Gardeners typically take nature's challenges in stride, but slopes are particularly difficult gardening situations. Preparing, planting and maintaining gardens on slopes may all present obstacles. Perennials are good choices for slope gardens if you select your plants carefully. Choosing plants for slopes means considering drainage, aspect and other conditions that may affect the growing conditions. Successful slope plantings will reward you with erosion control, lower maintenance and an aesthetically pleasing landscape feature.
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Assessing Planting Conditions
Choosing the right plant for the right place is essential to any perennial planting, but a slope adds another dimension. If you garden in the mountains, high altitudes and shorter growing seasons may narrow your range of possible plant choices. If your slope faces north, you may have heavier shade and much cooler growing conditions, while a south-facing slope can be brutally sunny and warmer. Most slopes will have quick drainage and may have poorer soils because of runoff or because a road cut or other human intervention created them, exposing subsoils. Rocky slopes have little soil, while steep banks have few rocks.
At high altitudes, winter hardiness may be the most important characteristic for perennials on slopes. Plant marginally hardy perennials at the top of a south-facing slope near a building, advise University of Idaho Extension specialists.
Rock Garden Approach
You don't need a slope to create a rock garden, but rocky slopes, especially those at high elevations, make ideal locations for rock gardens. Many traditional rock garden plants are alpine species, which survive above the tree line and require little soil for their roots. Colorado State University Herbarium Collections manager Jennifer Ackerfield includes alpine forget-me-not (Eritrichum aretioides), phlox (Phlox condensate) and sandwort (Minuartia obtusiloba) in her list of recommended native rock garden plants.
Even at lower elevations, rock garden perennials require sharp drainage, so they work well in soil pockets on rocky slopes. Washington State University Spokane County Extension Master Gardener Sydney McCrea recommends yarrows and penstemons, for example.
Flowers and Herbs
In a more conventional landscape where conditions don't include high elevations and rocky soils, consider planting low-maintenance perennial herbs and flowers that will tolerate poor, dry soil conditions. Ornamental and culinary thyme plants will create an interesting patchwork under taller herbs such as lavender and rosemary. Combine with tough flowering perennials, such as coneflowers (Echinacea spp.), day lilies (Hemerocallis spp.) and sedums.
When your slope is wooded your perennials will still need to tolerate well-drained conditions and will have to compete with tree roots and other understory plants. Sydney McCrea notes that epimediums and hostas are two plants that tolerate such conditions. Although both are flowering perennials, gardeners usually grow them for their interesting foliage. Hosta cultivars' variegated leaves and epimediums' patterned leaves create a mosaic of light and dark to add visual interest to shady areas.
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- Universtiy of Minnesota Extension; Steep Slopes; Connie C. Collison
- Colorado State University; Colorado Mountain Gardening Basics; L. Potts and I. Shonle; April 2008
- University of Idaho Extension; Gardening Strategies for Short-season, High-altitude Zones; Stephen L. Love, Kathy Noble and Stuart Parkinson; April 2009
- Washington State University; Spokane County Extension: Rock Gardens and Rock Garden Plants; Sydney McCrea; January 2005
- Colorado State University Herbarium; Gardening With Native Plants; Jennifer Ackerfield