The answer to 'why is my mulch erosion' is that mulch erosion is a common problem caused by factors like heavy rain, wind, or poor installation. Symptoms include thin or bare spots in your mulch layer, exposing the soil underneath. This can lead to weeds, soil compaction, and decreased moisture retention.
To solve mulch erosion, you'll need to replenish the mulch layer to the recommended 2-4 inch depth. Consider using larger wood chips or shredded bark that are less likely to wash away. Properly edging the mulch can also help prevent it from being displaced. If erosion persists, you may need to address underlying issues like poor drainage or sloping terrain, which may require the help of a professional landscaper.