Perennial flower gardens are as diverse as the gardeners who imagine them. That said, there's a wealth of information on garden design and plants out there to help you along your way. Explore online sources, gardening books and magazines as well as consult knowledgeable staff at a garden center or bend your neighbor's ear, the one who's been growing flowers for years. Here are some ideas to open the door to your own imagination.
Borders, Island Beds and Mixed Borders
According to the article "Stepping Stones to Perennial Garden Design" (author not listed) on the University of Illinois Extension site, the perennial garden was conceptualized in 1890 by George Nicholson, curator at Kew, England's Royal Botanic Gardens. The perennial border, with tall plants at the back of the bed, was introduced, followed much later by island beds, and finally the mixed border of perennials, shrubs and trees, and annuals that is popular today.
The Woodland Garden
The most unorthodox of perennials often grow in a woodland garden. They have such quirks as disappearing totally after flowering like Virginia bluebells (Mertensia) or shooting star (Dodecathon) and are called ephemerals because of this. They may be close to the ground like the delicate hepatica with its azure flowers or bluets that are vivid, but pale blue (if those two descriptions can at all go together). Then there are the those unto themselves such as Jack in the Pulpit (Arisaema). More intricate than the shady border, the woodland garden is for gardeners who appreciate dappled detail.
The Rock Garden
Rock gardens can be more involved because they often include rocks and walls and the plants have special soil requirements. Bobby J. Ward in an online "Horticulture" article "Ready to Rock" discusses soil qualities for rock gardens: well-drained but able to hold moisture, "lean" meaning "low-fertility." Rock gardens can also be in shade. Plants should fit the general idea: low-growing and mounding. Try pennywort (Cymbalaria) for shade and low-growing sedums in the sun.
Gardens with a Purpose
Some gardens have a purpose: to attract hummingbirds and butterflies, for fragrance or to showcase a specific color. With a little research, you can discover plants that can accomplish your goals. Hummingbirds go for flowers that are red. Paul deMarrais in the online "Horticulture" article "Plan a Butterfly Garden" mentions plants such as butterfly bush or (Buddleia) and coneflower (Echinacea). For fragrance, try dianthus and Oriental lilies (Lilium orientalis). Create a "moonlight garden" with white flowers.
Start with a site. Check light levels. Full sun is all day, partial shade is a half day and then there's shade gradients. Do you want a mixed border, adding shrubs and trees for structure and annuals for color, or an island bed? Amend the soil with top soil and organic matter and sand to improve drainage. Choose plants that will do well in your climate. Follow a plan or create your own design.