Date: Mon, 27 Nov 2000 10:17:10 -0500
From: Ron Rovansek
<Rovansek.Ronaldj@EPAMAIL.EPA.GOV>
Reply-To: BB for Hummingbirds and Gardening for them in
the Southeast
<HUMNET-L@listserv.lsu.edu>
To: HUMNET-L@listserv.lsu.edu
Subject: protecting plants from frost

HUMNETters,
Now that frosts are likely to occur way down in winter humer
land, I
thought I would mention a few ideas I have about protecting
plants from
frosts. What I am talking about here is protecting the flowers
and foliage
of plants from damage by subfreezing temperatures. This is not
the same
thing as protecting the bases/roots of plants from really cold
weather - in
fact it is almost the opposite. First, if you want to preserve the
flowers
and foliage of plants from frost I do not recommend mulching
around the
bases of your plants, as this could cause more damage than no
mulching.
Why? Because on a cold night, the ground is the main heat
source available
to plants, and the sky is the main heat sink. On a cold calm
night, heat
moves from warm things to cold things mainly by radiation.
All objects
radiate heat, and warmer objects radiate more heat that colder
objects. If
you cover the ground below the leaves of your plants you are
insulating the
ground and reducing heat loss from the ground. Above the
mulch, your plants
are still radiating their heat into the sky (which on a clear cold
night
doesn't radiate much heat back to the plants). So, if you reduce
the heat
reaching your plants from the ground and do not reduce the
heat leaving the
plants to the sky, you have created a net reduction in the heat
energy in
your plant, resulting in the plant reaching a colder temperature
and more
frost damage. Any object that is in the way of radiating heat
energy will
absorb the energy and then reradiate it back. Some of the
intercepted
energy will radiate upward, while some will radiate back
down. This makes
radiative heat transfer much less efficient and helps protect
plants. If
you want to protect your plants you could sprinkle mulch on
TOP of the
plant leaving the ground uncovered. This will reduce heat loss
to the sky
but not reduce heat gain from the ground, meaning a net
increase in heat
and warmer plants. Tarps or plastic have a similar effect,
reducing heat
loss and protecting plants, and trees; clouds, buildings, etc. do
the same.
Even clear plastic has this same effect, since long-wave
radiation - the
type of radiation emitted by objects that are at typical earth
surface
temperatures, does not pass through clear plastic the way
visible light
(shorter-wave radiation) does. Long wave radiation also does
not pass
through certain gasses like carbon dioxide, while shorter-wave
radiation
does - this is the famous greenhouse effect.

I have also had good luck protecting flowers by stringing
christmas lights
in them. These lights produce only a little heat, but it can make
the
difference on a cold night.

Once the tops of your plants are damaged beyond the point of
blooming
anytime soon, then you can mulch around the bases of the
plants to protect
the bases from really cold temperatures and to keep them
uniformly cool,
which seems to discourage resprouting during winter.
Resprouting during
winter and repeated freezing of the new sprouts is much more
likely to kill
your plants than a single freeze is.

Ron Rovansek