As the days get longer, gardeners become eager to put seeds in the ground, but run the risk of putting them in too soon. Seeds of some kinds of plants can grow while the ground is still cold, but others require warmer soil. Soil temperature is taken with a soil thermometer, in the morning, in the shade, and inch or so below the surface. Three weeks ago, the soil in the teaching garden at City College of San Francisco was 46 degrees F. Last week it was 42 degrees, made lower by the cold storm we had the previous week. This goes to show that the warming of soil is in spring is not necessarily a steady process. What will it be tomorrow? I will report.
In soil this cold, the snap pea seeds are germinating, though they aren't all up yet after 3 weeks. In soil at 60 degrees, we'd have seen them all in about a week. Warm season crops, like tomatoes or peppers, might not come up at all in soil this cold, so are best started indoors at the nice warm 80 degrees they prefer.
For a nice short article on what might have caused your seeds not to come up, see http://humeseeds.com/nogrow.htm an article on the website of Ed Hume Seeds.