Fleemo wrote:

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?

Depends on what you are needing to protect. Many early blooming plants
can have their flower buds damaged by a late hard frost - camellias for
example - but many other cold-sensitive plants will not survive a late
frost regardless of the protection you provide. Annuals like basil,
coleus and impatiens would be included here. Most perennials and woodies
will withstand a pretty good late frost without protection if they are
hardy for your zone. They might suffer some minor foliar damage, but the
roots should be unaffected. If you are concerned, try covering with remay
(aka harvest cloth) - this will ensure temps under the cloth are at least
5 degrees above the air temp.

I am a good bit north of you and we have had clear skies and frosts for
the last few days. I protect nothing in my garden - it must stand on its
own - and although many plants are already budding/blooming due to our
unseasonably mild winter, nothing appears to be any the worse for wear.

We gardeners tend to worry a bit unnecessarily sometimes - I can't
imagine that frosts in the Sacramento area could be too terrible at this
late date.

pam - gardengal