Denis Mitchel said:


Does the temperature have to be at or below freezing for frost to form on
plants?

In terms of the "official" air temperature, no.
In terms of the very immediate area of the plant, yes.

Beware of frost in open areas when the nights are clear, the overnight
temperatures are expected to be lower than 38 deg F, the dewpoint is
near (or lower than) 32 deg F and the winds are calm.

Heat lost to radiation to the open sky will drop the temperature near the
ground.

The dewpoint limits the amount of radiational cooling. When the dewpoint
is at or below freezing, frost will form.

This goes doubly so in lower lying areas, as cold air will flow downhill to
accumulate there. The bottom end of my vegetable garden is very slightly
downhill from everywhere at this end of the block. It may be the only
place that frosts on some days. The slope is very subtle, but it is enough
of a slope to create a frost pocket.

(A good meteorologist will tell you the dewpoint. Relative humidity is
no where near as useful, in my experience.)


--
Pat in Plymouth MI

"So, it was all a dream."
"No dear, this is the dream, you're still in the cell."

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