Spotted knapweed is a very aggressive invasive plant that releases toxins that reduce the growth of forage around it. This perennial has multiple stems growing from the rosette, and alternate sage green leaves. Flowers begin as bulb shapes, and flowers out with many purple or white. Spotted knapweed flowers are larger than Russian knapweed, and flower heads have a spotted appearance.spotted knapweed, noxious weed 8-2009 004