The answer to 'retaining wall yellowing' is that it is often caused by mineral deposits or efflorescence from the concrete or masonry materials used to build the retaining wall. This can occur when water seeps through the wall and evaporates, leaving behind salts and minerals that form a white or yellow hazy film on the surface. Symptoms include a discolored, stained appearance that may worsen over time.
The best solution is to first try cleaning the wall with a mild detergent or vinegar solution. This can help remove the surface deposits. If the yellowing persists, it may require more intensive treatment such as power washing, chemical cleaners, or even resurfacing the wall. In some cases, the root cause may be improper drainage or water intrusion, which will need to be addressed to prevent the problem from recurring. If the issue is extensive or the wall is structurally compromised, it's best to call in a professional landscaper or masonry contractor to assess and repair the retaining wall.