in pastures has led to this pressure creeping into corn and soybean fields. Make sure to pay attention to fields and properly identify your weeds to ensure effective control. For questions about poison hemlock or control of poison hemlock contact your local FS Crop Specialist.
Fig.2: Mature Poison Hemlock- U of KY Weed Science
Fig. 3: Purple blotches on the stem of Poison Hemlock