The answer to 'why is my retaining wall brown patches' is that it is likely due to mold, mildew, or algae growth on the surface of the wall. These discolored patches can be caused by excess moisture, poor drainage, or inadequate ventilation around the retaining wall. The brown or greenish patches may appear unsightly and can potentially damage the structural integrity of the wall over time if left untreated.
To address the issue, you can try cleaning the wall with a mild bleach solution or a specialized retaining wall cleaner. Make sure to wear protective gear and follow the manufacturer's instructions carefully. If the problem persists or the wall appears to be structurally compromised, it's best to consult a professional landscaper or masonry contractor who can assess the situation and recommend the appropriate course of action.