Erosion is the process by which topsoil or other soils are swept away by wind or water. Hillsides without vegetation are most susceptible to erosion. Hillsides with vegetation whose roots don't go deep enough to keep the soil in place are also susceptible to erosion.
Ground covers hold fine surface soils in place but may not offer deep enough roots to fully stabilize a hillside. The best species of ground cover will depend on your climate zone, but Yarrow, Dwarf Coyote Brush, Monkey Flower and Sage are often good ground covers. Any grassy ground cover with a tough, fibrous root system will work well to control erosion.
Small shrubs can help retain soil and, because of their deeper roots, help keep the soils stable at a deeper level than ground cover alone. Sagebrush, sunflowers and clumping grasses like buckwheat can create good shrubs that help prevent erosion (like poppies, elders and goldenbrush). Any small shrub with deep, tough roots suitable to your climate zone will help reduce erosion and stabilize hill soils.
Large Shrubs and Trees
Large shrubs and trees have deep roots that, when combined with ground cover and smaller shrubs, help keep hillsides stable. Lilacs or other very large shrubs, along with virtually any tree with deep growing roots, can help slow and stop erosion while also helping to prevent erosion related landslides. Pines and other evergreen trees are very cold hardy and will often grow well in many climate zones, ranging from very cold to sub-tropical.