Why Fences Turn Green and Black
Mold, mildew and algae all need moisture, and fences give them plenty of places to find it. Shaded north-facing sides, spots with poor airflow, boards touching damp soil or plants, and areas hit by sprinklers are the usual trouble spots. Dirt and pollen on the surface act as food. Algae typically shows as a green film, mildew as gray or black speckling, and mold as fuzzy or dark patches.
Most of this growth is a surface issue and mainly cosmetic, but on wood it can hold moisture against the fiber and encourage decay. That is one reason cleaning pairs well with other upkeep. Some people are sensitive to mold spores, so wear a mask and eye protection while scrubbing, especially for large or heavy growth.
Before You Start
- Check what the fence is made of. The right approach depends on the material, and the wrong one can strip finishes or damage surfaces.
- Protect plants. Cover or thoroughly wet nearby plants before using any cleaner, and rinse them afterward.
- Choose the day. A mild, overcast, dry day works best. Direct sun dries cleaners before they can work.
- Test first. Try any cleaner on a hidden spot and wait for it to dry before treating the whole fence.
- Read labels. Follow the manufacturer’s directions for any product, and never mix cleaning chemicals. Mixing bleach with ammonia or acids can produce dangerous gases.
Wood Fences
Wood is the most vulnerable to harsh cleaning. Start gently: wet the surface, scrub with a soft-bristle brush and a solution of dish soap and water, then rinse well. If growth remains, many people use an oxygen-based (sodium percarbonate) wood cleaner, which is usually gentler on wood and plants than chlorine bleach. Chlorine bleach can lighten wood, damage fibers and harm plants and metal fittings, so if you use it, dilute it as the label directs and rinse thoroughly.
Pressure washers can gouge soft woods, raise the grain and push water deep into the fibers. If you use one, choose a wide fan tip, keep a good distance and low pressure, and move with the grain. A garden hose and a brush are often enough.
Let the wood dry fully before any finish is applied. How long that takes varies with weather, but days are more typical than hours. A fence that was cleaned is then a good candidate for fence staining or sealing, since protective finishes can slow regrowth.
Vinyl Fences
Vinyl is nonporous, so growth stays on the surface and usually comes off easily. Soapy water and a soft brush or sponge often do the job. For stubborn spots, a solution of white vinegar and water is a common choice, or an oxygen cleaner diluted per the label. Avoid abrasive pads, stiff wire brushes and solvents, which can scratch the surface and leave marks where dirt collects. Keep pressure washing gentle.
Metal Fences
Aluminum, steel and iron fences can be cleaned with mild soap and water and a soft cloth or brush. Mildew sits on the paint or coating, not in the metal, so the goal is to clean without damaging the finish. Avoid harsh cleaners and abrasive tools on powder-coated or painted surfaces. If you uncover rust while scrubbing, deal with it separately using proper fence rust removal methods, since cleaning alone will not stop corrosion.
Composite Fences
Composite boards often come with manufacturer cleaning instructions, so check those first, as they may affect any warranty. In general, soapy water and a soft brush work, and some products tolerate gentle pressure washing while others do not. Mildew can still grow on the surface dirt of composite materials, so regular washing helps even though the boards do not rot like wood.
Slowing Regrowth
Cleaning removes growth, but moisture is what brings it back. A few changes help.
- Trim back plants so sun and air reach the fence. Keep shrubs and vines from touching it.
- Keep soil and mulch below the bottom board. Wood in constant contact with damp ground stays wet, and good fence drainage keeps water from pooling at the base.
- Adjust sprinklers so they do not spray the fence directly.
- Wash on a schedule. A light annual rinse is usually easier than removing heavy buildup every few years.
- Apply a finish suited to the material. On wood, a water-repellent sealer or a stain with mildewcide is typical. Products vary, so follow the label.
In deep shade or a damp climate, some regrowth is likely no matter what you do. Expect to repeat the cleaning periodically rather than treating it as a one-time fix.
When It Is More Than a Stain
Scrub a patch and press a screwdriver tip into the wood. If it sinks in easily, feels spongy or crumbles, the board may be decayed rather than just dirty, and cleaning will not restore it. Replacing rotted fence boards is the better route in that case, and you should check the rail behind them too. A persistent musty smell, soft wood or widespread patches in a damp area can mean a bigger moisture problem, so look for the cause.
If you have a health condition affecting breathing, or the growth covers a large area, consider asking a professional to handle the cleaning.