On almost every home we paint the exterior of, I can spot either mold, mildew or both. When repainting the exterior of your home it is very important to completely eradicate mold and mildew. If you don’t it can continue to grow under the new layers of paint.
When we prepare a home we use pretty strong chemicals that will require the home to then be painted. Even if you are not painting your home you should wash it at least twice a year and make sure to use the right chemical to remove dirt, mold and mildew.
There are several products available at all the big box stores. Whether or not it says it is safe around plants I would suggest you still cover them. You can make a pretty good cleaner yourself by taking 3 quarts water and adding 1 quart bleach and 1/2 cup TSP or washing soda. TSP is trisodium phosphate.
Wet the surfaces to be cleaned and them apply the cleaning mixture with a pump sprayer or with a power washer if it has a cleaner suction hose. Let it stand for several minutes and then rinse well.
Sometimes when you spray the cleaner on your siding you can actually see the mold, mildew degrade and run down the siding like mascara and tears. There has been times when I didn’t see any mold until the cleaner hit it.
Eradicate mold and mildew twice a year to keep your home, and your paint job, looking great. It will lengthen the life of the paint job.