Just as your mom might have told you, beauty is only skin deep. That’s the case with a trio of invasive plants that have recently been identified as problematic in Maine, according to a couple of plant experts here. Yellow iris, ornamental jewelweed and black swallow-wort may be pretty, but they can smother the state’s native flora and provide less-nutritious food for native fauna, according to Tori Jackson, a University of Maine Cooperative Extension associate professor. “This beauty comes at a steep price,” she wrote in the May edition of Maine Home Garden News, a monthly newsletter published by the University of Maine Cooperative Extension. She’s not kidding.