It's not the air temperature but the soil temperature that is critical in seed germination. Seeds begin growing when the soil reaches the optimal temperature for germination.
Certain vegetable seeds can be planted out in the early spring but they still need warm soil for good germination. You can help "Mother Nature" and speed up soil warming by covering your vegetable garden soil with black plastic for two weeks prior to planting. Depending on where you live, you can start this as early as March 15th. The black plastic will warm the soil much faster, allowing you to plant certain crops earlier and still have good germination.