Eupatorium coelestinum, (Mist Flower, Hardy Ageratum) is a rhizomatous plant reaching from one to three feet (30cm - 1 m) tall and up to 1.5 feet (45 cm) wide. It can be invasive. I once grew this one and enjoyed it until it started to commandeer a bit more space than I wanted it to. It will grow in sun or part shade and is a nice filler for late summer color, if you have the space. It is hardy from USDA zones 6 to 10 and tolerates the hot, humid summers in southern states like Florida.