TL;DR
A retaining wall is a structure holding back tons of wet soil, so cleaning it means low pressure, a wide tip, and no spray driven into the joints between blocks. Check the drainage before you check the dirt -- a wall that is green all over is usually a wall that is wet all the time, and that is a drainage story, not a cleaning one.
What a retaining wall is doing while you look at it
Most residential retaining walls are dry-stacked segmental blocks. There is no mortar. The wall stands because of block weight, the interlocking lip or pin at each course, the crushed stone behind it, and the drainage that keeps water pressure from building up.
That last part matters more than the rest. Soil holding water gets heavier and pushes harder, so every properly built wall has drainage gravel behind it and usually a perforated pipe at the base. Blast water into the joints of that wall and you are pushing fines into the drainage layer, which is the one thing keeping it standing.
Mortared walls and poured concrete walls behave differently but have their own soft spot: the mortar line, which is deliberately weaker than the units around it.
Source: Segmental retaining wall installation and maintenance guidance
Why is my retaining wall green?
Algae and moss grow where the surface stays damp, and a retaining wall is damp by design -- it has wet soil against its back face and it usually sits in the shaded, low part of a yard. Green growth is almost always a moisture and light condition rather than a sign the wall is dirty.
Check drainage before you clean anything
Walk the wall and look for the signs that this is more than a cosmetic job.
Bulging or leaning in a section means pressure behind the wall exceeds what it was built for. Stop and get it looked at by a wall or landscape contractor. Cleaning a leaning wall is putting a coat of paint on a structural problem.
Weep holes or a drain outlet that is blocked is the most common fix and the most commonly missed. If your wall has drainage outlets at the base, clear them. Water that cannot leave stays behind the wall.
Efflorescence, the white chalky bloom, tells you water is moving through the block and evaporating on the face. It is not damage on its own, but a heavy bloom means a lot of water is passing through.
Soil washing over the top means the grading above the wall is directing runoff at it. Regrading or adding a swale upslope solves what no amount of washing will.
The cleaning method: chemistry, dwell, gentle rinse
Treat this as a soft wash. The principle is the same one described in how soft washing works -- a cleaner with contact time does the work and water only rinses.
Clear the face. Pull ivy, weeds growing from the joints, and any vegetation rooted in the wall. Roots in a joint spread blocks apart over time and are a real problem worth solving properly, meaning removing the root rather than cutting the top off.
Pre-wet the wall and the plants around it. A dry block soaks the cleaner straight in and gives you nothing at the surface. Wetting the plantings first dilutes runoff.
Apply an outdoor cleaner and let it dwell 10 to 15 minutes. For algae and moss, an oxygen bleach product or a dedicated masonry cleaner works well and is gentler on the landscape than chlorine bleach. Keep the surface wet through the dwell.
Agitate with a stiff bristle brush on a pole. Work across the face rather than into the joints. Bristle contact is what replaces pressure on textured block.
Rinse from the top down at low pressure. A garden hose is enough on many walls. With a washer, use a 40-degree tip or wider, low pressure, held well back, and angle the spray slightly downward across the face rather than straight into a horizontal joint.
The joints are the whole risk
On a dry-stacked wall, spray driven into a horizontal joint goes somewhere. It carries the fine material that has settled between courses back into the gravel behind the wall, and it can push a block face out of alignment on a wall that has already settled.
Keep the wand moving and keep the angle shallow. If a joint is packed with soil and moss, clear it by hand with a joint tool rather than trying to flush it out with water.
The same caution applies to mortared walls for a different reason -- the mortar is softer than the block, and a concentrated stream cuts it. That is covered in more depth in cleaning brick without blowing out the mortar, and the general pressure limits by material are in what PSI each surface can take.
Chemical limits worth knowing
Never combine a chlorine bleach product with an acidic cleaner, a rust remover, or an efflorescence remover -- bleach plus acid releases chlorine gas. Never combine bleach with an ammonia-based cleaner. If one product did not work, rinse the wall fully and let it drain before applying another.
Acid-based efflorescence removers do exist and are effective, but they can etch and lighten concrete block and will kill plants below. Test any acid product on a low, hidden course first, protect the ground, and use it only when the white bloom is genuinely heavy.
Chlorine bleach solutions kill algae quickly but the runoff pools at the base of a wall, which is exactly where the planting bed usually is. Pre-wet, keep the mix weak, and flush the beds afterward.
For how different outdoor surfaces take different methods, see the exterior surface cleaning overview.
FAQ
Can you pressure wash a retaining wall?
Yes, at low pressure with a 40-degree or wider tip held well back and angled across the face rather than into the joints. On a dry-stacked block wall, spray driven into a horizontal joint pushes material into the drainage layer behind the wall and can shift blocks. Detergent dwell should do most of the work.
How do I remove algae from a concrete block wall?
Pre-wet the wall and surrounding plants, apply an outdoor masonry cleaner or an oxygen bleach product, keep it wet for 10 to 15 minutes, agitate with a stiff pole brush, then rinse from the top down at low pressure. Improving light and airflow is what keeps it from returning.
What is the white powder on my retaining wall?
Efflorescence -- mineral salts carried to the block face as water moves through the wall and evaporates. It brushes off dry and often weathers away over a season or two. A heavy, recurring bloom means a lot of water is passing through, so the drainage is worth checking.
Is a bulging retaining wall a cleaning problem?
No. Bulging or leaning means pressure behind the wall exceeds what it was built to hold, usually because drainage has failed or the wall was undersized. That is a structural issue for a wall or landscape contractor. Cleaning it changes nothing about the condition.
Should I seal a retaining wall after cleaning?
Usually not. A sealed face can trap moisture that is trying to move out through the block, which leads to spalling where the surface flakes off, especially with freeze and thaw. If you do seal, use a breathable product rated for segmental wall block and apply it to a fully dry wall.
Will bleach hurt the plants below my wall?
Chlorine bleach runoff collects at the base of a wall, which is usually where the planting bed sits, and it will damage foliage and roots. Pre-wet the plants with plain water, keep the mix weak, and flush the bed thoroughly afterward. Oxygen bleach products are gentler on landscaping.
How often does a retaining wall need cleaning?
Once a year is plenty for most walls, typically after the wet season. Walls in deep shade may want it more often. Clearing the drainage outlets and keeping soil and mulch away from the bottom course matters more to the wall's life than how often the face gets washed.
Wall height, block type, how much growth has established, and whether there is a drainage issue behind it all change the job, and a leaning wall needs a contractor before it needs a cleaner. Get a quote and we will look at yours, or read more about pressure washing services.