Hillsides are challenging places to landscape, prone to erosion and slumps if not treated with care. Since removing the grass, trees or shrubs that grow there naturally may accelerate erosion, it's important to replace them with design features such as terracing that prevent water from running off before it can sink into the soil and deep-rooted plants that will stabilize the ground.
Hills are never exactly the same. A gentle incline is more stable than a steep one, requiring less attention to stabilization and erosion control. A rock outcropping can be steep but erode quite slowly. Rain soaks into gravelly and sandy soil easily but runs off clay soils, eroding the slope. Know the underlying soil of your hillside before you plant. If the rich soil has washed away, leaving subsoil, you may need to improve it.
Minimizing erosion, maximizing slope stabilization and promoting tough, undemanding plantings are all important factors in landscaping hillsides. Erosion can be prevented by keeping bare ground covered with a mulch of straw, wood chips or bark. If you have a bare area and can't plant soon, seed with grass and kill the sod later by covering with straw. The roots of the grass will decay, enriching the soil. Plant as soon as possible for long-term erosion control. Hillsides can be stabilized by creating terraces and by planting deep-rooted, fast-growing shrubs and trees.
Design Features to Use
Terracing the hillside or installing boulders to create level areas can be expensive but may be worth the trouble on steep hillsides. Getting advice from a professional can save you both time and expense. Rock gardens are especially attractive on hillsides, mimicking the natural outcrops of alpine areas. Many rock plants appreciate the good drainage an incline offers.
Plants To Use
Depending on which part of Oregon you live in, you may have lots of rain or not much at all. Whenever possible, suit your plantings to the natural rainfall. Watering with hoses may be difficult on a hillside and if so, you should time your planting to coincide with the beginning of fall rains. Create level watering wells around each plant for maximum absorption. Use drought-tolerant plants whenever possible, trees like madrone and Douglas maple and shrubs such as ocean spray and salal. Groundcovers such as low varieties of Ceanothus, St. John's wort and junipers are all good choices.
Water drought-tolerant plantings well for the first two years to give them a good start and keep them mulched for weed and erosion control. Replant quickly whenever groundcovers or shrubs die. Add more plants to create a thick, lush cover.