Bryant RedHawk wrote:For such a slope the ideal would most likely be to terrace it so you had two flat areas for planting. No matter what you plant on such a slope, a heavy rain will cause erosion as you mentioned one rain already moving lots of soil to the ditch below.
Mulch can be washed away by a heavy rain, I've got gravel that was packed tight that now has a gulley from a rain storm that dropped 4 inches on us in 5 hours. I've even got a spot that was nice grass that was washed out during that rain event.
Terraces are not hard to build (just time consuming for the most part) and there are lots of different materials that can be used to build them.
In decades past many were built with old rail road ties, a decision that turned out to be just Ok, since the creosote ended up leaching into the soil of the lower portion(s).
On Buzzard's Roost we have a steep slope both north and south so what I am doing is using the "junk" trees, as I fell them I make stakes from the larger diameter branches then stack the trunks against the lower stakes, the up hill stakes are close enough to the lower stakes to hold the wall in place.
Other options are to grass seed the area at least three times so the plants end up tight to each other which will allow the water to ride over the root section of the grass plants and not make it to the soil in large enough quantities to wash out the soil.
You could use designer concrete blocks (the ones that have a face created to look like stone) and work up from the lowest point, making two block high mini walls but that would not leave much of a flat planting zone between the walls, three high is fairly usual for terraces on slopes like that one you showed.
You could even use 2x6 board pieces, sharpen one end and drive that into the soil along a string line, as long as the boards are driven down about a foot they should hold soil back fairly well for at least a few years.
I wish you great success in this project.