Dahlias (Dahlia spp.) bloom in summer in shades of red, yellow, pink, orange, purple and white. Hardy in U.S. Department of Agriculture plant hardiness zones 8 through 10, dahlias thrive in warm weather and full sun once they are established. Young seedlings need time to adjust to outdoor conditions before you can plant them outside. The process for this adjustment is called hardening off.
The best time to begin hardening off dahlias depends on your climate. The hardening off process takes about two weeks, and should begin after the last frost in your region. New plants can’t tolerate low temperatures and can be killed by light frosts. Hardening off is usually safe once soil temperatures remain around 60 degrees Fahrenheit, and daytime temperatures outdoors are around 70 F. In most climates, it is safe to transplant dahlias into the garden from mid-April through May.
Start the hardening off process by placing dahlias in a spot where they are protected from wind, heavy rain and extreme temperatures. A sheltered area, such as a covered patio, where your dahlias receive indirect sun is a good choice. Move your dahlias gradually so they are exposed to a little more sun each day. During the first few days of hardening off, keep the dahlias out of the sun, wind and rain. Dahlias that have not been properly hardened off may scald in bright sun.
Hardening off dahlias exposes them slowly to the more extreme weather outdoors and reduces the risk of transplant shock, which can kill tender young plants. Begin the process by placing your dahlias in a protected area outside in the mornings. Reduce watering so you water only when the soil feels dry to the touch. On the first day, leave your dahlias outside for one to two hours. Each subsequent day, increase the time outdoors by an hour, until you reach a point where your dahlias are outside all day. As you put them out each day, you should also gradually move them to a more exposed spot. If temperatures are expected to fall below 50 F at any point during the hardening off process, bring the plants indoors. After seven to 10 days, your dahlias should be ready for transplanting.
After hardening off, you can grow your dahlias in containers outdoors or plant them in the garden. To ensure your plants survive transplanting, move them into the garden on an overcast day. This allows the dahlias to adjust slowly to their new home without being exposed to too much sun. Surround the roots with as much of their old soil as you can to help reduce the chances of transplant shock. Keep newly transplanted dahlias moist but not soggy until the roots have established, which typically takes one to two weeks.