Wildflower planting dates largely depend on site location and geographic weather patterns.  The planting timetable should be decided by seasonal precipitation in your area rather than by temperature.  Wildflowers can be planted in the fall or early spring throughout all regions of the U.S. In the northern and northeastern geographic regions of the United States, USDA Zones 1 through 6, where extremely harsh winters are experienced, an early spring planting is recommended. In the Southern regions of the United States, USDA Zones 7 through 11, your wildflowers can be sown in early spring if desired.  Note: There are risks associated with an early spring planting in USDA Zones 1 through 11.  Warm spring weather and adequate rainfall will accelerate germination and seedling growth. However, if rainfall is sporadic after initial germination followed by an extremely hot, dry period, supplemental watering may be required to keep the ground from drying out and the seedling from dying.  Sowing In the southern and western portions of the United States, USDA Zones 7 through 11, the autumn months of September through December are the most favorable to plant your wildflowers. Many of the species will quickly germinate in order to allow the seedling enough time to establish a healthy root system before going dormant in the winter.  Some of the seeds may not germinate if the ground temperature is below 70 degrees Fahrenheit.  These seeds will remain dormant within the soil until early spring and will begin to emerge under more favorable conditions.  In the northern regions, USDA Zones 1 through 6, your wildflowers can be planted in late fall.  If you decide to plant your seeds in the fall in Zones 1 through 6, the seed will remain dormant during the harsh winter months and germination will begin at the first indication of spring.  NOTE: There are risks in sowing exotic garden varieties and "domesticated" species D in the fall.  Freeze damage may kill these varieties if unseasonably cold temperatures persist for long periods of time.