Why does frost form when the temperature never gets at or below freezing?

Answer 1

A little guy named Jack does it

Answer 2

It's called radiational cooling. Picture this; you sit in front of a fireplace and you can feel the heat from it even though there's no hot air coming your way. You are warmed by infrared radiation which we perceive as heat. Everything radiates heat this way; even us.

Now at night, even though the air is above freezing, the ground has the sky over it. On very cold nights, we are looking into space and the temperature of space is over a hundred degrees below zero.
The ground/your car/whatever radiates its heat into the cold of space and gets to the point where its temperature drops below freezing and frost forms. On cloudy nights, we don't have a straight shot at space and frost rarely forms.

Answer 3

It's called the Wind Chill Effect. It happens because there is water in the plant or whatever is getting frosted.