Why Is There Green Algae on My Siding (Usually Only on One Side)?

Why it grows on one side

In the Northern Hemisphere, north-facing walls get the least direct sun. They stay damp longer after rain, dew and humid nights, which is exactly what algae and mildew need. Walls shaded by trees, fences or a neighboring house behave the same way. Sunny south and west walls dry quickly, so growth struggles to take hold there.

What else feeds it

• Sprinklers hitting the siding
• Overflowing or clogged gutters
• Landscaping and shrubs pressed against the house
• Pollen and dirt that give spores something to grow on

Is it harmful?

It's mostly cosmetic at first, but left alone it spreads, can stain lighter siding, and makes surfaces slippery on steps and walkways. It also makes a well-kept home look neglected.

How to remove it safely

Scrubbing or blasting it with a pressure washer removes the visible layer but not the roots, and high pressure can force water behind vinyl siding. Soft washing applies a cleaning solution at low pressure that kills the algae and mildew, then rinses it away gently, so it stays gone longer.

How to slow it down

• Trim shrubs and branches back from the house
• Adjust sprinklers away from the siding
• Keep gutters clear and flowing
• Schedule a house wash every 1–2 years on shaded homes