Thanks to Connie Morris, we have an identification for the mystery plant: Lysimachia quadrifolia, whorled yellow loosestrife. Jane Floyd's hunch about loosestrife was correct. Connie used her keyed guide to the plants of North and South Carolina. The North Carolina Native Plant Society provides a picture and an explanation that the plant may turn red when it gets a lot of light, but the bloom will be yellow. Connie Morris teaches Appalachian Studies and history and furnished background information for the Blue Ridge Discovery Center's Whitetop Wildflower Walk (5/24).