Planting in Climates With Cold/Freezing Winters
Make certain that the ground temperatures have cooled enough so that when you sow there is no chance of the seed germinating. It they do, the wildflower shoots will die off with freezing temperatures. In cool climates, make sure ground temps are below 55 degrees. The biggest mistake people make with fall planting in cooler climates is sowing their seed too soon. You can wait and sow over snow in winter as well, as long as the ground has been prepared for seeding.
Planting in Warmer Climates
In warmer climates, you can take advantage of the rainy season for sowing wildflowers in fall. Your seed will germinate during the most optimal temperatures, rather than the often-times challenging high temperatures of very warm spring or early summer. If you live in a warm climate that still experiences frosts, you'll want to time your planting to be about 60-90 days before the first frost arrives, so your perennial wildflower establish strong root systems before frost. In warm climates, you can also 'winter sow' your wildflowers in January or February, taking advantage of the natural precipitation. Seeds will typically germinate about 2-4 weeks after planting.