A major factor that leads wood fences to turn green is moisture. When a wood fence is constantly exposed to moisture from rain, dew, sprinklers, wet soil, etc., the wood surface becomes an ideal environment for green algae, mold, and mildew to grow.
These organisms thrive on damp wood, using the cellulose and lignin in the wood as an energy source. As they spread across the surface of the fence, they impart their distinctive greenish color.
So why are wood fences so prone to algae and mold growth? The porous, absorbent nature of wood means moisture is easily soaked into the surface fibers. And fences have ample horizontal surfaces sunlight to fuel photosynthesis.
Controlling moisture is key to preventing the growth of green microbes. Ensuring proper drainage, reducing frequency of fence wetting, and allowing sunlight exposure helps keep wood fences drier.
Direct Sunlight Breaks Down Interior Brown Lignin
Another contributor to the green patina is ultraviolet (UV) radiation from sunlight. The lignin inside wood has a brown color that gives wood much of its appearance.
But over years of sun exposure, the lignin breaks down, causing color loss. This leaves behind the still-intact white cellulose, letting the white underlayer show through for a lighter, grayish appearance.
Any green algae or mold on the surface then stands out vividly against the faded background. Without the richer brown lignin, the wood’s color balance shifts to accentuate traces of green.
Fence orientation and protective stains/sealants can help slow the UV damage from sunlight. But some fading is inevitable with time.
Copper or Zinc In Hardware Leads to Blue-Green Copper Carbonate
Another potential source of blue-green discoloration comes from the metal hardware in wood fences. Galvanized screws or nails – with a zinc coating – or copper fittings will react with fence wood over time.
The metal ions interact with compounds in the wood, going through a chemical conversion resulting in blue or blue-green copper carbonate or zinc carbonate.
As moisture spreads the colored compound from the hardware into surrounding wood, runoff patterns of vivid greenish staining can occur underneath or around metal fixtures.
Avoiding direct metal contact with pressure-treated lumber or using stainless steel hardware reduces the likelihood of these reactions.
So while a number of factors may be at play, the primary cause of green wood fences comes down to simple organism growth enabled by ample moisture. Manage wetness levels through cleaning routines, maintenance repairs, or protective treatments – and a wood fence can maintain its natural woody hues longer before giving way to green.
What To Do When Wood Fence Turns Green
Once a wood fence has turned green, is there anything you can do about it? Thankfully, yes – you have a few options to potentially restore or renew the appearance of the wood.
Clean With Bleach, Borax, or Hydrogen Peroxide
Using a strong bleach, borax, or hydrogen peroxide solution, thoroughly clean the wood to kill and remove as much of the green growth as possible. This is especially effective on green algae and can restore surface appearance for some period of time.
Lightly Sand and Apply New Protective Stain/Finish
For more lasting effect, lightly sanding down the wood fibers to open the pores, followed by application of a heavy-duty stain, paint, or protective wood finish can block regrowth and lock in color. Be sure surface is fully dry before coating.
Replace Damaged Boards with New Untreated Lumber
For severely deteriorated boards with wood rot, replacing with new untreated lumber planks is necessary, as the old wood may be too far gone. This also brings back pristine surface conditions that resist organism growth.
Catching and remedying discoloration early on makes renewal easier. But even wood fences with established green patinas can often be revived through thorough cleaning, sanding, and refinishing. With some labor, that green can go away.
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