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

Tansy ragwort (Senecio jacobaea) is a Eurasian weed first reported in Oregon in 1922. It spreads primarily by seed--a single tansy ragwort plant may produce up to 150,000 seeds, which may remain viable for up to 15 years. All parts of this weed are poisonous. It cause liver damage to cattle and horses, while sheep are affected to a lesser extent.

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DESCRIPTION

Tansy ragwort is a biennialPlant that germinates in one growing season, then flowers, seeds, and dies during a second or short-lived perennialPlant that lives for more than 2 growing seasons. Seeds germinate the first year and form a rosetteA circular, normally basal, clump of leaves of raggedly lobedA cut into a leaf from the edge toward the center; greater than toothed, but not quite compound leaves up to 9 inches long. Flower stalks develop the second year, growing up to 6 feet tall, with many branches near the top. Stem leaves are 2 to 3 times pinnateWith 2 rows of leaflets, like a feather with blunt tips and blades that attach directly to the stalk. Numerous yellow, 1-inch wide, daisy-like flower heads with golden or light brown centers form at the tip of each branch from midsummer to fall. Seeds are tiny and are tipped by hairlike plumesA hairlike or featherlike structure, often on a seed that carry seeds in the wind for long distances.

DISTRIBUTION

Widespread on the coast and Cascade Mountains of Washington and Oregon.

CONTROL

Biological control agents (a seed head fly, a root/defoliating flea beetle, and a defoliating moth) provide fair to excellent control west of the Cascades, but have not been tested in Idaho. Herbicides are available.