I am no expert on the subject but I truly think that a little elbow grease and bleach would do the trick. I would make a spray bottle of bleach and water and spray the furniture and scrub it down with a brush and then hose it off. Of course, you will want to be careful about any nearby plants, grasses, ect., because the bleach will kill them. Let me know if it works, as I need to do the same to my outdoor furniture!
Patricia Wallwork