by Glenn Davis, UCCE
Master Gardener, El Dorado County
December 2002
In the old days gardeners used “cloches,” bell shaped jars, to
provide the necessary heat to start seeds and seedlings early in the year. They
lacked any ventilation exhaust system and were time consuming to monitor; in
fact they had to be checked daily to insure that the plants inside were
surviving. We’ve upgraded our garden practices and now we use hot-caps, wall-o-
water, row covers, plastic milk cartons, soft drink or juice bottles or any
semi-rigid container that will allow sun penetration, soil heating and
ventilation. All of these help the gardener to extend the garden season, either
by warming the soil early in the spring or by keeping the soil warm later in
the fall.
Extending the season became big business when growers found
they could provide our colder climates with produce during the winter by
building large greenhouses. This beat the cost of transportation but required a
lot of manual assistance to maintain the correct environment. Well, along came
thermostatic climate control and automatic watering systems. It was, however,
impractical for home gardeners, they needed something smaller and easier to
maintain.
The next obvious step was a small greenhouse, impractical for
a homeowner with a small backyard. It’s my guess that someone thought of taking
advantage of the insulation properties of soil and that with an old window they
developed what is known in the garden world as a “cold frame.”
Cold frames are boxes, made out of redwood, set into the
ground a few inches and covered with glass.
The box is usually about 3 x 6 feet and the class cover is an old window,
on hinges, to allow it to be opened for ventilation. While glass, with its
rigidity, is preferred, plastic, fiberglass or Plexiglas will do. Usually the
box faces south, but southeast or southwest will work. The top of the box to
the north is about 16 inches high and the bottom facing south is about 12
inches high. Since ventilation is so
important there should be something available to prop open the window.
Some cold frames are constructed to grow plants in the box,
while others are constructed to hold containers. The latter seems to be better for plant movement and soil
replacement. If the direct sun is too intense then some shade cloth or lathe
can be placed over the glass to protect the tender plants in the bed. Cold
frames can also be used to harden-off young seedlings before they are placed in
the garden.
Cold frames should be located on level ground and in a wind
protected area if possible. The heating will increase if the frame is near a
building where the sun can reflect into it. If that placement is possible then
careful water monitoring is essential. Manufacturers have developed portable
cold frames; these are usually made out of plastic and can be folded up and
stored during the summer. They are not as well insulated as the permanent
models, but they can be moved around to take advantage of solar heating.
It was only a matter of time until someone thought of a
permanent heating source in the cold frame. When this was developed the cold
frame became a “hot bed,” and the source of heat was from below the plants. Initially the heat was generated by layers
of fresh horse manure in the bottom of the frame; the heat generated was
inconsistent and the frame needed to be monitored constantly to insure the
survival of the plants.
Then along came the heating cable, a relatively cheap method
of providing constant, non-variable heat to the bottom of the hot bed. Heating
cables have built in thermostats, set to provide the correct soil heat for most
plants. If you use a heating cable, follow the installation instructions
carefully to make sure you don’t accidentally puncture the heating cable.
It’s a big jump from the historical cloches to a modern
cable-heated bed and maybe the next move will be a solar powered heating cable,
we’ll see.
For more information on this subject or any garden problem,
contact the El Dorado County Master Gardeners, 621-5512, Monday through Friday,
9:00 AM to Noon.