What causes green algae to grow on my deck in New Brunswick?

Green algae thrives on New Brunswick decks because the province's Maritime climate delivers the exact combination of persistent humidity, moderate temperatures, and frequent rain that algae needs to colonize wood and composite surfaces. It is one of the most common deck maintenance issues across the province, from the Bay of Fundy coast to the Saint John River valley.

Algae is a simple plant organism that reproduces through airborne spores. Those spores are everywhere, and they settle on every outdoor surface continuously. The difference between a deck that stays clean and one that turns green comes down to moisture and shade. New Brunswick's average relative humidity sits between 70 and 85 percent for much of the year, and the province receives approximately 1,100 mm of annual precipitation spread fairly evenly across all seasons. A deck surface that stays damp for extended periods after rain gives algae spores the sustained moisture they need to germinate and establish colonies.

Shade is the other major factor. A deck surrounded by mature trees or positioned on the north side of a house receives limited direct sunlight, which means the surface dries slowly after rain and morning dew. UV radiation from direct sunlight also kills algae naturally, so shaded decks lose both the drying benefit and the sterilizing effect of the sun. Decks in wooded areas along the Miramichi, Kennebecasis, or Petitcodiac river valleys are especially prone because the tree canopy blocks sunlight while simultaneously dropping organic debris that feeds algae growth.

The surface texture of your decking material matters as well. Rough-sawn or heavily weathered pressure-treated lumber has microscopic grooves and open wood grain that trap moisture and give algae something to grip. Smooth composite decking resists algae better, but the capped surface can still develop a biofilm layer, especially in persistently shaded areas. Horizontal surfaces are always worse than vertical ones because water sits on them longer.

To remove existing algae, mix an oxygen bleach solution at the concentration recommended on the product label and apply it generously to the affected deck surface. Let it work for 10 to 15 minutes, then scrub with a stiff deck brush and rinse with a pressure washer set to no more than 1,500 PSI. Higher pressures will damage the wood fibres and actually create more texture for future algae to grip. Oxygen bleach is the preferred cleaner because it kills algae effectively without harming nearby plants, whereas chlorine bleach damages vegetation, discolours wood, and breaks down the lignin that holds wood fibres together.

To prevent algae from returning, improve air circulation around the deck by trimming overhanging branches to allow more sunlight and airflow across the surface. Ensure the gap between the bottom of your deck boards and the ground is at least 18 inches for adequate ventilation underneath. Apply a deck stain or sealer with built-in mildewcide every 2 to 3 years, choosing a product rated for high-humidity environments. Keeping the deck clear of leaf litter and organic debris, especially in fall before the wet season intensifies, removes the nutrient base that algae feeds on.

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