If you've noticed dark streaks running down your vinyl siding — usually starting near the roofline and worst on north-facing or shaded walls — you're looking at algae, not dirt or mold in most cases. Here's what it is and what actually removes it.
What's Actually Causing the Streaks
Gloeocapsa magma is a blue-green algae that thrives in humid, shaded conditions — exactly what Indianapolis summers provide. It appears as dark streaking because the algae produces a dark, UV-protective coating as it grows. It spreads via airborne spores, which is why it often shows up on multiple homes in the same neighborhood around the same time, especially in areas with mature tree cover.
Why Scrubbing Doesn't Really Fix It
Scrubbing with a brush and soapy water can remove the visible surface layer, but algae has root-like structures that anchor into the microscopic texture of vinyl siding. Without killing those roots, the algae regrows — often within weeks — which is why DIY scrubbing tends to feel like a losing battle that needs repeating constantly through the summer.
Why Pressure Alone Isn't the Answer Either
It's tempting to think more force solves the problem, but high pressure on vinyl siding risks driving water behind panels and into the wall cavity, plus it can crack or warp siding, especially older or sun-brittled vinyl. Pressure can knock the visible algae off without killing the roots underneath — so it looks clean immediately but regrows fast. This is the same reasoning we cover in pressure washing vs. soft washing.
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The Soft Washing Process That Actually Works
Professional soft washing applies a low-pressure cleaning solution specifically formulated to kill algae, mold, and mildew at the root, then rinses at low pressure once the solution has had time to work. Because the algae's roots are killed, not just knocked loose, soft-washed siding stays cleaner significantly longer than a scrubbed or pressure-blasted surface — often a full season or more longer.
Why It Keeps Coming Back for Some Homes
Homes with heavy shade, mature trees, or a north-facing wall that rarely gets direct sun are naturally more prone to regrowth, regardless of cleaning method — the humidity and lack of UV exposure just favor algae. If this describes your home, an annual soft wash (rather than waiting until streaking is severe) keeps it from ever getting bad enough to notice. We see this pattern often in shaded, established neighborhoods throughout Speedway and similar areas.
Getting It Handled
If scrubbing hasn't worked or keeps coming back, that's the clearest sign it's time for a proper soft washing rather than another round of manual effort. We treat the algae at the root the first time, so you're not fighting the same streaks again by midsummer.