The gray on the foliage suggests that your hardy hibiscus or Rose of Sharon is suffering from a fungal disease and fall is the time for a fresh start in controlling this disease. First clean up all the fallen leaves so the disease spores do not overwinter. Once all leaves have fallen and been removed mulch with wood chips or gravel to keep rain from splashing spores up onto the plant. Poor air circulation adds to fungal problems so remove nearby plants if they crowd your hibiscus. If the infection returns next summer consider moving the plant to a area with more sun and better air circulation if you do not want to commit to constant spraying with a fungicide. Keep growing, Marianne Binetti