COOPERATIVE EXTENSION WORK IN
AGRICULTURE AND HOME ECONOMICS
(Acts of May 8 and June 30, 1914)
AGRICULTURAL EXTENSION SERVICE, UNIVERSITY OF FLORIDA
FLORIDA STATE UNIVERSITY
AND UNITED STATES DEPARTMENT OF AGRICULTURE
H. G. CLAYTON, DIRECTOR
Electric Hotbeds for Sweet Potatoes
A. M. PETTIS
Farm Electrification Specialist
Agricultural Extension Service
V. F. NETTLES
Agricultural Experiment Stations
It is recommended that growers of sweet potatoes in north
and central Florida utilize hotbeds for plant production. This
will assure them of a supply of plants which can be set in the
field at the earliest date after danger from late frosts.
Fig. 1.-Sweet potato plants growing in an electrically heated hotbed.
Cable needed: Two 60' lengths, 115 volts, 400 watts each, or 120' length,
230 volts, 800 watts
Cable needed: Two 120' lengths, 230 volts, 800 watts each
- 60-' -
Cable needed: Four 120' lengths, 230 volts, 800 watts each
Fig. 2.-Layout of soil heating cable for hotbeds of different lengths.
The use of electricity is the newest method employed for heat-
ing. Electric hotbeds require less labor and material than a
manure-heated or flue-heated bed. The temperature is automati-
cally controlled and thus the bed requires less attention than
other types. Manure-heated beds spread disease and heat from
these beds decreases as the season progresses, resulting in slower
Location.-The location should be near an electric outlet and
a water supply. The hotbed should be located also on well-
drained soil which has not been used for the growing of sweet
potatoes or tobacco. If a permanent bed is to be used, remove
the soil to a depth of 12 inches and drench the bed and frame
with a solution of 1 pint of formaldehyde per 15 gallons of water.
Next, refill the bed with new soil. It is best to face the bed to
the south, so it is fully exposed to the sun. This helps warm
the bed area; a high back can help protect it from northern
Fig. 3.-Wiring connections for an electric hotbed.
Size.-The size of the bed will depend on the number of plants
desired. A bushel of U. S. No. 1 potatoes of present commercial
varieties will usually produce 2,000 to 2,500 plants from 3 or 4
pulling. This bushel of potatoes will require from 12 to 15
square feet of bed area. When sweet potatoes are planted in the
Fig. 4.-Lead-covered electric cable is laid in bottom of bed on level
ground. Note that left or north side of bed frame is taller to protect from
wind and give exposure to sun on south side.
Fig. 5.-Sweet potatoes in foreground are placed adjacent to each other
without touching. They are then covered with 3 inches of sand or sawdust,
as shown in background.
field in 4-foot rows with the plants 12 inches apart, about 11,000
plants will be needed per acre. Thus a 6' X 15' bed filled with
6 bushels of seed potatoes will produce plants for 1 acre, using
the above spacings.
Hotbeds are usually constructed 6 feet wide and the length
may be 15, 30 or 60 feet.
Heating Cable.-Electric heating cable is made in 2 types-
lead-covered and plastic-covered. Standard cable sets are made
in a loop with both ends being attached to the male plug cap.
A cable for operation on 115-volts will be 60 feet and it will use
400 watts of power. A cable for 230-volts will be 120 feet and
it uses 800 watts of power. Diagrams are given in Fig. 2 for
the placing of cables in beds of different sizes.
Thermostat.-To conserve electricity and help maintain a uni-
form temperature, a soil thermostat must be used with each bed.
A thermostat is a switch device that is operated by the tempera-
ture of the bed. It turns on electricity when the bed needs heat
and turns off the electricity when the bed reaches the desired
temperature. The bulb of this thermostat is buried directly in
the soil. This thermostat is connected as shown in Fig. 3.
BILL OF MATERIAL FOR A HOTBED 6' X 30'
No. 2 or Rough Lumber For Constructing
/Q 1" X 6" X 15' Front and back
3 1" X 6" X 12' Ends
2 2" X 4" X 12' For posts to hold bed together
Lumber and glass or glass substitute for making several sash.
2 230-volt electric heating cables, each 120 feet.
1 Soil thermostat.
Frame Construction.-Make the frame exactly 6 feet wide.
As shown in Fig. 4, the north side of frame is made of three
1" X 6" X 15' boards, making it 18 inches high. The south
side of frame is made of two 1" X 6" X 15' boards, making it
12 inches high. Each end of the frame is made by using three
1" X 6" X 6' boards. The top board is tapered so the end is
about 18 inches high on the north side and about 12 inches on
the south side. All cracks and holes should be plugged to pre-
vent heat loss. If needed, a small "running board" can be nailed
on one side of the frame to support the sash.
Laying the Cable.-After the wooden frame is constructed,
remove 3 inches of soil. Part of this soil is banked around the
outside of frame. Next place the cable in straight rows about
3 inches from the frame and 7 to 8 inches between rows (see
Fig. 4). Do not kink the cable or make a sharp bend in it. When
completed, the ends of cable that plug into the outlet should be
conveniently placed close together at one end of bed.
Completing the Bed.-Cover the cable with 3 inches of soil.
Then use uniform size, carefully selected potatoes and place
them close to one another without touching, as shown in Fig. 5.
A layer of poultry wire can be placed over the bedded potatoes
to keep them in position when pulling plants. Then cover the
potatoes with 3 inches of sand or sawdust. The completed bed
is then covered with wooden sash enclosing glass or glass sub-
stitute. The dimensions for sash using glass are generally
3' X 6'. A convenient size sash using glass substitute is
6' X 6' or 71/2' X 6'.
Wiring to the Bed.-With cable in place, the soil thermostat
is mounted near the center of the end of bed. This is illustrated
in Fig. 6. Use weatherproof wire of the proper size to bring
115 volts or 230 volts to the hotbed. The weatherproof wire
should be fed from a switch box so it will be convenient to turn
Fig. 6.-Glass sash removed to show electrical connections and thermo-
stat, which will automatically control the temperature at 80-85 F.
the electricity on or off as desired. Follow wiring specifications
of the power supplier or the National Electrical Code.
Bed Operation.-Set the thermostat to 80 or 850 F. Covers
should be secured tightly on the bed and the switch turned on
for electricity. If the sash is covered with straw at night
electricity will be saved. The bed should be watered at first and
at intervals of about a week as needed. It is best to water it in
the daytime so the sun's heat will help warm the water, thus
saving electricity. To harden the young plants to sunlight
and prevent scorching when they are transplanted, remove the
covers on sunny days after sprouts appear.
Potatoes should be bedded about 4 weeks before the plants are
needed. The plants should be hardened several days before
pulling by removing covers and turning off the electricity.
A thermometer can be placed in the bed occasionally to make
sure the thermostat is operating properly and keeping the bed
at the right temperature. Good management of beds is essential
to cut down on the cost of electricity.
The above information should enable any farmer to success-
fully construct and operate electric hotbeds for sweet potatoes.
If more information is needed, see your county agent or write
directly to the Agricultural Extension Service, Gainesville, Flor-