Kentucky is in hardiness zone 6, with annual temperatures ranging from -10 to 105 F.
Frost free dates vary with longitude, latitude and mountain conditions. Lexington is April 17, while Louisville is not until May 10.
You can check the links below for either more information on temperatures in your area or for more regional gardening information:
USDA Plant Hardiness Zone - specific hardiness zones for any area in the U.S.
The Old Farmer's Almanac - information on Frost Free Dates around the country
Home Vegetable Gardening in Kentucky - a 49-page informative PDF document on home vegetable gardening in Kentucky from the University of Kentucky College of Agriculture