Idaho Weed Awareness Campaign
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Poison Hemlock                             

The Strategy - Poison hemlock is a biennial plant that grows up to 12 foot tall. This is the most toxic of all the plants that we have growing in the area. All parts of this plant are poisonous to livestock and humans, in fact the extract of this plant was used to execute Socrates of ancient Greece. A woman in Washington State was recently killed by the plant when she mistakenly put this into her salad. The plant hosts white flowers (donít mistake this plant for Water hemlock, wild parsley, or wild carrot). This plant is easy to distinguish as the stems have a purple striping up and down them that the other plants do. The leaves are fern-like and pinnately divided. The thick white taproot, which when sliced onto resembles walls with hollow chambers.

The Attack - This plant inhabits the riparian areas along streams and ditches or even where there are sub-waters near roadways. As the plant matures it block out sunlight and robs the soil of valuable nutrients and water. Wildlife cannot travel through the bamboo like stems (although they don’t turn woody) and obviously little or no animals can feed on it to help control it.

The Defense - There is one insect, a moth, that defoliates this plant. Mechanically digging up the plants can be effective in the first few years of establishment, just donít lick your fingers after the tugging contest. Cover your hands with a latex or nitrile glove then apply your leather gloves and get to removing. In pastures/rangeland herbicides such as Escort XP, or Telar XP is best. On outer banks of a ditch use Opensight. Treatments in early spring are best, but these products can be used right up to a late bloom stage of the plant.

PLEASE NOTE - The proper use and application of herbicides can be an effective way to control and eradicate noxious and invasive plants. Before using herbicides, always carefully follow the label and safety instructions on the label. While we recommend the use of herbicides as one of the effective tools for integrated pest management, the Idaho Weed Awareness Campaign assumes no liability for herbicide applications.

For more information, click on the link below to download the Idaho's Noxious Weeds Control Guidelines publication produced by the University of Idaho Extension.

U of I Idaho's Noxious Weeds Control Guidelines (183 KB PDF download)

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