Seed Germination and Soil Temperatures
Since Reed mentioned planting radish seed this weekend, I thought I'd link Tom Clothier's Vegetable Seed Germination Data Base -for anyone who might find the information useful. We refer to it a lot on this forum in the January-May planting season.
The information is useful because if you sow your seeds when the soil is way too cold for them, they will germinate very slowly and there's also a risk that they may rot if they sit in the soil for too long waiting for it to warm up. Conversely, with cool-season seeds, if you wait until the soil temps are too high for optimal germination, that can be a problem.
Sometimes we find ourselves between a rock and a hard place here in Oklahoma because the best soil temps for a given vegetable to germinate may occur much later than that crop needs to be planted in order for it to beat the heat and produce well before the temperatures are too hot for good production. Still, between the OSU Garden Planning Guide and the guidance on soil temperatures, most gardeners here manage to get a good stand of crops in late winter through late spring.
I plant based more on my soil and air temps than on a calendar date, and also based on my experience with the specific temperatures and freeze and frost patterns observed in our microclimate here where we live. The planting calendar is a helpful guide, but it is based on averages, and real life rarely mimics the statistical average.
Here is a link that might be useful: Seed Germination Data Base-Vegetables