You step outside one morning and notice it. A green haze creeping up the shady side of the house. Dark speckles under the eaves. A slimy streak along the north wall that was not there last summer. Most homeowners assume it is just dirt, but it is almost never just dirt. What you are looking at is a living layer of mildew, algae, moss, or lichen, and each one behaves differently, damages differently, and needs a different approach to remove. This is one reason professional power washing services do more than blast water at a wall. Proper exterior cleaning starts with knowing exactly what is growing on your home and treating it accordingly.
Here is a breakdown of the usual suspects, how to identify them, why they show up where they do, and what actually works to get rid of them.
Algae: The Green (or Black) Streak Maker
Algae is the most common culprit behind broad green or dark staining on siding. The strain most often found on homes is Gloeocapsa magma, the same organism responsible for the black streaks that run down asphalt roofs. On siding, it usually shows up as a green, gray-green, or charcoal-colored film that spreads slowly across shaded walls.
A few things make algae happy: moisture, shade, and a food source. Dust, pollen, and organic debris that settle on your siding give algae everything it needs to colonize. That is why you will almost always find it first on the north-facing side of a house, under tree cover, or behind shrubbery where the sun rarely dries the wall.
The good news is that algae is mostly a cosmetic problem. The bad news is that it does not stay cosmetic forever. Left alone, it holds moisture against the siding, which accelerates paint failure on wood and can etch stains into vinyl and painted surfaces over time. It also keeps coming back, because cleaning the visible stain without killing the organism leaves spores behind to regrow within a season.
Mildew: The Fuzzy Gray-Black Patch
Mildew is a surface fungus, and it looks different from algae. Instead of a broad even film, mildew tends to grow in patchy spots that are gray, black, or brownish with a slightly fuzzy or powdery texture. You will find it where water lingers: behind shutters, under window sills, along the bottom edges of siding panels, and anywhere gutters overflow or splash back onto the wall.
Unlike algae, mildew is more than an eyesore. It is a moisture indicator. If mildew keeps returning to the same spot on your siding, something is feeding it, and that something is usually a drainage problem: a leaking gutter joint, a downspout discharging too close to the wall, poor grading, or dense vegetation trapping humidity against the house.
A simple field test can help you tell mildew from plain dirt. Dab a little household bleach diluted with water on the stain, wait a minute, and rinse. If the spot lightens dramatically, it is organic growth, most likely mildew or algae. If nothing changes, it is likely dirt, pollution staining, or oxidation on the siding surface itself.
Moss and Lichen: The Heavy Hitters
Moss is easy to spot. It is green, raised, spongy, and actually rooted into the surface it grows on. On siding, moss usually starts where organic debris has piled up, such as behind bushes, at the base of walls, or in seams and corners where leaves collect and stay damp.
Lichen is the tougher cousin. It looks like flat, crusty patches in gray, green, or even pale orange, and it anchors itself firmly to the surface. Lichen is notoriously stubborn. Scrubbing it off by hand often damages the finish underneath, because the organism is bonded to the paint or the siding material itself.
Both moss and lichen are signs of chronic dampness. If they are growing on your walls, the bigger question is why that area of the house stays wet long enough to support them.
Why the Shady Side of the House Gets It Worst
Every one of these organisms shares the same basic requirements: moisture, shade, and nutrients. That combination explains the classic pattern homeowners notice.
- North-facing walls get the least direct sun, so they stay damp longest after rain.
- Areas under mature trees catch falling leaves and pollen while blocking the drying sun.
- Sections behind shrubs and hedges trap humidity and block airflow.
- Spots below leaky gutters or misdirected downspouts get wet far more often than the rest of the wall.
If the staining on your home follows these patterns, the growth is environmental, and it will return after cleaning unless you also address the conditions. Trimming vegetation back from the house, fixing gutter leaks, and extending downspouts can dramatically slow regrowth between cleanings.
Why Removal Method Matters More Than Most People Think
The instinct with green stains is to attack them with maximum pressure. That instinct causes a lot of avoidable damage. High-pressure washing can force water behind vinyl siding, strip paint, gouge wood, and etch softer materials. Worse, blasting algae or mildew with water alone often spreads spores rather than eliminating them, which is why the green film reappears within months.
The effective approach is the opposite of what most people expect. Professional exterior cleaning typically relies on low-pressure washing combined with the right cleaning solutions. Soft washing, for example, uses dedicated detergents and biocides applied at garden-hose pressure. The solution does the killing and lifting, and the water does only the gentle rinsing. This is the standard method for vinyl siding, painted surfaces, wood, and stucco, precisely because it removes the organism at the root without harming the surface.
Different surfaces and different organisms call for different treatments. Algae and mildew respond to surfactant and biocide mixes. Lichen needs dwell time and often a second treatment. Painted wood requires more care than vinyl. A cleaning crew that shows up with one setting and one solution for every house is a red flag, not a shortcut.
Prevention: Keeping the Green From Coming Back
Once your siding is clean, a few habits stretch the results:
- Trim trees and shrubs so sunlight and air can reach the walls.
- Fix overflowing or leaking gutters promptly.
- Keep firewood, mulch, and dense plantings away from the siding.
- Rinse or clean the shaded sides of the house more frequently than the sunny sides.
- Schedule exterior cleaning on a regular cycle, typically every one to three years depending on shade, climate, and tree cover.
Final Thoughts
That green film on your siding is not dirt, and treating it like dirt is why it keeps winning. Algae, mildew, moss, and lichen are living organisms with different causes, different risks, and different remedies. Identify what is actually growing, fix the moisture and shade conditions that invited it, and clean it with methods suited to the surface rather than brute force. Do that, and your home stays brighter, its finishes last longer, and the green stays gone instead of staging a comeback every spring.
