I responded to an identification request on Facebook yesterday of this plant, Spotted Knapweed. I knew it was a terrible invasive plant, but I hadn't paid that much attention. The person with the question was talking about pulling it up to remove it from their property. I checked a little closer, and was jolted out of my stupor with a warning about handling it. It strikes me as important enough to pass on. There is some evidence that simple contact with your skin, for example hands, can cause tumors. So, if you are working to remove it, be sure and wear gloves.