The key to working with difficult terrain such as a steep grade is to imitate and speed up the natural changes to the land. For example, a dark, shady spot in a yard may, in the future, turn into a moss-covered valley or a slope worn away by sun and wind may eventually turn into a rocky outcropping. Work with the land and its natural features to create a space you can live with and maintain with little effort.
If the slope is too steep to mow, it is too steep to garden, according to Time-Life's "How-To Landscaping Projects." The best bet is to build a low rock wall at the bottom of the slope and add plants in the spaces in between the landscaping stones. Mark the wall's location, making it as straight as possible. Dig a 14-inch deep shelf into the slope, saving all dirt, rocks and subsoil. Remove four inches of dirt from the bottom of the shelf, sloping so the front of the shelf is higher than the back of the shelf by two inches.
Spread a three-inch layer of gravel on the shelf, making sure the front of the shelf is two inches higher than the back. Place the largest and flattest landscaping stones on the gravel first, slanting them backwards so the top is higher than the front. Fill in the spaces around the landscaping stones with saved soil and clay, packing it down firmly. Add two or three layers of stones, making sure the crevices on each layer are packed firmly with clay and soil. When all the layers are in place, mix stone with planting soil and spread between the stones and the slope, placing plants in these pockets along the wall.
Consider a terrace, a level plateau built into a slope, to control erosion and to increase the amount of usable land. Unless you are experienced in building terraces, it is best to hire a professional. Connect multiple terraces with steps, suggests Julie Stillman in her book, "Deck & Patio Idea Book," using them to define different areas or zones, such as a seating area from a dining area. Determine how many terraces you would like to build into a hillside first.
Build a series of flat, level areas by cutting into the slope, adding a brace, either wood or bricks, and filling the area in with gravel or soil and leveling flat. Tile, set pavers or add plants to the terraces, depending on your design. Brace the edges of the terraces with wood, anchoring them with a tie driven perpendicular to the terrace into the hillside.
For steep slopes that are difficult or impossible to mow, consider planting ground cover, which will control erosion and result in a low-maintenance landscape. Setting plants in staggered rows will prevent rain from forming in gullies in the hillside, according to Barbara Damrosch in her book, "The Garden Primer." Set the plants lower than ground level to collect rainwater. Spread mulch around the plants after they are set.