PlantFiles: Tansy Ragwort
Senecio jacobaea

I want to say something in the defense of this beautiful wildflower. In the right place it is a very valuable plant, a very rich nectar source for hoverflies, bees, butterflies, moths etc..

It is a noxious weed but it is a great plant for butterflies and bees. If you have a wildflower garden, then I would not be against growing this plant. Cut the heads off as soon as the flowers have passed and you will prevent their self-seeding everywhere. This plant only reproduced by seed, not by rhizomes.

Colorado Class A Noxious Weed. Mandatory eradication.
All locations of this plant in Colorado should be immediately reported to the Colorado Department of Agriculture.

Out here on the Oregon Coast tansy is extremly invasive, the forest service has released a moth of some kind that eats it to help control it. It is very poisonous to horses, and the symptoms may not show up for years, I believe it eats away at their liver and kidneys. It's flowers aren't worth the pain to keep it in check, and to keep the animals away from it.

Highly poisonous. Invasive in untilled soils. Designated by many governments as injurious weed and controlled by legislation. Can cause extensive losses to cattle an horse farming. Can cause death to humans by ingestion or skin absorption leading to liver failure, cirrhosis or cancer.
Contains many Pyrrolyzadine Alkaloids