Landscape.com has some good ideas on how to deal with a sloped yard.
Landscaping through or near slopes means that homeowners have to deal with a concept called the angle of repose. The angle of repose is the maximum angle at which a granular material (in this case, soil) will remain at rest relative to the earth surrounding it. In other words, if the slope of a piece of land exceeds the local soil’s angle of repose, the soil will shift until it is at rest. What does this mean for homeowners who are landscaping into their hillsides, or moving large amounts of land to put down a hardscape? Typically, landscaping at a slope greater than the angle of repose necessitates fortifying structures known as retaining walls. These structures help support the pressure that is exerted by soil as it is trying to shift back to equilibrium.
When tackling hillside projects, homeowners need to carefully investigate the soil at the construction site. Is the slope itself contributing to soil erosion? Where does rainfall go as it runs down the hill? Is the soil sandy or does it contain a higher proportion of clay? DIY landscapers need to evaluate the size of the project as well. Major excavation frequently calls for specialized labor and large retaining walls reinforced with concrete and steel bars. More minor slopes, such as the gentle curve of a flowerbed’s soil as it meets the surrounding land, can be dealt with more easily. These smaller projects are an opportunity for DIY landscapers to employ decorative stone or brick without the risk of collapse from the massive weight of shifting earth. Being certain about the characteristics of the local soil (as well as its drainage) is critical during this stage. If homeowners are unsure about how they should proceed, consulting a professional landscaping firm or inquiring after other locally built retaining walls is a good place to begin.
After the integrity of the soil on the hillside has been addressed, DIY landscapers can start to brainstorm ideas for plant-based softscapes. There are a wide variety of groundcovers that can be used to provide coverage as well as help to prevent soil erosion due to heavy rainfall or wind. Steeper slopes may be supplemented with decks, terraces, stairs, or strategically placed rocks to control the flow of drainage (an especially important step if homeowners are worried about water damaging another person’s property). Homeowners must also take measures to ensure that any hillside plants are capable of dealing with a high degree of exposure to wind, rain, and natural light. For this reason, delicate flowers are not recommended for hillsides unless homeowners mean to reduce their exposure.