Perennials do not bloom all season long; rather, they have a season of bloom which can range from 2 weeks to 2 months or longer. When designing a perennial garden, it is important to truly understand just when and for how long a particular perennial blooms in your area.
Once you have pinpointed the season of bloom of a particular perennial, you need to "dig a little deeper". There are many other very important characteristics of plants that add to their value and role in your garden. Find out:
The foliage of a perennial is as important as the flowers! This is very hard for beginning gardeners to accept. When you first start out, all you can think of is color, color, color. If a perennial is in bloom for 3-4 weeks out of a 7 month growing season, and it's going to be taking up valuable space in your garden, you should know what it will look like after blooming. Some perennials have very attractive foliage that is a real garden asset. Some have colorful or variegated leaves that can be as valuable as flowers. Others need to be cut back or get very unkempt looking as the summer heats up. Find out the real story about each plant you choose.
Once you have honestly appraised the plants that will go into your garden, then you can work with their strengths and weaknesses to create a lovely picture every single week. Surround early bloomers that look awful in August with good foliage plants or strong late bloomers. By taking this approach, you can avoid the gaps and unsightly holes that most perennial gardens suffer from.
To draw a perennial garden plan, measure the garden bed and draw it to scale on a piece of graph paper. Place 8 sheets of tracing paper over this "master plan" and firmly attach them to the scale drawing with tape or staples. You will then have a tracing paper overlay for each of the 7 months of the growing season (April through October) and a bulb planting overlay.
As you choose your plants, draw them onto the appropriate month(s)' tracing paper overlay IN PENCIL. You will be changing and rearranging your plant placements many times before you are done.
To assure that you are drawing your plants to scale on your plan , cut cardboard circles to represent 18", 24" and 36" diameters. Most individual perennials will grow to 18" in diameter in 3 years. Many grow much larger than that. As you add plants to your plan, trace a circle of the appropriate size for each individual plant.
It is common practice, especially in larger gardens, not to use individual plants but rather to group plants together in drifts. This creates a more striking effect in the garden. The larger the garden, or the farther away from which you will viewing garden, the larger the drifts should be.
Make your garden as wide as possible. Very narrow borders do not allow you to layer perennials in drifts from tall to medium to short. Since the average plant takes up an 18" diameter space, even if you were to only layer 3 single plants in a garden bed, it would have to be 4 1/2' - 5' wide to accommodate them. If you are limited in space, make your garden wider and not as long.
A general rule of thumb about the heights of plants is: the height of the tallest plant should equal half of the width of the bed. If you have a very narrow garden and use 6-7' tall perennials in it, you won't have the room to layer your plants gracefully from tall to medium to short. Narrow beds can better accommodate shorter plants.
When placing your plants on the plan, use plant marriages. Place groupings of plants next to each other that bloom at the same time rather than trying to spread out and dot bits of color all around the garden each month. Plant marriages can be very effective if the colors are complementary or opposites, creating exciting contrast. There are many classic plant marriages that you will see in books, catalogs and slides that you can "plug in" to your plan in the appropriate month(s).
Place plants that are very different next to each other. Combine spiky plants that offer vertical accent with soft, billowy, mounded plants. Use broad textured, heavy foliage with delicate lacy foliage. Bold dramatic flowers look all the more dramatic if paired with light airy flowers.
Do not hesitate to repeat the same plants or combination of plants in a different spot in the garden. This will make the overall picture much more cohesive. Consider color echoing, i.e. repeating the same color or color combinations, throughout the garden, even if the actual plants are different. You can develop themes, such as using silver foliage plants or ornamental grasses for dramatic vertical accents, repeated in strategic spots in the garden.
Use spring, summer and fall blooming bulbs in between the plants. They go dormant and disappear after blooming. The perennials can easily mask the gaps. Once the plan is complete, use the bulb tracing paper overlay to embellish the plan with bulbs for added color.