Daniel ... WATER, WATER and MORE WATER. When we clean a roof with no gutters, we water everything under the roof's drip line first, as we are applying the mix to the roof, the ground guy keeps watering below me and when we are finished cleaning the roof, everything get's a final rinse. Irrigate all vegetation before starting the job, keep water on all vegetation during the job and give all vegetation a good rinse afterwards. We have not lost one plant, shrub or blade of grass implementing this method. Also, do not stop watering until the mix stops dripping from the roofs edge.
Tip: make sure your ground guy pays close attention to overspray and put water on those areas. A good ground guy can save you a lot of time, money and headaches!