Mulch a steep hill by installing low plastic edge fencing perpendicular to the hill's slope before laying down mulch. The edge fencing prevents mulch from sliding all the way down the hill during heavy rainfall. How many sections of edge fencing a slope requires depends on the hill's size and steepness.
For the largest, steepest slopes, several rows of plastic edge fencing may still not be enough to prevent serious mulch erosion during heavy rains. In these cases, use one or more retaining walls, or modify the terrain itself.
While installing a retaining wall is much more expensive than using plastic edge fencing, building one of these walls along the slope has several advantages. The retaining wall drastically reduces the angle of the slope above it, making it much easier for mulch to stay in place. The shallower slope also presents less of a falling hazard to humans.
Terrain modification, the most drastic solution for mulching a steep hill, involves adding or removing earth to create stepped terraces. This method can either reduce the slope between terraces or completely eliminate it. Because the terraces have reduced slopes if any at all, mulching them is as simple as mulching other flat areas.