The Roof Cleaning Institute Of America Training & Certification Forum
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One thing we have found very helpful is before you show up to clean a roof, is to have customers water the heck out of all their landscaping, the night before you come. This insures that the plants are not thirsty. The Last thing you want is dry ground, with thirsty plants. Remember, the plants can not "tell the difference" between the roof cleaning chemicals, and water. 

It is real simple, plants that are not thirsty will not drink, so you will have less to worry about.