Sunlight

• Algae grows best in direct sunlight, as any homeowner with a backyard pond will attest. For this reason you may find that algae grows best on the side of your house that gets the most sun. This is not always the cause, however, as algae also requires moist conditions for reproduction and survival. Algae can readily grow in lesser quantities in almost complete shade.
  
  

Dampness

• Algae will most readily grow on vinyl siding with a good mix of sunlight and constant moisture. This opens up more heavily shaded areas with only minimal sunlight to algae growth. Portions of outside walls that are consistently sprayed by sprinklers or kept moist by densely overgrown trees and shrubs are excellent places for algae to latch on to your siding.
  
  

Sugars

• Trees and shrubs that are located near to exterior vinyl walls will often drip sap on to the vinyl. This sap is a welcome source of sugars and nutrients for algae, and can help give it a foothold on your home. Prune back trees and shrubs so that they do not come in contact with your walls, and rainwater that reaches the siding does not filter through their bows.
  
  

Dirt Accumulation

• Algae needs a place for moisture to accumulate in order to gain a foothold on your siding. Vinyl siding that is left dirty, or has moss, mold or mildew growing on it, provides an excellent opening for algae. Keep your siding clean by pressure washing it regularly, and wiping down any parts that get dirty during yard work or construction projects.