Manual or mechanical methods of control are not recommended because they will expose the operator to the plant sap and spread seeds. If you must get near the plants use gloves, protective clothing and eye protection.
Foliar applications of glyphosate work best if applied to the rosette stage in the spring or fall. Keep in mind that glyphosate kills all plants. Keep it away from desirable plants in the area; read and follow the product label instructions completely.
Several years of treatment may be necessary to bring large populations under control. Continue to scout the area for new plants that may develop from latent seeds.
ID photos: Megan Pistolese-SLELO PRISM, Bruce Ackley, The Ohio State University, Bugwood.org; Leslie J. Mehrhoff, University of Connecticut, Bugwood.org; Leslie J. Mehrhoff, University of Connecticut, Bugwood.org