It may destroy the seed stalks but will cause the plant to turn perennial and grow hundreds of rosettes from the roots. If the infestation is small, hand pulling the plants before they seed can help with control. Water the ground before pulling, because it's necessary to get all the root fragments out. The plant will regrow from root fragments. After hand pulling on a small site, mulch the ground with about 3-4 inches of an organic mulch to keep tansy ragwort seeds from germinating.
Several herbicides are effective against tansy ragwort.
During the flowering stage, when the stalk is tall, use dicamba. Be very careful with this pesticide. It can damage shrubs and trees if misused. Read the label carefully. For more information about noxious weeds, call the Noxious Weed Board in your county.