Answer: Without knowing how much sun the hillside receives it is difficult to make specific suggestions. Ornamental grasses need full sun or at least a minimum of six hours of direct sun to really grow well. Some perennial flowers that look natural together with ornamental grasses would include purple coneflower (Echinacea purpurea) and black eyed Susan (Rudbeckia fulgida "Goldstrum"), yarrow (Achillea), sages (Salvia) and sedums. Spring flowering bulbs such as crocus and daffodils would do well there too. You might also consider adding some shrubs to add height to the planting all season. Some to consider might include burning bush for a good red coloring in the fall or possibly a forsythia for wonderful burst of spring color. How many to use depends on the overall size of the area. All of these plants will all do well in a sunny spot and well drained soil, which a hill provides.
If you are going for a more natural look on the hillside you might want to use a more natural edger material than brick. Used railroad ties, stones collected on the property, or even tree logs would be very in tune with the natural look.
I hope this gives you some ideas.
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