Here is a handy chart courtesy of Seed Savers Exchange. Seeds or transplants of cool season veggies can be planted when the temperatures are at least 40 degrees, which is March and April in Virginia. There are two types of cool season veggies. Hardy types can withstand a heavy frost and temperatures as low as 40 degrees so they can be planted two to three weeks before the average last frost. In Northern Virginia, the average last frost date is between April 10 and 21 so I arbitrarily pick April 15 to be able to remember. That means that I can either directly sow seed into the ground the weekend of March 25 (because I work during the week) or (having started the seeds indoors) I can plant the small plants into the ground. Semi-hardy plants can withstand a light frost and prefer slightly warmer temperatures toward 50 degrees so they have to be planted later, two weeks before average last frost date which would be the weekend of April 1. If a severe temperature drop would to occur, I would protect the plants by covering them with empty 2-liter plastic soda bottles that had bottoms cut off.
