How to Stop Mold and Algae Coming Back on a Patio (Humid South Edition)
You're seeing: Green or black growth on pavers, concrete or stone that comes back a few months after every pressure wash, worst in the shade and along the joints.
TWO DIFFERENT PRODUCTS DO TWO DIFFERENT JOBS. A rinse-off cleaner (bleach or oxygen) removes what is there today and leaves nothing behind. A leave-on spray (Wet & Forget, a quat; BioAdvanced, an EPA-registered soap) keeps working after you walk away — BioAdvanced's label claims several months to a year, Wet & Forget's maker says once a year. So: (1) take away what growth lives on — shade, ponding water, leaf litter in the joints (UF/IFAS: mildew grows best at 77 to 86 degrees, in dark, still, damp places); (2) clean once with a rinse-off cleaner; (3) spray a leave-on product on the dry, clean surface and leave it; (4) seal the slab if it soaks up water. Keep leave-on sprays off plants and out of ponds and pools — both labels warn about aquatic life.
If you pressure wash the patio every spring and it is green again by fall, nothing went wrong with the cleaning. Cleaning removes growth; it does not prevent it. The fix is three things in order: take away what the growth lives on, clean it once, and leave something on the surface that keeps working.
Why it comes back
UF/IFAS Extension puts numbers on it: mildew “grows best when the temperature is 77 to 86 degrees”, and it thrives “in dark areas and places where there is not much air circulation.” In the Gulf South that describes most of the year — and specifically the north side of the house, the corner under the oak, and the joints where leaves rot. Moss and algae want the same thing: damp, shade, food.
So before any product:
- Light and air. Trim back branches over the patio. Sun and moving air dry the surface between rains, and a surface that dries does not grow much.
- Standing water. Low spots, a downspout dumping onto the slab, pavers that have settled into a dip — fix the drainage, or the wettest square foot will always be the greenest.
- Food. Sweep leaves and blow out the joints. Organic litter in the gaps is what growth feeds on; TimberTech’s own care guide makes the same point about the gaps between deck boards.
Rinse-off vs leave-on — two jobs, not two brands
| Rinse-off (bleach or oxygen) | Leave-on (quat or soap-based killer) | |
|---|---|---|
| What it does | Removes the growth there today | Keeps killing growth after you leave |
| How fast | Minutes | Days to months |
| Rinse | Required | No |
| Left behind | Nothing | Residual on the surface |
| Plants | Wet them before and after | Keep it off them |
| Near a pool | Bleach is just chlorine; keep overspray out | Keep it out of the water |
You usually want both: one rinse-off clean, then a leave-on spray on the clean, dry surface.
Step 1: clean it once
For concrete, pavers and brick, a bleach wash is the fast route. On travertine and other limestone, use a mild bleach solution, never vinegar — the cleaning guide has the row for your material.
Step 2: leave something on it
This is the step most patios never get. Two products, two chemistries:
Spray either on a dry day with no rain forecast, keep it off the plants, and keep it away from ponds, fountains and pool water. The Wet & Forget data sheet rates its active ingredient toxic to fish; the BioAdvanced label warns about aquatic invertebrates.
Step 3: make the slab drink less
A porous slab that soaks up every rain stays damp longer. A penetrating sealer makes it shed water instead, without changing how it looks or grips, and one adds a biocide to the mix:
On pavers whose joints keep filling with green, a joint-stabilizing sealer with an antifungal in the film does two jobs at once — see the sealer guide.
The order, on one line
Trim, drain, sweep → rinse-off clean → let it dry → leave-on spray → seal if the slab soaks up water → leave-on spray once a year after that.
Around a pool, the same order applies, with a few extra rules about runoff and slip — they are in Pool School’s pool deck care guide.
Frequently asked
Why does mold keep coming back on my patio?
Because the conditions that grew it are still there and nothing is left on the surface to stop it. UF/IFAS Extension says mildew grows best at 77 to 86 degrees and in dark places with little air circulation — most of a Gulf summer, in the shady corner of a patio. A pressure wash or bleach wash removes the growth and rinses away, so the next crop starts on a clean surface. Fix the shade, standing water and debris, then use a leave-on product that keeps working.
What kills mold on concrete and keeps it from coming back?
Use two products. A bleach-based wash removes the mold you can see today. A leave-on spray keeps it from returning: BioAdvanced 2-in-1 Moss & Algae Killer is EPA-registered and its label says one application controls moss and algae on concrete for several months to one year; Wet & Forget, a bleach-free quaternary ammonium cleaner, is designed as a once-a-year maintenance spray. Penetrating sealers with a biocide, such as Ghostshield Siloxa-Tek 8500, also claim to resist mold and mildew.
Is Wet & Forget or bleach better for a patio?
They do different jobs. Bleach removes growth in minutes and must be rinsed; it leaves nothing behind. Wet & Forget is left on, works slowly with the weather — its maker says green algae often fades within a few weeks and darker mold can take months — and is meant to keep the surface clean between sprays. Many owners use bleach once to clean, then Wet & Forget once a year to maintain.
Are patio mold and algae killers safe around plants and pools?
Read the label. Bleach washes ask you to wet plants before and after and keep runoff out of storm drains; a little bleach in a pool is just chlorine. Leave-on products are different: Wet & Forget's data sheet rates its active ingredient toxic to fish, and the BioAdvanced label says it may be hazardous to aquatic invertebrates and must not run off into drains or surface water. Keep them away from ponds, fountains and pool water.