Green on the driveway. Black streaks down the north wall. A slick film on the pool deck that shows up every summer no matter what you do.
Welcome to Florida. Here is how to actually get rid of it, and why the thing most people try first makes it worse.
The short version: algae and mold are alive, so you have to kill them, not scrape them. Apply a cleaning solution, let it sit long enough to work, then rinse gently. Pressure alone knocks off the top layer, leaves the roots, and the green is back in a month. On concrete you can add some pressure after the solution has done its job. On walls, roofs, and pavers, keep the pressure low or you will damage the surface.
Key Takeaways
- Green film on flat surfaces is algae. Black streaks on walls and roofs are mold and mildew. Both are living organisms.
- Kill it first with a cleaning solution and dwell time. Rinse second. That order matters.
- Pressure alone is a scrape. It misses the roots and can etch concrete or strip paint.
- Shade, poor drainage, and salt air are why it keeps coming back. A cleaning rhythm beats a one time fix.
- The wrong pressure on the wrong surface is the number one way DIY goes wrong here.
First, Figure Out What You Are Looking At
| What you see | What it is | Where it lives |
|---|---|---|
| Green film, sometimes slimy | Algae | Driveways, walkways, pool decks, north facing concrete |
| Black streaks or dark spotting | Mold and mildew | Siding, stucco, soffits, fences, roofs |
| Dark lines in the joints | Mildew in paver sand | Paver patios and pool decks |
| Chalky white film | Salt and mineral residue | Waterfront homes, anything near the bay |
None of it means you did anything wrong. It means you live somewhere warm, wet, and salty. The same climate that grows the palms grows the film on your pavers.
Why Blasting It Does Not Work
This is the part that saves you the most frustration, so I will be direct.
Algae has a root structure that works into the pores of concrete. Mold does the same in stucco and paint. When you hit it with a pressure washer, you remove what the water can reach and leave everything below the surface alive. Two to four weeks later, it is back.
Meanwhile the pressure itself leaves lines, swirls, and a rough etched texture on concrete that never goes away. On siding it strips paint and drives water behind the panels.
So you did the work, damaged the surface, and solved nothing. That is the DIY story we hear most.
How to Actually Remove It, Step by Step
Step 1: Wet down everything green that is not a target
Plants, grass, mulch beds. Soak them with plain water before any solution comes out. Saturated leaves absorb almost nothing.
Step 2: Apply the cleaning solution
Low pressure, even coverage. On concrete you can use a pump sprayer. On walls, keep it gentle and work bottom to top so you do not get clean streaks running through dirty areas.
Step 3: Let it dwell
This is the step everyone skips. The solution needs time on the surface to kill the growth. Ten to fifteen minutes on concrete. Do not let it dry, especially in the sun. If it starts drying, mist it.
Step 4: Rinse from the top down
On walls, roofs, pavers, and screens: low pressure, like a strong garden hose. On concrete: you can step the pressure up now, because the growth is dead and you are just clearing it. Use a surface cleaner attachment on driveways if you have one. It stops the striping a wand leaves.
Step 5: Rinse the plants again
Everything you soaked in step one gets soaked again. Nothing sits on foliage.
That is the whole method. The driveway cleaning guide and the house exterior guide go deeper on each surface.
Which Surfaces Can Take Pressure After the Solution
| Surface | Pressure after dwell? | Why |
|---|---|---|
| Concrete driveway | Yes, with a surface cleaner | Dense enough to take it, and it lifts the dead growth evenly |
| Sidewalks and walkways | Yes, controlled | Same as driveways |
| Siding and stucco | No, rinse only | Strips paint, cracks stucco, forces water behind panels |
| Pool deck and pavers | Light at most | Blows out joint sand and pits soft pavers |
| Roof | Never | Lifts shingles, cracks tile, voids warranty |
| Fences and soffits | No, rinse only | Wood furs, paint chips |
The driveway, sidewalk, and pool deck service pages each use the method that fits that surface, and the house exterior and siding page covers the soft wash side.
What Your Symptom Is Telling You
Green film back within weeks of a rinse? The roots were never killed. Pressure without solution and dwell time. Do it again, properly.
Black spotting climbing the north wall? That side stays damp longest. It needs a soft wash, and it will need one more often than the sunny side.
Pool deck slick underfoot? Algae has taken hold and it is now a slip risk. This one is worth handling promptly. We wrote about why in pressure washing and slip and fall safety.
Keeping It From Coming Back
Honest answer: nothing keeps a surface spotless forever in St. Petersburg. The moisture, warmth, and salt never take a break. The goal is to stay ahead, not to win once.
- Get on a rhythm. Light growth is easy. An established mat is not.
- Fix drainage where you can. Anything that stays wet and dark turns green first.
- Trim back shade if it is optional. Sun is your friend here.
- Use the same people. Whoever cleaned it last knows which corner goes first.
There is also an upside beyond looks. Mold and mildew right outside your door have real health and allergy implications, and clearing them regularly matters more than most people think.
When to Call Instead
If it is a driveway and you have a weekend, you can do this. If it is stucco, a roof, a pool cage, or a two story wall, the downside of getting the pressure wrong is a repair bill that dwarfs the cleaning. We are two people who do this every day and we would rather you call than call a painter after.
The complete guide to pressure washing in St. Petersburg has the full picture on every surface.
Frequently Asked Questions
Is the green stuff on my driveway algae or mold?
Green film on flat, damp concrete is almost always algae. Black or dark streaking on walls and shaded surfaces is mold or mildew. Both need to be killed at the root, not rinsed off the top.
Will pressure washing alone get rid of algae for good?
No. It removes the visible layer and leaves living roots in the pores, so it grows back fast. A cleaning solution with proper dwell time is what actually kills it.
Can high pressure damage my home?
Yes. Too much pressure etches concrete, strips paint, gouges stucco, forces water behind siding, and blows out paver joints. Match the pressure to the surface.
Why does the growth keep coming back on the shaded side?
Shade stays damp far longer after rain, and lingering moisture is exactly what algae and mold need. That side will always turn first and needs cleaning more often.
Can you remove black mold from stucco and painted walls without harming them?
Yes. Low pressure plus a cleaning solution that treats the mold directly. It protects the paint and texture while removing the growth completely.
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