When you plan your flower garden, perennial flowers in cool shades -- such as purple, blue and green -- should form the backbone of the design. In fact, garden design guidelines suggest using cool colors for about 85 to 90 percent of the plants in a landscape, according to the University of Vermont Extension Department of Plant and Soil Science. When planting medium-height purple perennials, or plants from 3 to 6 feet tall, consider their cultural requirements such as moisture and sun, as well as their U.S. Department of Agriculture plant hardiness zone ratings.
For cool climates, plant delphiniums (Delphinium elatum). These striking perennials grow to 4 feet tall and bloom with spires of deep to light purple blossoms in summer. Hardy in USDA zones 2 to 7, the delphinium prefers sunny sites with rich, well-draining soil. The purple conflower (Echinacea purpurea) grows to 3 feet tall and is hardy in USDA zones 3 to 9. This adaptable perennial tolerates drought and heat and blooms from summer through frost with daisy-like, purple flowers. Deadhead the flowers for more abundant blooms. Purple coneflowers attract pollinators to the garden.
Moderate Temperature Areas
The milky bellflower (Campanula lactiflora) grows from 3 to 6 feet tall and is hardy in USDA zones 5 to 7. An upright perennial, the milky bellflower blooms from summer through fall with clusters of bell-shaped blossoms. It prefers moist, well-draining soil and grows in full sun or partial shade. The obedient plant (Physostegia virginiana) also produces purple flowers from midsummer through fall. This North American native grows to 4 feet tall and produces spires of purple-pink blossoms. It's hardy in USDA zones 2 to 8 and grows well in a range of site conditions. Hummingbirds love this native plant.
For late summer blooms, plant a New York aster (Aster dumosus). This perennial grows well in full sun to partial shade and reaches heights from 3 to 6 feet tall. Its purple, daisy-shaped flowers have yellow centers and attract butterflies that the kids will love to watch. The cardoon (Cynara cardunculus) creates a dramatic effect with thistle-like purple flowers that appear atop 5-foot-tall gray-green stems in the middle of summer. Hardy in USDA zones 7 to 8, this perennial thrives in sunny, dry sites. It's related to the artichoke and has edible stems.
Warm and Dry Conditions
The hyssop (Agastache foeniculum) grows to 3 feet tall and is hardy in USDA zones 5 to 9. This sun-loving perennial attract bees and butterflies with its tall blue-purple flowers. It flowers through the summer and prefers moist, well-draining sites. For aromatic scent, plant lavender (Lavandula angustifolia). This 3-foot-tall perennial blooms with purple flowers through the summer atop fragrant, gray-green foliage. Hardy in USDA zones 5 to 9, lavender grows well in sunny sites with slightly alkaline soil.