Dahlias (Dahlia spp.) are versatile flowers that vary widely in size, shape and color. One thing they all have in common, however, is that they bloom best in full sunlight -- with one important caveat: They don't like very hot temperatures. If your dahlias can't take full sun, the issue could be the temperature of the sunlight. Other environmental issues might also be affecting the flowers.
Dahlias are perennials in U.S. Department of Agriculture plant hardiness zones 8 through 10. The plants bloom in midsummer and continue to bloom through early fall -- until the first frost, that is. In fact, dahlias seem to dislike extremes, as they also wilt under hot temperatures. High temperatures cause flowering to slow down, according to the North Carolina Cooperative Extension Service. Dahlias grow best in the cool, wet, mild summers of the Pacific Northwest, where they are sometimes inches larger than the same cultivar grown in a warmer climate.
Dahlias that fail under full sunlight might be lacking moisture. The plants thrive in soil that is consistently moist, but not soggy, as this can cause the tubers to rot. To keep the soil moist, surround the dahlias with 3 to 4 inches of compost, pine bark or straw mulch. Extend the mulch to a 2-foot diameter around the plants, which is how far the roots stretch from the tubers.
Light or dappled afternoon shade can help dahlias thrive in even the hottest climates, according to Old House Gardens. And although they don't like the air to be too hot, dahlias prefer warm soil. Too much shade can cool the soil down enough so that the roots rot, which is why the flowers can't be planted until all danger of frost has passed.
The best way to ensure that your dahlias tolerate full sun is to choose cultivars that have been bred for the heat. "Kidds Climax" is an award-winning, heat-tolerant cultivar that features pink and yellow blooms up to 10 inches across. "Prince Noir" has deep purple blooms and is highly recommended for hot climates. "Jersey's Beauty" is a popular, heat-tolerant dahlia that features bright-pink flowers on stems that can reach heights of 6 feet or more.