"Denis Mitchel" wrote:

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

No. The dew on plants can freeze without the air temperature dropping to
below 32ºF... this often occurs with lawns and other low growing plants
because even though the air near the ground is heated by the radiant heat to
above 32ºF yet the fine droplets will freeze... fog can freeze and settle on
plants (like snow). But just because the dew freezes on the plant doesn't
mean the plant freezes, many plants contain chemicals that act as
antifreeze, and many plants will suffer 'frost' damage above freezing,
especially young tender seedlings. There's good reason why weather
forcasters mention "dew point", has to do with barametric
pressure/elevation.

Why do you ask?