Remediating wood can be difficult, particularly if it’s unsealed. Like a weed, mold grows roots called hyphae. Thanks to the porosity of wood, those moldy roots can grow into the surface and require some elbow grease to get rid of.
Oftentimes, when a deck turns green, it means that mold has settled in due to high moisture in the structure. This isn’t ideal because as the mold grows, it reproduces by releasing microscopic spores into the air. Some species of mold also produce microscopic toxins called mycotoxins when threatened. A colony near a door or window that leads into the home means a higher number of particles will be making their way inside, decreasing the indoor air quality, contaminating any surfaces, and increasing the likelihood of a colony developing in the home.
Typically, mold spores only need two major components to transition into a living colony: a food source and a moisture source. Wood, glue, or other organic particles on the deck itself offer perfect edible options. Most woods are porous or semi-porous, which means that water can get trapped and build up within these small pockets.
Read the full article here: https://www.rmfp.com/blog/2022/10/11/why-is-my-wood-deck-turning-green/