An annual flowerbed won't feature early spring flowers but will have flowers all summer long once they start blooming. Begin planning your rectangular garden with edging plants, such as begonias, in the front. Place petunias or snapdragons behind the edging plants. Create some focal points with groupings of tall plants such as gladioli, cosmos or sunflowers towards the center of the bed, then fill the spaces between with plants including marigolds, zinnias, African daisies, geraniums and stocks. Depending on your personal preferences, you can plant flowers of similar colors, or create a flower rainbow.
You can achieve a very formal look with some perennial shrubs and herbs that have a compact, neat appearance, such as lavender or Potentilla. Or create a more natural look with mid-season blooming perennial plants such as tickseed coreopsis, butterfly bush and mints like bergamot. Place tall plants--such as butterfly bush--in the center of your rectangular bed. Group three or more medium-sized plants--such as purple coneflower--together in the middle ground, and plant shorter perennials in the front, such as dragon's blood sedum. With a bit of planning, you can have flowers in bloom all season long. Windflowers and crocus will bloom early in the spring, daffodils mid spring, and tulips late spring. Add some late fall blooming perennials such as asters and sedum "Autumn Joy" to have a complete growing season of blooms.
Rectangular flowerbeds lend themselves to raised bed gardening. The material that you use to create the raised bed will affect its durability and appearance. Cement blocks create an urban look but with the proper flowering plants--especially those that arch and break the plane of the cement blocks--the final look can be more casual and attractive. Wood-edged raised beds create an earthy, country look in the garden, if you allow the wood to age naturally, or can provide an oriental look if they're treated with a cherry stain. Painting the blocks or wood can change the whole appearance of the raised bed. Lighter colors can help reflect light in darker corners while darker colors will heat up the soil sooner in the spring.
If the rectangular garden is located in a less prominent position, planting cut flowers in an island raised bed allows you access to cut flowers for bouquets without compressing the soil. Some flowers are better suited for cutting. Zinnias are a classic cut-flower favorite. There are many varieties, colors and sizes of zinnia including some species that are referred to as "cut and come again" because of their prolific blooms. Other cut-flower favorites include roses and bulbs. When selecting roses, look for straight stemmed varieties instead of climbing types. Many bulbs are appropriate cut flowers. With some planning for bloom time, daffodils, tulips, gladiolas, crocosmia and other bulbs can contribute most of the summer to a cut-flower garden.