Summer can be tough on annual flowers in the South. Heat, insects, and diseases cause flowers to "burn out" before summer's end. That's why gardeners in USDA Hardiness Zones 7 and 8 may want to replant a fall crop of annual flowers to keep the flowering season going into winter. Recent evaluations conducted at Mississippi State University revealed some top performers among colorful annuals.
After studying 27 cultivated varieties planted on three test dates (August 15, August 30, and September 13), researchers at Mississippi State University in Starksville (zone 7b) found that the annuals did best and seemed to have the fewest pest problems for that region when planted in mid-August.
They periodically evaluated the annuals for flower color and number, canopy cover, leaf color, and overall performance and came up with these top picks:
For full sun: Begonia semperflorens: 'Varsity Bronze Scarlet', Catharanthus roseus: 'Cooler Raspberry', Petunia: 'Purple Wave', P. grandiflora 'Pink Storm', Tagetes patula: 'Bonanza Harmony', T. p.: 'Marvel Gold', and Zinnia: 'White Pinwheel' were the best choices.
Based on their research, gardeners should set out six-week-old, 4-inch transplants in mid-August. (Zinnias performed well after direct sowing at the same time.) They emphasize you should water well, especially right after planting. For a lush, vivid display, they found that spacing the plants 8 inches apart, rather than the commonly recommended 12 inches, resulted in better color until frost.