How to Remove Green Algae Stains from a Wood Deck

Have green stains on a wood deck? Remove them using simple, effective methods. Below you’ll find step-by-step guidance on how to clean a deck and which common cleaning products work well to keep your decking looking fresh.

before and after of cleaning green stains on wood deck.

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If you live in a humid area and own a wood deck, you’ve probably seen this problem: the wood turns green in patches. It looks unpleasant and can feel a bit unsafe underfoot, but the good news is that cleaning it is straightforward. With a little preparation and the right products, you can restore your deck and keep it maintained with minimal effort.

Wood deck with green algae stains and outdoor furniture on a sunny day.

About a year after we moved into our home, our deck started looking like the photo above. I try to keep up with an outdoor cleaning schedule, but when algae and mildew take hold, sweeping alone isn’t enough. The deck looked messy no matter how often I tidied it, and we stopped using it for outdoor time — which was frustrating.

Why is my deck turning green?

The short answer: moisture. The green stains on wooden decks are typically fungi, mildew, mold, or algae that thrive in damp, shaded environments. Humid climates make this problem worse, and moisture trapped under leaves, furniture, or rugs accelerates growth.

We live in Mississippi, where humidity is high and trees drop pine straw and leaves onto our deck. That combination creates the perfect environment for green algae and mildew. Beyond appearance, mold spores can pose health concerns, so it’s worth treating the deck promptly.

How to Prepare Your Deck for Cleaning

A few quick prep steps will help your cleaning be more effective and faster.

Remove patio furniture and planters.
Sweep away pine straw, leaves, and larger debris.
Use a blower to remove dust and small debris from cracks and crevices.
Rinse the deck with clean water and allow it to dry before applying most treatments.

Quick Results Method: OxiClean

If you need immediate improvement, an OxiClean solution is a fast, effective option to lift algae and mildew stains from wood surfaces.

Supplies Needed

OxiClean powder
A 2-gallon (or larger) bucket
A big, sturdy push broom
A hand-held scrub brush for railings and cracks

Hand scrubbing green stain off wooden deck with soap and water.

Mix 1 cup of OxiClean (powder) into 1 gallon of water. Work in manageable sections: pour a small amount of the mixture onto the deck and scrub vigorously with the push broom. Use the hand-held brush to reach along railings and between deck boards. As the stains loosen, rinse thoroughly with a garden hose until residue is gone.

Cleaning a wooden deck with a pressure washer to remove green stains.

This method delivers quick, visible results and can dramatically improve the look of your deck within a single session. However, those results may be temporary; in humid areas algae can return in a few weeks.

Wood deck with green algae and moss stains from outdoor moisture.

Best use case: Use OxiClean when you want the deck to look great immediately for a gathering or before applying a long-term treatment.

Long-Term Method: Wet & Forget

For a longer-lasting solution that requires minimal effort, consider a product designed to treat mold, mildew, moss, and algae and to keep them from returning for months.

Wet & Forget moss mold mildew algae stain remover for outdoor surfaces.

Supplies Needed

Wet & Forget or a similar outdoor stain remover
An attachable garden sprayer or a garden spray bottle

Sprayer use: If your sprayer allows, set the dilution to about 8 oz per gallon. With a simple spray bottle, mix 1 part Wet & Forget to 5 parts water and set the nozzle to a wide fan pattern for even coverage.

Apply the solution on a dry, cloudy day when no rain is expected for at least 12–48 hours. Spray the dry deck evenly and then leave the product to work—do not rinse. The cleaner activates when it gets wet, so a few rain showers will gradually lift and remove the green growth over the following weeks.

This method takes less than five minutes to apply and typically keeps a deck cleaner for months. I usually reapply two to three times a year for ongoing protection.

Note: Wet & Forget does not remove heavy stains instantly. If you want immediate brightness and longer-term protection, use the OxiClean method to remove surface stains, then follow up with Wet & Forget to maintain results.

Before Using Wet and Forget

After Using Wet and Forget

Clean wooden deck with green stains and weathered planks.

After a few rain cycles and the product doing its work, our deck looked like new again. Wet & Forget is also effective on patios, driveways, and other outdoor hard surfaces for algae removal.

How NOT to Clean a Pressure-Treated Wood Deck

Avoid the following techniques, which can damage wood or give disappointing results:

Skip the pressure washer: High-pressure water can gouge, splinter, or remove wood fibers and finish, permanently damaging deck boards.
Avoid household chlorine bleach: Regular bleach can strip natural wood color, degrade fibers over time, and leave the surface dull and brittle.
Redwood caution: Do not use OxiClean on redwood. OxiClean can react with redwood tannins and oils, causing blotchy discoloration and uneven fading.

Wood Deck Maintenance

Use Wet & Forget regularly: Reapply every 4–6 months or seasonally to keep algae and mildew under control.
Keep the deck clear: Sweep leaves, pine needles, and debris regularly so moisture doesn’t become trapped.
Move furniture and planters: Shift items occasionally to prevent hidden buildup and trapped dampness underneath.
Eliminate standing water: Use a broom, mop, or blower to remove puddles after rain so the surface dries quickly.
Maximize sunlight: Trim back overhanging branches and position furniture to allow more sunlight exposure, which helps discourage algae growth.

FAQs

Does vinegar kill algae on decks?

Vinegar can help reduce mold and mildew in some cases, but most household vinegar is not strong enough to reliably eliminate stubborn outdoor algae. Results are often temporary, so vinegar is not usually the best long-term solution.

Can I use Wet & Forget on a stained or painted deck?

Yes. Wet & Forget is generally safe for use on stained and painted decks and is effective at removing surface mold and mildew without stripping stain when used as directed.

Can you stain or paint over algae on wood decking?

It’s best to clean the deck thoroughly before staining or painting. Treat the algae with one of the cleaning methods above, allow the wood to dry and weather briefly if needed, then proceed with staining or painting for even adhesion and a better finish.