A listener asked, “What is a ‘hard freeze’?” The answer is a bit complicated. In general, a ‘hard freeze’ is said to occur when temperature drop low enough for a long enough period of time to seriously damage or kill seasonal vegetation. For most of the US, a hard freeze is when temperatures drop below 28 °F for several hours during the growing season. In some areas the threshold is as low as 25 °F, and others it is 32 °F. One definition also notes that a hard freeze would result in ice forming on standing water, and the ground hardening. If you have gardens, or ornamental plants outside, watch for those cold temperatures that can still create damage.
Mary Knapp, Weather Data Library