Tansy ragwort

About

A yellow-flowered European weed of pastures and roadsides in the coastal Northwest, northern California and New England. It slowly destroys the liver of horses and cattle that eat it, fresh or dried in hay.

Also called stinking willie, ragwort

How to identify it

Look for

First-year plants form a rosette of dark green, deeply lobed, ruffled leaves. Flowering stems grow 1½ to 4 feet tall with flat-topped clusters of yellow daisy-like heads, usually with about 13 petals, and black-tipped bracts underneath.

Look-alikes

Common tansy has fern-like leaves and button flowers with no petals; St. Johnswort flowers have only 5 petals. Tall butterweed has fewer petals (5 to 8) and toothed leaves.

Where it grows

Pastures, roadsides, fields, logged or cleared forest land and other open, disturbed ground with plenty of rain.

When

Grows as a rosette the first year and flowers from midsummer into fall the second year; mowed plants can come back for years.

How it spreads

Spreads mostly by seed, which can live up to 15 years in the soil, and can regrow from root pieces.

Most common in

Where it is regulated

Washington lists it as a Class B noxious weed, and Oregon as a Class B and target (T) weed.

What to do, gentlest first

Protect yourself first. Weed killers are a last step; always read and follow the label.

1. Keep pastures thick and hay clean Prevent

   Good pasture care and reseeding bare spots crowd out rosettes. Check and don't feed hay that contains ragwort.

   Dried plants in hay are still toxic.

2. Pull or dig the whole plant By hand

   Pull or dig out the entire plant, roots included, ideally before it flowers. Hoeing or tilling also works in gardens.

   Wear gloves. Seal flowering plants in a plastic bag for the trash. Mowing alone won't kill it.

3. Cinnabar moth, flea beetle and seed-head fly Natural enemies

   Three released insects, the cinnabar moth, the ragwort flea beetle and a seed-head fly, have knocked it down across much of western Oregon. Sheep and goats can also graze it.

4. Treat rosettes with 2,4-D, dicamba or glyphosate Conventional spray

   Spray rosettes in spring or after fall rains; in landscape beds use glyphosate only as a spot treatment.

   Read and follow the label. Many lawn and brush herbicides also kill garden plants, so keep spray and drift off anything you want to keep.

Uses, wildlife and pets

Pets

Ragworts (Senecio species) are toxic to dogs, cats and horses (ASPCA), causing liver damage that can show up suddenly.