I have a timber tech gray composite deck with white azek custom made large flower boxes. I have used the Corte Clean product every spring since I built the deck before I take out our outdoor furniture and accessories. After the fall and winter here in southern Connecticut the deck and flower boxes have green algae stains and are very stained from the falling leaves. This product works outstanding, as long as you follow the instructions. It does not harm our Koi in our pond which is located right off of our deck or my wifes numerous flowers surrounding it. It its well worth the investment. I am a Corte Clean user for life !!! Give it a try I think you might be surprised with the results..