Also called "tansy ragwort" or "stinking Willie," ragwort belongs to the plant species Senecio jacobaea, which is in the aster or sunflower family. This noxious weed is highly toxic to both humans and animals, particularly grazing cattle and horses. Tansy ragwort can become a problem in many different areas, especially in pastures. You can use certain herbicides and cultural control methods to get rid of ragwort.
Ragwort has upright, slightly spreading stems that are usually unbranched and covered with pubescent hairs. The leaves are dark green on the upper surfaces and whitish on the undersides with toothed edges and deeply-lobed margins. The leaves are arranged alternately along the stems. Ragwort blooms in flat-topped terminal clusters of yellow flowers with many petals that resemble daisies. Tansy ragwort plants reproduce vigorously by seed, with each plant producing more than 150,000 seeds, but they can also reproduce from root fragments and crown buds.
Ragwort plants are biennial and die after they go to seed, but sometimes the plants become short-lived perennials by staying in the rosette stage. The seeds can remain viable in the soil for several years. You can identify ragwort easiest when the weed is flowering, typically during early summer through early autumn. Although tansy ragwort plants are native to western Asia and Europe, the invasive weeds are found throughout the United States.
You can use a selective herbicide to effectively control ragwort weeds. Herbicides containing picloram, dicamba, aminopyralid or 2,4-D will control tansy ragwort, particularly when you apply the herbicide while the weeds are actively growing and in the rosette stage. If you're trying to kill off more mature ragwort plants, select an herbicide containing dicamba. Applying herbicides to tansy ragwort weeds can make the plants taste better to livestock, so remove all grazing livestock from the pasture before spraying the herbicide. Don't allow the livestock to graze in the pasture for at least three to four weeks after spraying the herbicide. Always carefully follow the application and safety instructions on the herbicide label.
Aside from herbicides, you can prevent and control the growth of ragwort by keeping desirable vegetation and grasses growing strong and healthy. Ragwort weeds tend to grow most profusely in disturbed areas with thinning vegetation. You can hand-pull smaller infestations of tansy ragwort, but be sure to remove all the roots because the weeds can propagate by root fragments left in the soil. Another non-chemical control option for tansy ragwort is introducing the seed head fly, which feeds on the weeds and can disrupt the spread of ragwort plants.