Since flowers provide the bulk of the color in a garden, one of the most important aspects of planning for color is the timing of bloom. When designing, you must know when each plant will bloom and for how long. Different plants bloom at different times throughout the season and for different periods of time. The bloom times of some plants coincide and others overlap. Timing must be coordinated to produce the effect you want.
The object is to have new flowers appearing as others fade. For many gardeners this is the most challenging, most interesting, and most exciting feature of flower gardening. Many flowering shrubs, trees, and other woody plants bloom for short periods in the spring or summer, although some flower for months. Most perennials usually bloom for two to four weeks, whereas annuals generally bloom throughout the growing season. In general, even when you plant for a succession of bloom, there will be three or four peak periods of bloom during the season, interspersed with periods of quiet.