At what point should one take action to prevent frost damage? When
the forecast calls for temperatures to dip to 32 or below? Or should
you play it safe and take action any time the temps dip below 40?

Also, is spraying plants with water for frost prevention as effective
as covering them with material?
And at what point in the evening should preventative measures be
taken? Right after sundown, or is late in the evening ok?