Heats up and dries out quickly in spring. Allows seedlings to germinate and become better established earlier in spring. Roots have had time to grow deeper, and seedlings are more resistant to summer dry spells. Hard to work in the spring if it is wet. Clay dries out obstructing root development.
Earlier germination means greater seedling establishment before summer heat.
Fall or spring are both acceptable. If wildflower seeds are planted in fall, they go through winter in the soil, breaking seed dormancies naturally, promoting higher spring germination rates compared to spring or early summer seedings. Divide the mixture in half. Firm the seed in the soil by rolling over the site with a cultipacker, roller, or drive across site with a truck or tractor tires. Firm the seed in the soil by rolling over the site with a cultipacker, roller, or drive across site with a truck or tractor tires, unless your seeder already firms the seed into the soil.