Why does my shaded deck or fence turn green and black, and how do I get rid of it?

What you are looking at

The green film that makes a shaded deck slick in the morning is algae. It lives on the surface, feeds on dust and pollen, and wipes off easily when wet. The black speckles and smudges are mildew, a mold that roots into the top fibers of the wood or into a finish and is harder to lift. Moss shows up in the gaps and on the north face of fence boards, and old fences grow pale gray-green lichen, which is slow and mostly harmless.

None of these are rot. They do not eat sound wood. What they do is tell you the wood is staying wet long enough for things to grow, and that is exactly the condition that lets rot start.

Why shade near the coast grows it

In North County the marine layer settles overnight and burns off from the inland side first, so coastal yards can stay damp until late morning for months of the year. Wood that also sits in shade never gets the few hours of direct sun it needs to dry. Add the organic dust that trees drop, a hedge pressed against the fence that blocks air, or a planter that never moves, and you have a surface that is damp and fed every day.

That is why the same fence can be clean on the south face and black on the north, and why the boards under the dining table look different from the boards in the open.

Cleaning it off

Mix an oxygen-bleach wood cleaner as the label directs, wet the surface, apply the cleaner and let it work for ten or fifteen minutes without drying out. Scrub with a stiff synthetic brush along the grain, then rinse thoroughly. Heavy mildew may take a second round. On gray wood, follow with an oxalic acid brightener to restore color and neutralize the cleaner.

Skip chlorine bleach. It kills the growth but damages the wood fibers, corrodes hardware and burns the plants that are usually right next to a shaded fence. Be careful on the deck while it is wet; algae is slippery before it is gone. Composite decking grows mildew in its texture too and takes the same cleaner and a medium brush.

Finishing so it stays away

Clean wood with no finish will grow mildew again within a season in a shaded coastal spot. Once it has dried for two or three days, apply a penetrating oil or stain that lists a mildewcide among its ingredients. Penetrating products are the right choice here because they do not form a film that can trap moisture underneath, and they can be touched up spot by spot each year.

Film-forming finishes in deep shade tend to peel from the back as moisture works under them, and then mildew grows behind the film where you cannot clean it.

Changing the conditions

Cleaning treats the symptom. The fix is more sun, more air and less water. Prune branches that shade the deck in the morning. Cut the hedge back so there is a hand's width of air between it and the fence. Lift planters onto feet and move them around. Redirect the sprinkler head that wets the post every morning, and extend the downspout that splashes on the boards.

You will not change the marine layer, but even a small improvement in airflow shortens the hours the wood stays wet, and that is usually enough to keep growth to a quick spring cleaning instead of a constant battle.

• Prune for morning sun on the shaded side.
• Keep plants and hedges off the wood.
• Lift planters, rugs and furniture so air moves underneath.
• Fix sprinklers, hose bibs and downspouts that wet the wood.

When black means more than mildew

After cleaning, press a screwdriver into any area that was heavily blackened, especially the bottom of fence boards, post bases and the ends of deck boards. If the wood is firm, you had a surface problem and it is solved. If the tip sinks in or the wood is spongy and dark inside, the dampness has already gone further than mildew and that piece needs repair rather than cleaning.

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