Idaho OnePlan recommends the website of the Idaho Weed Awareness Campaign as the best resource for up-to-date information about Idaho's noxious weeds, and their control.
Source for this page: Idaho's Noxious Weeds by Robert H. Callihan & Timothy W. Miller

BACKGROUND

Poison hemlock (Conium maculatum) is native to Europe. It contains highly poisonous alkaloids toxic to all classes of livestock and humans. It has poisoned many who have mistaken it for parsley. Poison hemlock is often found on poorly drained soils, particularly near streams, ditches, and other surface water.

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DESCRIPTION

Poison hemlock is a biennialPlant that germinates in one growing season, then flowers, seeds, and dies during a second that grows up to 10 feet tall. Stems are stout, hollow, ridged, and mottled with purple spots. Leaves are shiny green, 3 to 4 times pinnatelyWith 2 rows of leaflets, like a feather compoundLeaves with 2 or more distinct leaflets, and claspLeaves that appear to wrap around the stem at their base the stem at the obvious nodes. Crushed foliage has a disagreeable, mousy odor. Flowers are small, white, and borne in umbrella-shaped clusters about 3 inches across in early summer. Seeds are ridged and flattened, with 2 seeds borne together. The plant has a thick, white taprootA thick, central root with minimal branching.

DISTRIBUTION

It grows throughout the U.S. except from eastern Montana to northeastern Minnesota and south to Nebraska. It has an extensive presence in Idaho.

CONTROL

A biological control agent (a defoliating moth) provides good to excellent but inconsistent control. Herbicides are also available.