Some annuals have a relatively short bloom season, just as some vegetables are harvested once and then are gone. To have a continual supply of color or food, plant seeds of these in succession, a couple of weeks apart, from spring to mid summer. I have harvested big, juicy Tomatoes in November from a July planting, and enjoyed fresh, vigorous Marigolds in late summer from summer planting. Beans and Corn are also excellent for succession planting, and a second planting of Zinnia makes a wonderful late summer display.