Yellow Loosestrife (which is not closely related to Purple Loosestrife, of the genus Lythrum) is a little-known perennial with yellow flowers in early Summer that originate in a whorled fashion from the leaf axils on the vertical stems. Yellow Loosestrife prefers damp soils in sunny to shady sites, where it will quickly spread by underground rhizomes and become invasive. Drier sites in partial shade keep it more in check.