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Patio Keeps Going Green? Kill the Algae So It Stays Gone

Algae thrive on damp, shaded, porous patio stone. Scrub the surface clean, then use a leave-on algae cleaner to deal with the rest.

By Jamilur Rahman Updated November 5, 2026 · 6 min read

Time
2 hours
Cost
$20-$50
Difficulty
Easy
A concrete patio with green algae spreading across the shaded corner beside a garden wall and a push broom
In this guide
  1. Quick answer
  2. Free vs. permanent fix
  3. What you'll need
  4. Why it happens
  5. Try this first (free)
  6. When free fixes aren't enough
  7. Buy the fix
  8. What suits your surface
  9. What to skip
  10. FAQ
  11. Related problems

Quick answer

Why does my patio keep going green? Algae grow wherever concrete, pavers or stone stay damp and shaded, and a quick scrub removes only the top layer. Sweep, scrub with a stiff brush and a mild cleaner, then apply a patio algae and mildew cleaner that kills what is left. That is what keeps the green from returning in a few weeks.

Free fix vs. permanent fix

Free fix · 1 to 2 hours

Sweep, soapy scrub and rinse

  • Sweep away leaves and soil
  • Scrub with dish soap and water or an oxygen bleach mix
  • Rinse well and let it dry
  • Do not use vinegar on limestone or travertine

How long it lasts: Weeks to a couple of months. Shade and rain bring the green back.

Most readers choose Permanent fix
Patio algae and mildew cleaner (illustration)
Illustration

A patio algae and mildew cleaner

Scrubbing removes what you can see and leaves algae rooted in the surface pores. A cleaner made to kill algae and mildew reaches into the pores, so the patio stays clear longer and takes less scrubbing next time.

What to get: Patio algae and mildew cleaner

How long it lasts: Often a season or more where the patio stays damp, when used as directed.

Check price on Amazon

As an Amazon Associate I earn from qualifying purchases.

What you'll need

  • Stiff push broom

    For sweeping and for scrubbing in sections

    You have it

  • Garden hose

    To wet and rinse the surface

    You have it

  • Gloves and eye protection

    For mixing and applying cleaner

    You have it
  • Our pick
    Patio algae and mildew cleaner (illustration)
    Illustration

    Patio algae and mildew cleaner

    Pick one that is labeled for your surface

    Check price
  • Long-handled stiff scrub brush (illustration)
    Illustration

    Long-handled stiff scrub brush

    Gets into texture a broom skips

    Check price
  • Pump garden sprayer (illustration)
    Illustration

    Pump garden sprayer

    Applies cleaner evenly over a big patio

    Check price

Why it happens

1

Shade and standing damp

Algae need moisture. Patios shaded by walls, fences and trees dry slowly and turn green first.

2

Porous surfaces

Concrete, pavers and stone have tiny pores that hold water and give algae something to root in.

3

Slow drainage

A flat patio, one that slopes toward a wall or one that gets runoff from a downspout stays wet longer.

4

Leaves, soil and pollen

Organic debris feeds algae and mildew, and also makes the patio look dirty.

Try this first (free)

Do these in order, starting with the simplest.

  1. Clear and sweep

    Tool used: Stiff push broom

    Move furniture and planters. Sweep leaves and soil, and pull weeds from the joints.

  2. Wet and scrub in sections

    Tool used: Stiff push broom

    Wet the surface and mix dish soap and warm water, or oxygen bleach per its label. Scrub a few square feet at a time with a stiff brush, then move on.

  3. Rinse and check

    Tool used: Garden hose

    Rinse until the water runs clear and let the surface dry. Whatever still looks green or dark is the algae that soap did not reach.

  4. Treat what is left with a cleaner

    Tool used: Gloves and eye protection

    Apply a patio algae and mildew cleaner as the label says. Many are sprayed on and left to work, and some are rinsed off while others are left for the rain.

  5. Cut the damp

    Trim branches that shade the patio, redirect downspouts away from it and sweep weekly. Keep planters on feet so water does not pool underneath.

When free fixes aren't enough

If you scrub every spring and the green is back within a month, the algae are living in the pores and shade. A cleaner that kills the growth lasts longer than soap, and many are no-scrub or low-scrub. If you pressure-wash, use a wide fan tip and keep it moving. A narrow, close jet can etch concrete and blast sand out of paver joints, which then needs polymeric sand. Check any cleaner label for your surface, since acids can etch limestone and travertine.

Buy the fix

Still stuck after the free fixes? These are the products we would look at.

Our picks compared
PickProductFixesWho it's forBuy
Best pick
Patio algae and mildew cleaner (illustration)
Illustration
Patio algae and mildew cleaner
algae that scrubbing leaves in the pores of the surface Anyone whose patio or driveway goes green again every season. Check price
Budget pick
Long-handled stiff scrub brush (illustration)
Illustration
Long-handled stiff scrub brush
scrubbing time and uneven cleaning Anyone who only needs a seasonal scrub on a small patio. Check price
Upgrade pick
Pump garden sprayer (illustration)
Illustration
Pump garden sprayer
slow, uneven application on a large patio or driveway Owners of a big patio, or a patio plus a driveway, who treat large areas. Check price
Best pick
Patio algae and mildew cleaner (illustration)
Illustration

Patio algae and mildew cleaner

Fixes: algae that scrubbing leaves in the pores of the surface

Look for: Labeled for algae, moss and mildew on concrete, pavers or stone; Works with low scrubbing, or no scrubbing at all; Clear coverage area and dwell time; Safe around pets and plants once dry, per the label.

Who it's for
Anyone whose patio or driveway goes green again every season.
Honest downside
Your surface is delicate natural stone and the label does not list it. Test a small hidden spot first.
Check price on Amazon
Budget pick
Long-handled stiff scrub brush (illustration)
Illustration

Long-handled stiff scrub brush

Fixes: scrubbing time and uneven cleaning

Look for: Stiff bristles in a material that suits your surface; A long handle for scrubbing standing up; A head wide enough to cover ground quickly.

Who it's for
Anyone who only needs a seasonal scrub on a small patio.
Honest downside
You already own a stiff push broom. A brush alone will not kill algae that has taken root.
Check price on Amazon
Upgrade pick
Pump garden sprayer (illustration)
Illustration

Pump garden sprayer

Fixes: slow, uneven application on a large patio or driveway

Look for: Seals that hold up to cleaners, per the maker; An adjustable nozzle with a fan spray; A capacity of one to two gallons.

Who it's for
Owners of a big patio, or a patio plus a driveway, who treat large areas.
Honest downside
Your patio is small. A watering can or brush does the job.
Check price on Amazon

As an Amazon Associate I earn from qualifying purchases. We don't show prices because they change, so use the button to see today's price.

How we picked: How we picked: this page is based on concrete and paver care guides, algae cleaner labels, and the problems and fixes people report in reviews. We do not test products by hand.

What suits your surface

  • Concrete: stiff brush, oxygen bleach or an algae cleaner. Skip a narrow jet.
  • Pavers: a gentle scrub and a wide fan tip. Re-fill the joints with polymeric sand if it washes out.
  • Natural stone: mild soap and an algae cleaner labeled for stone. Avoid acids such as vinegar on limestone and travertine.

What to skip

Vinegar on natural stone and a narrow-jet pressure washer

Acid can etch limestone, travertine and some concrete. A close, narrow jet can blast sand from paver joints and scar the surface. Use a mild cleaner and a wide fan tip.

Frequently asked questions

Why is my patio green?
The green is algae. Damp, shaded and porous surfaces such as concrete and pavers hold the moisture algae need, and leaves and soil feed it.
Why does my patio go green again right after I clean it?
Soap and scrubbing remove the film you can see, but some algae stay rooted in the pores. Shade and slow drainage bring the green back. A cleaner that kills the growth lasts longer.
Why does my patio have black spots?
Black spots are usually mildew, black algae or lichen. A scrub with an algae and mildew cleaner clears them. Tire marks and leaf stains look dark too but come out differently.
Why is my driveway turning green, or why is my driveway turning black?
The cause is the same: algae and mildew on shaded, damp concrete or asphalt. Treat it the same way, with a stiff brush and an algae and mildew cleaner. Oil spots need a degreaser instead.
Why does my patio get so dirty?
Soil splashes up in rain, and pollen, leaves and bird droppings settle on a porous surface. A weekly sweep and keeping planters on feet cut most of it.

As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure

Patio algae and mildew cleaner (illustration)
Illustration

Our pick: Patio algae and mildew cleaner

Check price