"I know a bank whereon the wild thyme blows/Where oxlips and the nodding violet grows." Even before Shakespeare's day, gardeners understood the charm of a sunny hillside tumbling with fragrant herbs and flowers, or an Alpine slope covered in boulders and tenacious shrubbery. For both aesthetic and practical reasons, variety is key when deciding how to landscape a hillside. Choose species with a wide range of foliage size, plant height and root depth to add visual interest while preventing rainfall from carving soil-eroding paths into the hillside.
Many of the plants best-suited for holding a bank straddle the line between being ground cover and dwarf shrubbery. Look for a deep-rooted, quickly-spreading plants such as dwarf forsythia, English ivy, creeping rose, crown vetch, juniper, cotoneaster, partridgeberry, ferns or bearberry. Mulch the plants until they are well established. Planting in staggered rows helps the plants look good until they grow large enough for their branches to touch.
While steep banks make harvesting and pruning impossible, gentle slopes are often ideal places for fruiting plants, especially trees. Most fruits are vulnerable to frost damage, even in moderate climates. Slopes mitigate the ravages of the frost by draining cold air away. Avoid planting on hillsides sloping more than 20 degrees, because the steeper the hill, the harder it is to tend the plants. Because "frost pockets" gather at the bottom of hills and the summits experience high winds, planting part-way up a bank is best. In general, south-facing slopes work best for apples, trellised grapes and melons, peaches, apricots, bramble fruits, pears and cherries. However, in colder climates many growers of tender peaches, apricots,plums and cherries opt for a northern slope, which keeps the fruit from ripening before all danger of frost passes.
Herbs happily colonize smaller slopes. Because soil is significantly dryer at the top of even a short slope, with moist loam at the bottom, use this "microclimate" to its best advantage. Creeping thyme, dwarf comfrey, rosemary, lavender, chamomile, houseleek, sage and other "Mediterranean" herbs thrive in the dryer, higher parts of a sunny bank. The middle and higher parts of north-facing slopes favor periwinkle, sweet woodruff, alpine strawberry and angelica. For the bottom of either sunny or shady slopes, consider violets, primroses, mint, angelica and cowslip.
Flowering Perennials, Annuals & Bulbs
Flowering perennials that stand up to wind and water runoff work best on slopes. Hardy choices include coneflower, wild indigo, goldenrod, prairie clover, black-eyed Susan, yarrow, sedum varieties, globe thistles and sea pinks. Smaller bulbs like crocuses, jonquils and some tulips also thrive. While waiting for these perennial flowers and other plants to establish themselves, consider adding annual bedding plants -- marigolds and zinnias add inexpensive color in most sunny situations, and impatiens in shady sites.