Answer: There are two ways to do this. One is to plant them relatively close for faster coverage but risk them becoming overcrowded eventually. The other way is to plant them to allow for mature size, and patiently wait for coverage. In my experience the second choice is better in the long run, which means putting them about six feet apart measuring from the center of each plant to the next. In the meantime you can use burlap to cover the slope and top it with a large particled mulch such as pine bark nuggets. The mulch will help hold down weeds as well as help with the erosion. You can also use boards held into place with pipe or pegs to terrace the area slightly, help slow the runoff and catch any sliding mulch. Another option might be to look into changing the drainage pattern to help slow the water. I hope this helps.
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