Richard Tyson, County Extension Agent - E-mail Richard Tyson
Twenty eight (28) degrees F is the temperature most plant tissue freezes, but vegetable fruits like squash, tomato, eggplant, and snap bean will freeze at 30°F degrees. The critical temperature for vegetables to turn on frost protection irrigation would likely be from 32°F to 34°F depending on whether fruit was present or not on the plants.
For information on critical temperatures for vegetables check the EDIS publication Frost and Freeze Protection for Vegetable Crops Grown in Florida in the BMP Era By Eric Simonne and George Hochmuth