Prickly Lettuce, a non-native, has been added to the USWildflowers database (09/30/2010.) Scientific name is Lactuca serriola. It’s also known by the common name China Lettuce. Photo below was taken in downtown Chattanooga, TN on May 20, 2010.
Side note: This is the 100th species added to the USWildflowers database in 2010.