Turtles eat a wide variety of plant life and common garden staples. They will eat what they can access and like low-lying greens like lettuces, carrot tops, collards, turnip greens and watercress. They'll also eat vegetables like squashes and pumpkins and fruits like strawberries, blueberries and tomatoes. Tomatoes contain phosphorus and small amounts of calcium, important parts of most turtle diets, so that may be one appeal of your plants.
A wide variety of box turtle species thrive in the wild throughout the U.S. The American box turtle, or one of its subspecies like the eastern box turtle, may be your garden culprit. The young are omnivores and generally look for a diet rich in animal matter like slugs, snails, worms and crickets. Adults, though, are generally vegetarians and will each anything they can reach in the garden.
Box turtles like to feed in early mornings and are likely more abundant on rainy days when worms and slugs are most easily available. You probably won't find them in your garden on really hot days. Turtles are reptiles and will avoid extreme temperatures of cold or hot.
If you are not willing to share your produce with your garden thief, then you should enclose the plot of land. Most turtle species are protected by law, and even moving them to a new location is against the law in some states.