Tansy Ragwort

What is tansy ragwort?

Tansy ragwort is a weedy member of the aster family from Eurasia that has become a serious pasture and forest weed in the cool, wet parts of the western United States. You will see it listed as Jacobaea vulgaris or under its older name, Senecio jacobaea, which USDA PLANTS still uses. tansy ragwort identification comes down to ruffled, deeply lobed leaves and flat clusters of yellow daisy-like flowers with roughly 13 petals each.

Ranchers and horse owners take it seriously because it is poisonous. Its alkaloids damage the liver of cattle and horses, the harm cannot be reversed, and the toxins persist in dried hay, where animals cannot sort them out. It also multiplies fast: one plant may shed 200,000 seeds that can wait in the soil for about 15 years. Before biological control insects were released, Oregon estimated the weed cost its livestock industry about $5 million a year.

Tansy Ragwort identification: what to look for

Most people notice it in its second year, in flower. Count the rays on a few heads and look at the leaves. As a first-year rosette, look for dark, crinkled, deeply cut leaves lying close to the ground. Wear gloves while you inspect or pull it.

• Leaves: Twice-divided, dark green on top and whitish-green beneath, with rounded lobes that make the leaf look tattered. Lower leaves have stalks; upper ones do not.
• Stems: One to several stout, upright stems from the crown, branching near the top.
• Flowers: Yellow heads about half an inch wide, a yellow center ringed by roughly 13 rays (10 to 15), grouped in flat-topped clusters.
• Bracts: About 13 dark-tipped bracts at the base of each head.
• Rosette: First-year plants form a low, ruffled, dark green rosette that dies back once flowering is well along.
• Roots: Thick taproot with fleshy lateral roots; leftover pieces can resprout.

Where it grows in the United States

Tansy ragwort was first recorded in California in 1912 and in Oregon in 1922, and it is now well established in the Pacific Northwest, mostly west of the Cascade Range. Oregon alone has roughly 125,000 infested acres. Where it is still uncommon, as in eastern Oregon, agencies aim to wipe it out; in western Oregon, biological control is the main tool.

It does best in cool, wet, cloudy climates and moves into land where grazing, logging or fire has opened up the existing plant cover. Overgrazed pastures, clearcuts, roadsides, riverbanks and rangeland are typical, and it is not fussy about soil. It is usually not a lawn problem, though regular mowing can keep plants alive as nonflowering perennials.

Tansy Ragwort look-alikes

A few plants get mixed up with it:

• Common tansy (Tanacetum vulgare): Has fern-like leaves and button-shaped yellow heads with no rays at all. Tansy ragwort has ragged lobed leaves and a ring of yellow rays on each head.
• Common St. John's wort (Hypericum perforatum): Its yellow flowers have five petals rather than the roughly 13 rays of a ragwort head.
• Tall butterweed (Senecio serra): A native relative in eastern Washington with toothed rather than ruffled, lobed leaves and only about 5 to 8 rays per head.

How to get rid of tansy ragwort

Pulling, digging and mowing: Pull small infestations when the soil is moist and get the whole root; rosettes come out more easily and leave less to dispose of. Wear gloves. Bag flowering plants and put them in the trash, not compost or yard waste, because wilted ragwort is more appealing to animals and cut flowers can still set seed. Mowing only buys time: it stops that year's seed but makes plants regrow with several stems and live longer, so follow it with another method before they flower again. Hoeing or rototilling kills plants in cultivated ground, though digging can bring buried seed up to sprout. Check for seedlings and resprouts for years.

Cultural control matters most, because the right conditions keep it from coming back.

• Keep pastures thick and avoid overgrazing; ragwort moves into thin, disturbed sod.
• Sheep grazing on rosettes in early summer is effective and sheep are not poisoned the way cattle and horses are; stop before seed production starts.
• Use certified weed-free hay and seed, and never feed hay that contains ragwort.
• Three biological control insects (a root-feeding flea beetle, a seed-head fly and the cinnabar moth) have sharply reduced ragwort in the Pacific Northwest over 5 to 10 years. They reduce it rather than eradicate it, and the moth also feeds on native relatives, so further spread of it is discouraged in many areas.

Herbicides: Selective broadleaf herbicides can take ragwort out of grass pastures without killing the grass. Extension sources name 2,4-D for rosettes in spring or on regrowth after fall rains, dicamba for larger rosettes and bolting plants, and glyphosate as a spot treatment; WSU lists dichlobenil and glyphosate for landscape beds. Spraying plants in full bloom will not stop seed, so pull and bag those. Keep grazing animals off sprayed areas for two weeks, or longer if the label says so. Use a product labeled for your site and follow the label; it is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Buy certified weed-free hay and seed, keep equipment from carrying seed between infested and clean ground, and clean animals before moving them to clean pastures. Walk pastures every year, pull new rosettes while they are few, and work with neighbors so a cleaned field is not reseeded from next door.

Dense grass is the best defense, so avoid grazing to bare soil and reseed thin spots after logging or fire. Where control insects are working and eradication is unrealistic, leaving a few plants as their food source helps keep numbers down.

Questions we get about tansy ragwort

Similar weeds

Common TansyTanacetum vulgareCommon St. John's WortHypericum perforatumBuffaloburSolanum rostratum