Dahlias can be grown as annuals in zones 2–7 even though they are only winter hardy in zones 8–11. The plants will bloom by mid-to late-summer if the tubers are planted in the spring.
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Hardiness Zone 2: These plants can only handle severe colds for short periods. They need to be grown in a greenhouse or another warm place.
Hardiness Zone 3: These plants can handle short periods of very cold weather, but if they are exposed to cold weather for a long time, they should be moved to a warmer climate.
Hardiness Zone 4: is a transitional area between Zones 3 and 5, and it is usually found in the middle of the U.S. Plants in this zone can handle short periods of cold weather. Still, they should be moved to a warmer climate if they will be exposed to cold weather for a long time.
Hardiness Zone 5: is the most common zone in the U.S. and Europe. Plants in this zone can handle the moderate cold for a short time, but they should be moved to a warmer climate if they are exposed to cold weather for a long time.
Hardiness Zone 6: is where most plants in American gardens grow. Plants in this zone can handle short periods of cold weather, but they should be moved to warmer climates if they will be exposed to cold weather for a long time. Even though broadleaf evergreens like those with thick bark eventually settle in zones 5 and 6, it’s important to remember that these trees and shrubs will sometimes go through the worst months.
Hardiness Zone 7: These plants have fully adapted to the cold weather, but they may not be able to handle long or moderately cold periods.
Hardiness Zone 8: These plants can handle harsh winter winds for a short time, but they should be brought inside if they are out for a long time.
Hardiness Zone 9: it’s best not to replant plants that have been badly damaged by wind or snow. If a plant survives heavy damage from winter winds, it can handle short periods of cold weather, but if it is out in the cold for a long time, it should be brought inside.
Hardiness Zone 10: Only a few states near the equator are in Zone 10. The plant can handle very cold temperatures for a short time.
Hardiness Zone 11: is the farthest north and has the weakest plants. The plant can’t handle being outside in cold weather, so it needs to be grown inside.
USDA Plant Hardiness Zone Temperature Chart
|Zone 2a||-50 to -45°F or -45.6 to -42.8°C|
|Zone 2b||-45 to -40°F or -42.8 to -40.0°C|
|Zone 3a||-40 to -35°F or -40.0 to -37.2°C|
|Zone 3b||-35 to -30°F or -37.2 to -34.4°C|
|Zone 4a||-30 to -25°F or -34.4 to -31.7°C|
|Zone 4b||-25 to -20°F or -31.7 to -28.9°C|
|Zone 5a||-20 to -15 °F or -28.9 to -26.1 °C|
|Zone 5b||-15 to -10°F or -26.1 to -23.3°C|
|Zone 6a||-10 to -5°F or -23.3 to -20.6°C|
|Zone 6b||-5 to 0°F or -20.6 to -17.8°C|
|Zone 7a||0 to 5°F or -17.8 to -15°C|
|Zone 7b||5 to 10°F or -15 to -12.2°C|
|Zone 8a||10 to 15°F or -12.2 to -9.4°C|
|Zone 8b||15 to 20°F or -9.4 to -6.7°C|
|Zone 9a||20 to 25°F or -6.7 to -3.9°C|
|Zone 9b||25 to 30°F or -3.9 to -1.1°C|
|Zone 10a||30 to 35°F or -1.1 to 1.7°C|
|Zone 10b||35 to 40°F or 1.7 to 4.4°C|
|Zone 11a||40 to 45°F or 4.4 to 7.2°C|
|Zone 11b||45 to 50 °F or 7.2 to 10 °C|
Dahlias can be successfully planted in a wide range of climates, as long as you consider this flower’s specific growing requirements. While Dahlias are happiest in warm, dry climates with plenty of sunshine and little humidity, they will still grow well in milder zones if provided with adequate water and regular sun exposure. Cool-season dahlias (Zones 6-10) should be planted during the late fall or early winter when daytime temperatures are between 50°F – 55°F(-45.6-, while warmer-climate dahlias (Zones 9b through 11) should be sown from April to May when average daytime temperatures range from 75°F – 85°F.