to protect against frost / freeze - WATER
On another post, someone included this great link to Cornell on Understanding Frost:
I'm copying this bit because I found it really helpful:
Irrigate before the frost Ã¯Â¿Â½ A moist soil can
hold four times more heat than a dry soil. It will
also conduct heat to the soil surface faster than
a dry soil, aiding in frost prevention. In a study
performed years ago, the air temperature
above a wet soil was 5 degrees F higher than
that above a dry soil and the difference was
maintained until 6 a.m. the next morning.
Thus, plants should be well watered the
evening before a frost.