Annuals such as pink petunias can provide a splash of color to "plug the gaps" between your perennials' blooming times.
No matter how well you've planned the sequence of bloom for your perennials, there will almost inevitably be gaps in color: periods when no perennial in your garden is offering a particularly showy display. Besides pink petunias, you'll find varieties in colors such as red, white and purple.
To achieve a colorful four-season landscape, cold-climate gardeners can supplement their perennial plantings with annuals for three of the seasons. For example, in spring and fall, use pansies; in summer, use petunias and impatiens.