The green film on your siding is algae, it always colonises the north side first, and the reason is sunlight — or the lack of it. Understanding why it grows where it grows tells you a lot about how house washing keeps it away.
Shade, moisture and why north matters
Algae needs moisture to establish. A wall that dries quickly after rain or dew is hostile territory; a wall that stays damp for hours is a habitat. North-facing walls in this hemisphere get the least direct sun, dry slowest, and so green up first — followed by any wall shaded by trees, and the strips behind shrubs where air moves poorly.
This is why two identical houses on the same street can look completely different. It is not the siding, and it is not the owner. It is orientation and tree cover.
Underneath all of it sits one variable: drying time. Three things set it — hours of direct sun, how freely air moves across the wall, and how much water reaches it that did not come from the sky. Sprinkler overspray, a downspout dumping at the foundation, a gutter overflowing at one corner. Each turns a stretch of wall into permanently damp ground, and the growth pattern shows you where.
Dew does more work than rain
Rain arrives and leaves. Dew arrives most clear nights and sits until the sun reaches it, which on a north wall means it never leaves, it only thins. That nightly cycle, not the occasional storm, keeps a shaded wall wet enough for a colony to hold. Walk the house at eight in the morning: the walls still visibly damp are the ones that will green up next.
What the colony costs you, and what each wall will tolerate
Algae is not eating your siding. It is holding water against it, and water ages exterior surfaces. On vinyl that costs appearance. On painted wood it costs paint — a coating that never dries from behind loses adhesion at board edges and around fasteners, and that is where peeling starts. Mould and mildew under the eaves work the same way: different organism, same requirement of damp that lasts.
So method follows material. What is safe on one wall is destructive on the next.
- Vinyl — takes chemistry well and force badly. Water driven upward gets under the laps and into the wall cavity, the one thing lapped siding exists to prevent. Low pressure, rinse downward, never up under the courses.
- Painted wood — the coating is the surface you are cleaning, so the wash has to be gentler than the paint. Sound paint takes a soft wash without complaint. Paint already chalking or lifting will come away, and finding that out is useful even when it is unwelcome.
- Brick — durable, but the mortar joints are the weak line. Soft old lime mortar erodes under a wand long before the brick face shows anything.
- Fibre cement — hard, stable and forgiving of chemistry, with the same caution as painted wood, since what you are cleaning is a factory finish. Cut edges and open joints let water in.
- Stucco — porous and unforgiving. Pressure turns hairline cracks into real ones and drives water into a system with no easy way to drain. Dwell time does the work; force does the damage.
Why it comes back faster after a bad wash
Pressure washing algae removes the visible growth and leaves the root structure in the siding’s texture. The colony regrows from that base within a season, which convinces homeowners that washing is pointless.
A soft wash kills the organism outright. Regrowth then has to start from new airborne spores rather than an established base, which is the difference between months and years.
You can read which one you got a year later. A pressure-washed wall greens back in the pattern it had before; a killed wall returns slowly and unevenly, from wherever new spores land. Worth understanding first: the difference between cleaning with chemistry and cleaning with force.
What a wash day actually involves
Most of the work is preparation and rinsing; the cleaning is the quick part. Landscaping gets wet before anything goes on the wall. Pre-wetting is not a courtesy gesture — plant damage from wash chemistry is a concentration problem, and foliage already saturated cannot take up much of anything else. The beds get rinsed again at the end. Plants that come through a wash badly are almost always plants that went into it dry.
Windows and screens are the other predictable complaint. Rinse water carries dissolved minerals; left to dry on glass in the sun it spots, and screen mesh holds it until it dries as a grey haze. The fix is sequence — work the elevations the sun is off, rinse glass last, keep it wet enough to sheet rather than bead.
Then the fittings. Dryer vents and exhaust hoods are open holes into the house and get covered or worked around. Meter boxes, disconnects and exterior outlets are electrical and get treated as electrical. Light fittings fill with water, hold it, and deliver it to the wiring weeks later. None of it is difficult — it is the difference between washing a house and spraying one.
Gutter faces are cleaned on the same visit, but the inside of the trough is a separate job. Gutter clearing is about where water goes, and a gutter overflowing onto a shaded wall undoes a wash faster than anything else on the property.
What a wash cannot fix
A wash removes what is growing on a surface. It does not repair the surface, and that is the honest ceiling.
Oxidised vinyl is the most common disappointment. Years of UV break down the outer skin of the panel and leave a chalky layer — run a hand down the wall and it comes back white. Cleaning takes off the growth and loose chalk and leaves a flat, faded panel. Better, genuinely. New, no.
Failed caulk and failed paint are the same story told twice. A wash finds them; it does not fix them. Split caulk means water was already getting behind the trim; the wash only makes it visible.
Staining under the surface stays under the surface — rust bled through from a fastener, tannin from oak leaves that lay wet against a board. Those are in the material rather than on it. So are artillery fungus spots thrown up out of mulch beds, which are embedded and come off one at a time or not at all.
Southeast Missouri gives algae almost everything it wants
Humid summers, mature oak and pine cover on most older lots, and river-valley fog that settles into the low ground and stays past mid-morning. A wall in that fog is wet long after a wall on a ridge has dried, and growth follows wetness exactly.
Spring compounds it. Heavy pollen and oak drop coat every elevation, then rain glues the film to the wall and hands the colony a moisture-holding layer. A house under a big oak in Marble Hill or Patton shows a green tint by midsummer that a house on an open lot past Sikeston will not show at all.
Water source changes the finish as much as the cleaning does. Iron-rich well water is normal across the rural parts of the service area, and rinse water carrying iron spots glass and leaves a faint orange cast on pale siding if it dries there. That is a sequencing problem, and it is why rinse discipline matters more here than on soft municipal supply.
Freeze-thaw sets the other end of the calendar. Solution and rinse water both need surfaces above freezing to behave predictably, which closes the window through deep winter — and road salt thrown onto elevations along the state routes gives spring washing a second job.
Slowing it down between washes
Trim vegetation back from the walls so air can move and the surface can dry. Redirect sprinklers that hit siding — irrigation water on a shaded wall is an algae subsidy. And accept the geometry: a north wall under oaks will always need attention more often than a south wall in full sun.
Annual washing keeps most homes ahead of the growth. Heavy shade sometimes justifies twice a year, and that is a property fact rather than a failure. To work out which one you own, how often a house here actually needs washing goes through the signals worth watching.
While you are at that elevation, look up. The roof slope above runs the same conditions, and the black streaking on north-facing shingles is a different organism doing the same thing for the same reason.
Common questions
If only the north wall is green, can I just wash that one?
You can, and on a large house with one problem elevation it is sometimes the sensible call. Two catches: the other walls carry early growth you cannot see yet, and one cleaned elevation next to three dirty ones makes the rest look worse.
Does green siding mean there is mould inside the wall?
No. Surface algae lives on the outside face and needs light. What it tells you is that the wall stays damp, and persistent damp is worth tracing — usually to a gutter, a sprinkler or vegetation, all fixable.
Can I do this myself with a bleach mix from the hardware store?
On a single-storey wall you can reach from the ground, carefully, yes. What goes wrong is dilution and dwell — too strong burns plants and marks trim, too weak looks like it worked until the colony regrows from roots that never died. Ladders are where DIY stops being worth it.
Will the wash kill my shrubs and flower beds?
Not if the beds are saturated before work starts and rinsed again after. Damage happens on dry plants, or where run-off pools. Point out anything fragile or newly planted so it can be covered rather than merely wetted.
If the north wall has gone green, the rest is coming. A photo gets you a quote the same day. Read more about house washing →