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?





It depends. What are you trying to protect?

Some plants will show damage with air temperatures of 35F or below with
clear skies at night. Radiation cooling will produce local frost and
damage the plant. Basil is the best example of this.

Other plants will take temperatures down to 20F with no damage. Lettuce
is one of these (not all varieties however -- the red edge seem to do
better than green lettuce).

If you are dealing with radiation cooling (air temperatures above 32F)
you can cover the plants. If you do this, support the cover so that it
doesn't touch the plant. The cover will get cold and can damage the
plant where it touches.

If you are dealing with really tender plants such as basil, a water
spray won't provide much protection, since it will freeze. If the
temperature is really marginal it might help, but such plants really
don't like the low temperatures and may just decide to die anyway.

The way a water spray works is that water requires a certain amount of
energy to change from a solid to a liquid. It gives off this energy when
it changes from a liquid to a solid. The water releases this energy,
called the "heat of fusion" when it freezes. This means that in the
process of freezing, while you have a mixture of ice and water, the
local temperature is kept at 32F. Once all the water is frozen, the
local temperature will go lower. This is why you have to keep spraying
liquid water. However, the best you can do is to keep the local
temperature from going below 32F.

In the case of the cold tolerant plants, their fluids have a lot of
dissolved solids (sugars, starches, various nutrients). The dissolved
solids lower the freezing point of the liquid below 32F. The plant will
eventually freeze, however, and since the plant fluids are largely
water, the water will expand on freezing and break the cell walls,
damaging the plant.

As far as the time of night to apply protective measures, unless the
temperature is really dropping rapidly, late in the evening is OK. The
coldest part of the night is usually just before dawn. However, if you
have a rapidly moving cold front coming through it could drop below
freezing quickly (even during the daytime hours).

It's a bit late in the winter (northern hemisphere) to worry about frost
protection.