Raised bed flower gardening is an excellent alternative to in-ground planting where soil is rocky or poor. Plan your garden in winter, then cart in enough garden soil in early spring to create a level bed at least 30 to 37.5 cm (12 to 15 inches) deep. Border it with stones, bricks or railway sleepers to hold the bed in place and keep it from washing away. Give the soil a week or so to settle before you plant a mix of annual and perennial flowers.
Plant tall perennials in your raised bed for a low-maintenance supply of colour and light shade for other plants. After your initial investment, the flowers will pay for themselves by coming back year after year. Low-maintenance, tall perennials like rudbeckia, black-eyed Susan, hollyhock, iris, gayfeather, red-hot poker, lupine and loosestrife make beautiful, tall backdrops for smaller plantings and will crowd the space with blooms of every colour from spring to late summer. Many of these species will grow to a height of more than 90 cm (3 feet), with equal spread.
Leave room for planting tall annuals when all threat of frost is past. In many areas, this can be as late as the last week of May. Plant sunflowers from seed in May for towering stalks of flowers up to 3.6 m (12 feet) high in August. Fill in bare spots with clumps of cosmos that top out at 1.2 m (4 feet) with daisy-shaped flowers in magenta, pink and white. Zinnias and dahlias also come in taller varieties in a wide range of colours, needing very little care other than watering and deadheading to promote continued blooming.
Plant smaller perennials in front of taller flowers to direct the eye down while keeping interest and variety. Sea lavender has a clumping growth habit with countless tiny flowers in summer. Dianthus, related to the carnation family, creeps in mounds along the ground. Creeping phlox has the same mounding habit, with lavender-blue blossoms in the late weeks of spring. Scabiosa also hugs the ground, with purple, pincushion-like flowers in midsummer. Maltese cross with its scarlet blooms can be trained up onto a stake, or left to creep and tangle itself through other plantings.
Plant small annuals to fill in bare spots with bright colour or to refresh areas where perennials have faded for the year. Impatiens come in every colour and need nothing but water and partial shade to bloom all summer. Petunias, snapdragons, marigolds, blanket flower and lobelia bring added colour with little care needed other than deadheading. Coleus fills out the roster with inconspicuous flowers and brightly coloured, variegated leaves. All of these small annuals will continue blooming until the first frost of autumn.