beds and cold frames are used by gardeners for propagating vegetables,
flowers and ornamentals. Hot beds are used for starting the plants
and cold frames for tempering or hardening plants to outdoor conditions
before transplanting. For most home gardeners, the same frame can
serve both purposes. The principle difference between the two is
that hot beds have a heat source.
Traditionally, hot beds and cold frames were built even with or
slightly below ground level and covered by glass sash. Present-day
frames are often completely above-ground and plastic covered because
film is generally available at a reasonable cost.
Hot beds and cold frames should always be located on well-drained
soil, free from flooding during heavy rain. A location with a southern
exposure and adequate wind protection on the north and west is ideal.
Locate close to water and heat sources.
any size hot bed can be electrically heated; however, most are from
5 to 6 feet wide and 6 to 12 feet long. The ultimate size is dependent
on planting requirements, kinds of plants, number of plants and
spacing. If glass sash will be used to cover, the length is usually
in multiples of 3 feet; however, plastic coverings do not limit
beds are constructed with wood sidewalls; more permanent beds can
be made of poured concrete or masonry blocks. Wooden walls and supports
should be painted or treated with copper napthenate--not creosote
or pentachlorophenol, as both are harmful to plants.
Excavate the bed area 8 inches deep. After walls are built, apply
6 inches of gravel or coarse sand for drainage. Add a layer of burlap
or other material to prevent sand from sifting down. Add a 2-inch
layer of sand on which the heating cables are laid. Two more inches
of sand should be applied over the heating cables and the sand covered
with 1/2 inch hardware cloth to protect the cables. Place either
propagating medium or flats over the hardware cloth. Construct the
back or north wall 18 inches above the level at which the heating
cable is placed. Side walls usually slope toward the front about
1 inch per foot of width. A 6 foot wide bed will be 12 inches high
in front. The footing for concrete or block walls must be placed
below the frost line. Nail a 1 x 4 inch board to the outside top
edge of the back and side walls. (the sashes extend over the edge
of the front wall to shed water). The boards serve as weather stripping
and reduce heat loss between walls and sashes. Bank soil against
the outside of the walls to prevent air leakage. The sash or plastic-covered
frames, sometimes hinged at the back, are lifted in front and braced
open for ventilation.
Plastic Covered Type
Units of this nature may be of most any form, from arch, A-frame
to quonset, with the structure of wood or thin-wall electrical conduit.
These frames are inexpensive to build and easy to construct. The
frames are covered with 4-mil clear polyethylene plastic film designed
to be rolled down the ridge or up the sides to allow for adequate
Although steam, hot water and even manure have been used to heat
hot beds, most home gardeners use electric cables. A thermostat
is needed to control the temperature in the bed. Although heating
cables operate on either 240 or 120 volts, most small beds of 10
feet or less can be satisfactorily operated on a 120 volt system.
One 60 foot cable is required for a 6 x 6 foot bed and two 60 foot
cables for a 6 x 12 foot bed. The cables should be arranged in the
beds. Heating cables and thermostats are available from mail order
and garden supply centers.
A soil temperature of 70 to 75 degrees F is ideal for germination
of most seeds. Following germination, adjust the temperature to
suit the particular plant. Cool-season crops such as lettuce, cabbage
and cauliflower require an air temperature during the day of 60
to 65 degrees F. Warm-season crops such as tomatoes, peppers, and
melons require an air temperature of 65 to 75 degrees F. Night temperatures
are usually 5 to 10 degrees F lower than day air temperatures. If
the air temperature in the bed goes above 85 degrees F, ventilation
will be necessary. The beds usually require ventilation on all mild,
sunny days. Electrically-heated beds tend to dry the medium rapidly,
and attention to watering is a must. The soil should be kept moist
at all times but not wet. Apply water in the morning so the plant
foliage will dry before evening.
from Elton M. Smith, Ohio State University Extension, 2000