Every neighborhood in Green Bay has one: the front walk on the north side of the house that never quite dries out. By July there's a green film along the edges, moss in the joints, and a spot by the downspout where everybody steps carefully.
Patios under big maples get it too. So do basement floors, as gray or white fuzz and a musty smell.
Some of this is a Saturday job with a bucket and a brush. Some of it means water is going somewhere it shouldn't. Here's how to tell which, and how to clean it without killing your hostas.
What's Actually Growing There?
- Algae: A thin green film, sometimes so dark it looks black. Slimy when wet, almost dusty when dry, and the slipperiest of the bunch.
- Moss: Small, soft green plants that form cushions in joints, cracks, and edges where soil and water collect.
- Mold and mildew: Fungi, not plants. Black, gray, green, or white spots, flat (mildew) or fuzzy (mold). Outdoors, it feeds on the film of dirt on the surface. Indoors, it shows up on damp basement floors.
- Lichen: Crusty gray-green or orange patches that look painted on. A fungus and an alga living together. Slow-growing and the hardest to remove completely.
One lookalike fools a lot of people. Efflorescence is white, powdery salt left behind when water moves through concrete and evaporates, and it's common on basement floors. The test: mist it with water and wait a minute. Efflorescence dissolves or fades and feels gritty. Mold stays put and feels soft. (Brown leaf-shaped prints are tannin stains, covered in our patio cleaning guide.)
Why the Same Spots Turn Green Every Year
Growth needs moisture, shade, and something to feed on. Wisconsin yards supply all three.
- North sides and tree canopies. A walk on the shady side may never get direct sun, and the snowbanks there melt last, keeping the concrete wet well into spring.
- Water with nowhere to go. Downspouts emptying onto the walk, sump lines pointed at the patio, and slow-draining clay soil keep slab edges damp for days.
- Sunken slabs. A settled slab holds a puddle after every rain, and the puddle line is often exactly where the green starts. See voids under concrete.
- Organic debris. Leaves buried by the first snow, grass clippings, washed-out mulch. That layer holds water and feeds growth.
A Green Walk Is a Slip Hazard
Wet algae is slick, and it gets worse at the wrong times: under wet leaves in fall, and in early spring when meltwater runs across it by day and freezes overnight. For a landlord, an HOA board, or a business with a shaded entrance, a walkway everyone knows is slick is a liability question, and "we clean and treat it every spring" is a much better answer than "we meant to."
Don't fix it with a glossy, film-forming sealer, either. A film can make a damp walk slicker.
Removal Options, Gentlest First
Sweep and scrape off what's loose, and pry moss out of joints with a putty knife. Light mildew spots may need nothing more than dish soap and a stiff nylon brush. Beyond that, go only as far down this list as you have to.
Oxygen bleach
Powdered oxygen bleach is sodium percarbonate. In water, it splits into hydrogen peroxide and sodium carbonate (washing soda), and the peroxide breaks down into water and oxygen as it works. It's much gentler around plants than chlorine bleach, though it still belongs off foliage and out of the storm drain.
Mix it in warm water at the label's ratio and use it fresh, since it's strongest in the first couple of hours. Keep the surface wet through the dwell time, scrub, and rinse. For most outdoor algae and mildew, start here.
Cleaners labeled for mold, algae, or moss
Some outdoor cleaners are based on quaternary ammonium compounds. Some are scrub-and-rinse; others are left on to work over days or weeks. Products sold to kill mold, algae, or moss are regulated as pesticides, and federal law requires using them as the label directs.
Diluted chlorine bleach
It works on stubborn black mildew and algae, and it comes with the most rules:
- Never mix bleach with ammonia (it releases toxic chloramine gas) or with any acid, vinegar included (it releases chlorine gas). Don't combine it with other cleaners, and rinse thoroughly between products. Some rust removers and masonry cleaners are acidic.
- Dilute it. Federal mold-cleanup guidance says no more than 1 cup of household bleach per gallon of water. Stronger doesn't work faster.
- Gear up. Gloves, eye protection, old clothes. Stay upwind.
- Protect your plants. Bleach whitens and kills leaves it lands on. Soak beds and lawn with plain water first, loosely cover delicate plants, and rinse everything afterward.
- Test colored concrete. Bleach can lighten some colored and stained finishes.
What to skip
- Vinegar. It's an acid, hard on concrete with repeated use, and it releases chlorine gas if it meets bleach residue.
- Lawn moss killers. Many contain iron, which stains concrete rust-orange. Use only products labeled for hard surfaces.
- Muriatic acid. Not a mold treatment. It etches concrete.
Mind the runoff. Green Bay's storm drains carry water to local waterways and the bay with no treatment. Mix only what you need, rinse with plenty of water, and keep runoff in your yard instead of sending it down the driveway.
Scrub Brush or Pressure Washer?
| Method | Good for | Watch out for |
|---|---|---|
| Stiff brush and bucket | Small areas, stamped or colored concrete, spots next to plants | Slow on big areas |
| Pressure washer wand | Fast removal on plain, sound concrete | Too much pressure or a narrow tip etches the surface and leaves stripes; overspray carries chemicals onto plants |
| Rotary surface cleaner | Large flat areas; even result, no stripes | Can't reach joints, step edges, or corners |
Washing alone removes what you can see, and what's left in the texture turns green first. Treat, let it dwell, then wash.
Growth Type Cheat Sheet
| Growth | Where it shows up | How to remove it | How to keep it away |
|---|---|---|---|
| Algae | North-side walks, shaded patios, puddle lines, steps | Oxygen bleach or algae cleaner, scrub, rinse | More sun, better drainage, less debris, penetrating sealer |
| Moss | Joints, cracks, edges next to lawn and beds | Scrape out by hand, treat with a hard-surface cleaner, rinse | Clean out and caulk joints; keep soil and mulch off the slab |
| Mold and mildew (outdoors) | Under trees, beneath planters and furniture, shaded steps | Dish soap for light spots; oxygen or diluted chlorine bleach for stubborn ones | Rake leaves before snow, set pots on feet |
| Mold (basement floor) | Along walls, under stored boxes and old carpet, near the sump | Small patches: detergent, then dry fast. Large areas or after flooding: a mold pro | Fix the moisture source first |
| Lichen | Old, undisturbed concrete, steps, retaining walls | Labeled cleaner or oxygen bleach, long dwell, repeated scrubbing; faint marks may stay | Clean before it gets established |
Keeping It From Coming Back
- Move the water. Extend downspouts well past the walk or patio and point the sump discharge where it can soak in. Here's how to extend downspouts.
- Raise slabs that hold puddles. Foam lifting raises a sunken slab so water runs off. The holes are 5/8", about the size of a dime, and you can walk on it right away.
- Let in sun and air. Trim low branches and shrubs crowding the walk.
- Get leaves off before the snow. Leaves left under snow all winter feed growth.
- Close up the joints. Soil-packed joints are where moss starts. Clean them out and seal them with a flexible sealant. See filling concrete expansion joints.
- Use a penetrating sealer. Silane and siloxane sealers soak in instead of forming a film. They cut how much water the concrete absorbs, so it dries faster after rain, and they resist deicing salt without making the walk slick. They won't make shaded concrete growth-proof, but they help. Seal only clean, dry concrete, within the label's temperature range.
Doing all of it? Order matters: clean, let it dry, lift, patch the holes, caulk the joints, then seal once the caulk has cured.
Basement Floors: When Cleaning Isn't the Fix
Mold on a basement floor means moisture is getting there: humidity condensing on cool concrete, seepage through cracks or at the wall, a sump that can't keep up, a leak, or a past flood.
Rule out efflorescence with the water test first. If it's mold, EPA's guidance comes down to this: control the moisture and you control the mold. A patch under about 10 square feet is usually a homeowner job. Larger areas, significant water damage, or anything involving sewage or floodwater call for a professional with mold experience.
Don't seal or coat over it. A sealer or coating on a damp floor traps the problem and is set up to fail.
We don't do mold remediation or waterproofing, and we don't give health advice. But if a walk or patio has sunk toward your foundation and sends water at the basement, that part is a lifting job, and it's ours.
Do It Yourself, or Call Us?
Do it yourself if it's a few squares of walk or a small patio, the concrete is plain and sound, and you're careful around plants. You don't need us for that.
Call us if:
- You want it cleaned and sealed in one visit, with any lifting and caulking done in the right order.
- The green comes back every summer because a slab holds water.
- It's stamped or colored and you don't want to risk stripes or faded color. (We'll clean it gently; a glossy reseal on decorative concrete is a job we refer to a specialist.)
- There's a lot of it, like walks across rental units or an HOA.