Answer: Although zinnias are considered full sun plants, they do better with some afternoon shade in hot climates. As a rule you need to keep soil somewhat moist and mulch around the plants to help slow evaporation and to keep the soil cool. Zinnias can survive in soil that's on the dry side but will wilt in very dry conditions.
To keep your plants growing strong, trim faded flowers, or cut flowers often for bouquets, to promote more and longer blooming.
At this point I would prune off the faded flowers and feed the plants with a liquid fertilizer to help boost their health. Hope they recover soon!
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