Green walls, black streaks, and a house that seems to grow them faster than the one across the street. The usual explanation is Tauranga’s rainfall.

That’s the wrong place to start. Water arrives here from the air as much as from the sky, and the city’s humidity decides how long it stays on your walls. Sun doesn’t prevent it either, which is the part most homeowners never hear.

Here’s how it actually works, and why some Tauranga sections grow it and others don’t.

Tauranga’s problem isn’t rain. It’s drying

Rain falls, then the surface dries. In Tauranga the second half of that is the problem.

Humid air holds a lot of water already, which leaves it very little capacity to take up more. Tauranga’s mean morning relative humidity sits at 78.3%, based on NIWA data from 1991 to 2020. Air that full absorbs moisture off a wall extremely slowly.

So the wall stays damp long after the rain has stopped. Not because more rain fell than somewhere else, but because the air around it can’t take the water away. A wall facing a humid day dries far more slowly than the same wall on a dry one, and most of Tauranga’s days are humid ones.

Here’s the part that catches people out. Sun and humidity pull in opposite directions, and humidity wins on a wet wall. A week of fine weather does dry a surface, but not nearly as fast as the forecast suggests.

In its 2022 climate summary, NIWA recorded Tauranga as both the wettest and the sunniest of the six main centres. Sunny and damp, in the same year and the same place.

A Tauranga wall doesn’t need rain to stay wet.

What happens on a Tauranga wall overnight

Every surface is losing heat to the sky after dark.

A roof or wall faces open sky, and on a clear night it radiates heat away faster than the surrounding air can replace it. The surface ends up colder than the air around it, and once it drops past the point that air can hold its moisture, water condenses straight onto it.

That’s dew, and on a Tauranga house it forms on roofs, cladding, window sills, fences and railings. No rain involved.

Cloud does the opposite. Overcast nights trap heat and keep surfaces warmer, so less water condenses. Clear nights give surfaces nothing, so they cool further and the dew is heaviest.

So the weather that looks best for your house produces the most water on it. Fine days bring clear nights. Clear nights bring the wettest mornings.

The surface is wet every night before the sun is up, and then faces Tauranga’s humid morning air, which is in no hurry to dry it. Two mechanisms pushing the same way, most days of the year.

Dew shows up first on roofs, on walls in shade, under eaves and on anything sitting still — the surfaces morning sun reaches last.

Why winter doesn’t give Tauranga houses a break

In colder parts of the country, winter does some of the maintenance for you. Cold slows biological growth, and cooler air holds less moisture. Growth that was building through summer stalls, and some of it dies back. The house effectively gets a reset.

Tauranga doesn’t get one.

Winters here are mild, so growth doesn’t stop. It slows a little and continues. There’s no cold enough spell to knock it back, which means whatever established over summer keeps developing through the season.

And winter is the most humid part of the year here. The air carries more moisture in June than it does in January, so the drying problem is at its worst exactly when a cold snap would otherwise be doing some good.

The two effects work against you. The season that acts as a break in colder places is the season your surfaces stay wettest longest.

Growth accumulates across twelve months instead of resetting. A colder climate takes something back each winter. This one doesn’t.

There’s no season here that cleans the house for you.

Why the black is different from the green

Green and black aren’t one problem at different ages. They’re two organisms, and they succeed in opposite places.

Green algae needs moisture and shade. It grows where the surface stays damp and out of direct sun — the sheltered side of a house, walls under trees, anything below an eave.

The growth behind most black staining works the other way round. It’s a cyanobacteria, Gloeocapsa magma, and it makes its own protection instead of avoiding the sun. It produces a dark sheath that shields it from UV, and that sheath is the black you’re looking at.

So black growth doesn’t need the shade green algae needs. A sun-exposed wall is a poor site for green algae and a perfectly good one for this.

That’s the pattern you’ve probably noticed without knowing why. Green on the shaded side, black on the sunny side of the same house. It looks like one problem getting worse. It’s two organisms each taking the side that suits them.

Practical difference: that sheath is what resists the cleaning that works on green growth. A shaded wall and a sun-exposed wall aren’t the same job.

The black is there because it built its own shade.

Why your house and not the one next door

Same city, same weather, different result. What differs is how well each wall gets rid of water.

Airflow decides how fast that happens. A section with a breeze running through it dries surfaces quickly. A wall hemmed in by a fence, a hedge or a neighbouring building sits in still air, and the damp lingers.

Tree cover does two things at once. Overhanging branches shade the wall and drop debris onto it, and planting hard against the house slows airflow there. Both keep the surface wet for longer.

Orientation then decides which walls recover. Some get sun for most of the day and dry out between wettings. Others sit in morning shade and are still damp when the next evening’s dew lands on them.

Then there’s what covers the wall. A deep eave keeps rain off, and it keeps the wall dark and still too. Shelter from rain isn’t shelter from water. A wall under a generous eave can be the wettest on the house — nothing washes it and nothing dries it.

All four are visible from your own driveway.

The city decides whether a house can grow it. The section decides whether it does.

Why it came back after you had it cleaned

Nothing about the surface changed. It still gets wet every night and still sits in the same air.

Cleaning removes growth. It doesn’t change what the wall does. So the growth that arrives now comes from the air rather than from anything left behind.

Removing and killing are different jobs, which is where most of the frustration comes from. Washing takes the visible growth off. A treatment works on the organism itself, so the surface has to be recolonised rather than growing straight back. That’s why growth blasted off a wall resets the clock instead of stopping it, and why the shaded side returns first — it never dried in the first place.

It also explains an uneven result. A clean done before a humid spell looks worse by the end of it than one done in a run of dry weather, without anything being wrong either time. Same work, different drying.

The honest answer for a Tauranga exterior is that this isn’t a one-off job. The conditions are permanent. Keeping on top of it is a schedule, not an event.

Tauranga mould, algae and black stains: quick answers

If you’re not sure which side of your house you’re dealing with, send us a photo and we’ll tell you what you’ve got.