Richard, In Birmingham, AL you can get 12% sodium hypochlorite (chlorine) at Industrial Chemicals Inc. I'm in Mobile, AL and I used to get 2 drums twice a month from their location in Mobile, but it comes from their B'ham headquarters. It was $1.16 / gal when I stopped buying from them. I now get it from a pool store in Pensacola, FL for $0.89 / gal. Mike Hughes is right about the shelf life, but I know it will hold its strength for a month if you keep the lids on it and keep it out of sunlight.
Mark, you want to use a strong enough solution of chlorine to kill the mildew, mold and fungus, but no stronger. I apply 1.44% of chlorine to the surface and make sure you have enough suds from your detergent to keep the chlorine in place (dwell time) or it just runs off. Wait 2 or three minutes and use a low pressure rinse. If surface is very bad, a second application may be needed before rinsing. I have no problem with plants and flowers at this percentage and with a good rinse.