Water in that soil has to go somewhere. If the drainage behind the wall is not working, it will come through the wall, and it will take the render with it. That is why we look at the back of the wall before we quote the front.
Drainage comes before render, every time
A properly built retaining wall has an aggregate drainage layer behind it, an agricultural drain at the base running to somewhere useful, and weep holes through the face. Those three things let water out at the bottom instead of pushing it through the wall.
When they are missing or blocked, hydrostatic pressure builds up behind the wall. Water is forced through the masonry, emerges on the face, and pushes the render off from behind. No render system survives that, and no amount of surface waterproofing fixes it, because you are trying to hold back water with a coating rather than drain it away.
If we look at your wall and the drainage is not right, we will tell you. It is a more expensive conversation than just quoting the render, and it is the honest one.
Weep holes stay open
This gets us called out more than anything else on retaining walls. Someone renders the wall and, wanting a clean unbroken face, renders straight over the weep holes.
Those holes are the only exit route for water behind the wall. Blocking them turns a functioning wall into a dam. Within a season or two the render is blistering and the wall face is wet.
We keep every weep hole open and we form a clean reveal around it. It is barely visible from a few metres away and it keeps the wall working.
Where the render stops at ground level
Render should never run down into the soil. Where it does, it wicks ground moisture straight up into the coating and the wall behind it, and it holds that moisture against the masonry permanently.
We stop the render clear of the finished ground level with a clean edge. On a wall where the client wants the render to appear to continue into a garden bed, there are ways to detail that which do not involve burying the coating.
Choosing the coating for a retaining wall
Retaining walls take more moisture load than any other wall on a property, so we lean towards flexible acrylic systems rather than a plain cement finish. Acrylic copes better with the moisture cycling and it is more forgiving of the small movements a retaining wall goes through.
Colour choice is worth thinking about too. A retaining wall is usually at ground level, close to gardens and irrigation, and it will get splashed with soil. Mid tones hide that far better than a bright white does.