"Fleemo" wrote in message
om...
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?

Play it safe. We had a minimum at 34 degrees on Wednesday morning (low
desert). I had a frost blanket over the tomatoes but it blew off on
one corner and one of the plants' leaves froze. It may recover but will
now take more time to produce fruit than the others and will probably
never be as healthy. 32 degrees for freezing is not an absolute. It
can also depend on air density (less dense in the desert), humidity and
several other ambient conditions. We sometimes get light freezes with
no white frost deposits because of the low humidity. We never need to
protect leafy crops or root crops but occasionally the frost-tender
varieties like tomatoes, peppers, corn, etc. need protection

Also, is spraying plants with water for frost prevention as effective
as covering them with material?

I believe a frost blanket is best. Water droplets freezing on the
leaves of frost tender plants can't be good for them.

And at what point in the evening should preventative measures be
taken? Right after sundown, or is late in the evening ok?

In our area, the coolest time of day is at sunup. I usually cover mine
tomatoes at sundown to help hold in the heat. But any time it is at or
near freezing and dropping is okay if you are only worried about frost
and not concerned about keeping the plants warm.

Note that my perspective is from the low desert and may not apply
everywhere. For some practical tips, see:
http://ag.arizona.edu/pubs/garden/az1002.pdf

Olin (Near Phoenix AZ)