If you've noticed green patches creeping across your deck or dark black streaks along the boards, you're not imagining things, and it's not just dirt. That buildup is alive, and understanding what it is will help you understand why it keeps coming back and what you need to do to actually get rid of it.
What's Actually Growing on Your Deck?
The two most common culprits are algae (the green) and mold or mildew (the black or dark gray). Both are microorganisms that thrive in moist, shaded environments, exactly the conditions most decks provide.
- Green algae, photosynthetic organisms that colonize wood and composite surfaces. They love north-facing or shaded decks that stay moist. The growth is usually uniform and slippery underfoot.
- Black mold and mildew, fungal growth that typically shows up in streaks along the grain of wood boards or in the low spots between boards. It tends to appear after algae has already taken hold.
- Lichen and moss, less common, but older wood decks near wooded areas can develop lichen (grayish-green crusty patches) or moss, which is a sign of sustained moisture.
⚠ It Gets Worse Over Time
Algae and mold don't just look bad, they retain moisture and accelerate wood decay. Left untreated on a wood deck, they can shorten the life of your boards by years.
Why Does It Keep Coming Back?
Here's the thing most homeowners don't realize: if you just pressure wash your deck with water, the biological growth will be back within a season. That's because high-pressure water removes the visible surface growth but doesn't kill the spores embedded in the wood grain.
Algae and mold reproduce from spores. If the spores aren't killed at the cellular level, they germinate again as soon as conditions are right, which is basically every time it rains. On the South Shore, with our humid coastal climate, conditions are almost always right.
What Conditions Make It Worse?
Several factors accelerate biological growth on decks:
- Shade, decks that don't get direct sun stay moist longer after rain
- Proximity to trees, organic debris (leaves, pollen, seed pods) provides nutrients for algae
- Coastal location, salt air keeps humidity high even on dry days
- Poor drainage, boards spaced too tightly, or a slightly pitched deck that pools water
- Age of the wood, older, weathered wood is more porous and holds moisture longer
Does This Happen on Composite Decks Too?
Yes, and this surprises a lot of people who chose composite specifically to avoid maintenance. Composite decks don't rot, but their textured surface traps moisture, pollen, and organic debris, giving algae and mold exactly what they need to establish. The black streaks and green patches you see on a Trex or TimberTech deck are the same biological growth as on wood, it's just sitting on the surface rather than penetrating the material.
The good news: composite decks are actually easier to clean because the growth can't penetrate deep into the material.
How Do You Actually Fix It?
The right solution is soft washing, low pressure combined with professional-grade biodegradable cleaning solution that kills the biological growth at the cellular level. This is fundamentally different from pressure washing with plain water.
The cleaning solution we use contains sodium hypochlorite (the active ingredient) at the appropriate concentration for the surface, plus surfactants to help it penetrate and cling. It doesn't just remove what you can see, it kills the spores, which is why soft-washed decks stay clean significantly longer than pressure-washed ones.
After cleaning, a penetrating sealer on wood decks provides an additional barrier against moisture and biological growth, extending the time before you need to clean again.
Can You Prevent It?
You can slow it down:
- Keep the deck clear of leaves and debris, don't let organic material sit on the surface
- Trim nearby trees and shrubs to increase sun exposure and airflow
- Apply a quality sealer to wood decks after cleaning to reduce moisture absorption
- Clean regularly, catching early-stage growth is much easier than removing established colonies
But if you live on the South Shore near the water, you're fighting a climate that strongly favors biological growth. Regular cleaning, every 1 to 2 years, is just part of deck ownership here.