June Weed of the Month: Poison Hemlock
Poison hemlock (Conium maculatum) is a toxic member of the carrot family. It can grow to heights of eight feet and has clusters of small white flowers that bloom from May to August.
The leaves are fernlike and the stems have purple spots. All parts of the plant are toxic if ingested. 2017 was an important year for poison hemlock in Minnesota.
Prior to 2017, it was confirmed in isolated pockets of only eight counties. Accurate mapping and reporting led to confirmation of infestations in 17 additional counties.
A rigorous evaluation using a weed risk assessment model, was conducted on poison hemlock by the Noxious Weed Advisory Committee (NWAC).
Press from a subsequent MDA media release also helped increase awareness and interest in the toxicity of this plant.
The NWAC risk assessment was informed by the mapping and reporting in 2017. The risk assessment is a decision tree based on scientific literature conducted for each plant species on the Minnesota Noxious Weed List.
Based on the results of the risk assessment, a species may be recommended for listing on the Prohibited – Eradicate, Prohibited – Control, Restricted, or Specially Regulated categories.
After the poison hemlock risk assessment was completed, NWAC recommended that the species be emergency listed as a Prohibited – Eradicate species, the most severe category on the list.
Because of its current status on the Noxious Weed List, landowners are required to destroy all above and below ground parts of the plant. Most infestations are small, with just a few plants.
Effective management must prevent seed production and exhaust the seedbank.
Mow before seed set to prevent movement of seed to new locations. Clean equipment, clothing, and shoes after moving through an infestation.
Herbicide applications that target rosettes in the early spring and fall can be very effective.
Spring treatments of rosettes should be timed well in advance of flowering and follow-up treatments may be necessary for several years to exhaust the seedbank.
If using herbicide treatments, check with your local co-op or certified landscape care expert for assistance and recommendations.
There are several businesses throughout the state with certified herbicide applicators that can be hired to perform chemical applications.