It's not plants with deep roots that you need, it is plants with wide spreading roots. But start just with a deep layer of mulch such as grass clippings about 3 inches deep. THis will prevent rain splashing on the surface of the soil and starting the erosion. Then get something vigorous and put it in holes that you dig in the mulch. I have a ton of some little unknown plant and you can have as much of it as you wish to start your bed. It likes shade and ihas small purple blooms in the shade. I can send a photo if you wish. . I am in Hilo 805-545-0642 Laura