First, unless you love the natural or native appearance, a perennial garden is not a maintenance free garden, but it does require less work, yields more return for your efforts and will bloom year after year. Second, a well designed perennial garden does not require a degree in horticulture or years of experience. A good garden design starts with thinking about what you like and want to accomplish, and most importantly - remembering that your garden is an evolving project and that a perennial garden will take 2 to 4 years to mature.
Choose your garden location before designing it; work with the land, not against it.
Consider sun, wind, soil type, soil amendments and water.
Plan the area to scale on paper.
Consider the amount of time needed to maintain the garden.
If needed, add organic matter to improve soil aeration and drainage - incorporate organic matter 12 inches deep. A general recommendation is 3 cubic yards of organic matter to every 1,000 square feet.
Make a list of the plants you like and group them by color, texture and form. Also chart them by season of bloom. Consider both flowers and foliage.
Narrow perennial designs are not as effective as wider ones. A 3:1 ratio is a good rule of the green thumb - 1 foot wide for every 3 feet in length
Use plants in clumps. Place like plants in groups of three, five or seven (odd numbers) of each type to increase the effect of color and texture.
Repeat groups of the same plant type two or three times throughout the space, to give it unity.
Use tall plants at the back of a one-sided garden or in the middle of an island bed.
Complement tall plants by gradually placing shorter plants towards the outer perimeter, ending with low border plants at the edge of the bed. Bring an occasional plant forward from their height line to increase variation.
Use various colors, textures and forms to add interest to the garden.
Consider bloom time and interesting foliage to create a succession of color and interest throughout the season.
Complement perennials with annuals and bulbs for bright focal points and accents during low bloom periods.
Leave room for plant growth and allow for individual plant growth habits. In general, plant tall perennials 18 to 36 inches apart, intermediates 12 to 18 inches apart, and dwarfs 6 to 12 inches apart. Don't place plants in straight rows -- use a scattered or triangular spacing so one mass blends into another.
Plant perennials so they can become established before dormancy and the onset of winter.
The most important consideration in selecting plants for a perennial garden is to group them according to their environmental and cultural requirements. A drought-tolerant plant may not thrive in moist conditions.
When you have your basic garden plan, mark out the proposed areas for planting and examine them from every angle - from different parts of the garden and the various viewing areas - consider the garden in terms of interlinking shapes rather than flowers and shrubs.