A simple tool used by flower bed designers is to number the weeks of the year from 1 to 52. Week one being the first week of January. The first week of April is week 14. The first week of July is week 27 and on through the year. The plant chart on our Prairie Bloom list of perennials has a column providing the week of the year each of the plants begin to bloom here in Kansas. The dates are an average over time, so depending on any particular year's weather the actual date will vary. Remember that here on the prairie the average only represents the middle of the extremes and it is the extremes that actually occur.
Use the week of first bloom data to select your plants to have something in bloom throughout the growing season. Annuals are simpler as they bloom for the entire season or maybe we plan for a cool season and a warm season planting. Thirty weeks of color may require us to plant fifteen to twenty different perennials. But perennials offer a much greater depth and breadth of color, form and texture, a richness of palette choices, that annuals cannot match and they don't have to be replanted each year. Just be sure to consider bloom date when choosing which perennials to plant. If you have a big annual 4th of July barbecue for friends and family you may want to consider plants that will be in bloom week 27 to help decorate your yard for the party.
An ideal flower garden should have color spots of annuals for season long color and the uniqueness of perennials for all the visual interest and changing of the season they provide.