Every winter is a little different, so I use a soil thermometer to keep an eye on the temperature of the soil and plant accordingly.  The thermometer pictured here is actually a meat thermometer, bought for less than $10, but it works just as well as the more expensive soil thermometers you'll find in gardening stores.

To check the soil temperature, get up early before the sun has hit the ground and insert the thermometer into the ground.  Wait a few minutes, then take a reading.  If you have garden areas that are more sunny than others, you'll want to test the soil temperature at several places–I usually find that our soil is two to five degrees colder in the shade of our hillside compared to in the sunnier parts of the garden.

The table below lists the germination temperatures for common spring crops.

To plant earlier than cold soil will allow, many gardeners are tempted to start seedlings indoors.  However, unless you have grow lights or a heated greenhouse, I recommend that beginners stick to growing their plants entirely in the earth for the first year.  A quick hoop might be enough to let you plant a couple of weeks earlier than you otherwise could have, then you can transfer the same protection to a new bed in April to jumpstart your summer garden.