By the EverGlow Exteriors Team

The green film on the north side of your house is algae — living organisms that grow wherever siding stays damp and shaded, and the north wall of an Iowa home is the one face the sun never fully dries. It’s the same reason moss grows on the north side of trees. It’s not dirt, it’s not paint failure, and scrubbing the surface won’t keep it gone — you have to kill it.

Here’s what it actually is, whether it’s damaging your siding, and how to get rid of it in a way that lasts.

What exactly is growing on my siding?

Usually a mix. Green algae is the most visible — a flat green film or streaking that starts near the ground, under eaves, and behind shrubs, then spreads up the wall. Mildew shows up as gray-to-black spotting. And the same family of organisms behind roof staining — Gloeocapsa magna, the cyanobacteria that causes black streaks on roofs — thrives in the same shaded, damp conditions.

All of them need the same three things: moisture, shade, and something to cling to. Your north wall offers all three, every day.

Why Iowa homes get it worse

• Humid summers. Central Iowa summers keep dew on shaded walls well into the morning. That daily wet-dry cycle is an algae buffet.
• Mature tree cover. Shaded streets in Urbandale, Clive’s Greenbelt corridor, Beaverdale — beautiful canopy, permanent shade, faster growth. Trees also drip organic material that feeds the film.
• Sprinklers. Irrigation overspray hitting the bottom three feet of siding keeps it wet on a schedule. (It also leaves hard-water spots on windows — a separate headache.)
• Freeze-thaw doesn’t kill it. Algae goes dormant over winter and wakes up in spring. Meanwhile the moisture it holds against your siding freezes and thaws all season.

Is it actually damaging my house?

Slowly, yes. Algae itself doesn’t eat vinyl, but it holds a film of moisture against the wall through every freeze-thaw cycle, it feeds mildew that can work into paint and wood trim, and over years it permanently stains lighter siding. On painted surfaces it shortens the repaint cycle. And it never gets better on its own — the colony spreads.

The bigger near-term cost is what it does to the house’s appearance. A green north wall reads as neglect from the street, and if you’re selling, buyers see it before they see anything you fixed inside. Curb appeal is worth real money at sale — up to about 7% by some real estate estimates — and this is the cheapest piece of it to fix.

How do I get rid of it — and make it stay gone?

Don’t pressure wash it

Blasting a green wall makes it look better for a few months, but pressure only removes the surface layer — the organism survives and regrows. Worse, high pressure on siding cracks panels and drives water behind them, where Iowa winters turn it into real damage. Full breakdown: soft washing vs. pressure washing.

Soft wash it instead

A soft wash house washing applies a low-pressure cleaning solution that kills the algae, mildew, and cyanobacteria at the root, then rinses the dead growth away. Because the organism is dead rather than trimmed, north walls typically stay clean 1-2 years even in shade — versus a few months for blast-only cleaning.

Slow the regrowth

• Trim shrubs and trees back from the wall so air and light reach the siding
• Aim sprinkler heads away from the house
• Keep gutters flowing — overflow streaks down walls and feeds growth (gutter cleaning helps more than people expect)
• Rewash on a schedule instead of waiting for full green — lighter growth means gentler, cheaper cleaning; it’s the core of our seasonal plans

Is it a health concern?

Outdoor algae on siding is mostly a building problem, not a medical one — it’s not the toxic indoor mold that makes headlines. That said, heavy exterior growth is a signal worth reading: it means that wall stays chronically damp. Persistent moisture at the siding can eventually mean moisture in the wall assembly, and mildew colonies on the exterior shed spores right next to windows and intake vents. Allergy-prone households notice. Cleaning the growth and fixing the moisture pattern (trimming, sprinklers, gutters) addresses the cause, not just the color.

Can I DIY it?

On a single-story wall: maybe. Garden-hose siding cleaners plus a soft brush can knock back light growth for a season. The limits show up fast — you can’t get proper solution coverage or rinse above the first story without equipment, and working a ladder over shrubs against a slick, soapy wall is exactly how homeowners get hurt. A rental pressure washer (about $90/day) is the wrong tool entirely; see above.

If you hire it out, vet the company on two questions: do they soft wash siding (not pressure wash it), and can they send a certificate of insurance. Both answers should come easily.

The short version

Green north wall = algae living where the sun can’t dry the siding. It’s normal in Iowa, it spreads, and pressure washing is the wrong fix. A soft wash kills it for a year or two; trimming, sprinkler tweaks, and clean gutters stretch that further. We soft wash north walls every week across Urbandale, Clive, Johnston, and the rest of the metro — send a photo of yours and we’ll quote it sight unseen.