Question

why do farmers fill their feids with water on a cold winter night

Answer 1

During a cold night, when the temptrature falls below 0°C,  water in the veins of plants will freeze. Due to the increase in volume of water on freezing, the veins will burst and the plant will be destroyed. To prevent this, farmers fill the fields with water. 

Answer 2

to freeze their field. if the field is frozen, the fruit will not rot.