Though arranging a flower bed is a matter of personal taste and style as much as anything else, there are several elements that can guide you in planning for your perennials. Whether you favor a more formal flower bed or a casual, cottage-style mass of blooms, consider the following features before you plant.
Start with size. The tallest perennials, whether trees, vines, or shrubs, should go at the back of the flower bed so they don't cover the smaller plants. The exception to this rule is certain taller but streamlined plants, such as some dwarf conifers, and taller plants you really wish to highlight, such as tree roses. These plants could be attractive in a front corner, or be centered in the flower bed as the focal point.
Create a color plan. After size, color is one of the most dominant traits of flowering plants and those perennials with colored foliage and/or fruits. The lime green stripes on certain hosta varieties can be as eyecatching as summer blooms, and in autumn and winter the orange and red berries of various evergreens will become more prominent. Group plants by a single color for maximum impact; all white in front, all pink in the back. Or choose two to three colors that work with your hardscaping and paint colors. Or you can choose a single color family, such as shades of blue, and stick to that for your flowering plants.
Think about winter interest. Deciduous perennials will lose their foliage in the fall, leaving a bare brown form. Include some evergreens in your flower bed, especially those with bright berries or seedpods, that will brighten up the winter landscape. Consider installing an evergreen on each side of your flower bed to balance the design.
Balance it out. Do you have your main bloomers, that great rose and the bright azalea, stuffed on one side of the flower bed and nothing bright on the other? Once you have things sorted by size, color, and winter interest, you still need to step back and notice any gaps or overfilled places.
Give it a finished look. Plan on small plants as the "edging" inside your perennial flower beds for a polished look. Miniature roses make an elegant edging statement; lamb's ears, with their silvery-gray foliage, highlight the area; and evergreen creepers, such as barberries, provide winter greenery and will grow to fill in spots beneath larger perennials in the flower bed.