Cold Frame, Hot Bed Construction And Use

By The Old House Web

Hot beds and cold frames are used by gardeners for propagatingvegetables, flowers and ornamentals. Hot beds are used for startingthe plants and cold frames for tempering or hardening plants tooutdoor conditions before transplanting.

For most home gardeners thesame frame can serve both purposes. The principle difference betweenthe two is that hot beds have a heat source. Traditionally, hot bedsand cold frames were built even with or slightly below ground leveland covered by glass sash. Present-day frames are often completelyabove ground and plastic covered because film is generally availableat a reasonable cost.


Hot beds and cold frames should always be located on well-drainedsoil, free from flooding during heavy rain. A location with asouthern exposure and adequate wind protection on the north and westis ideal. Locate close to water and heat sources.


Almost any size hot bed can be electrically heated; however, most arefrom 5 to 6 feet wide and 6 to 12 feet long. The ultimate size isdependent on planting requirements, kinds of plants, number of plantsand spacing. If glass sash will be used to cover, the length isusually in multiples of 3 feet; however, plastic coverings do notlimit dimensions.


Most beds are constructed with wood sidewalls; however, more permanentbeds can be made of poured concrete or masonry blocks. Wooden wallsand supports should be painted or treated with copper napthenate butnot creosote or pentachlorophenol, as both are harmful to plants.


Sash Type

Excavate the bed area 8 inches deep. After walls are built, apply 6inches of gravel or coarse sand for drainage. Add a layer of burlapor other material to prevent sand from sifting down. Add a 2-inchlayer of sand on which the heating cables are laid. Two more inchesof sand should be applied over the heating cables and the sandcovered with 1/2 inch hardware cloth to protect the cables. Placeeither propagating medium or flats over the hardware cloth. Constructthe back or north wall 18 inches above the level at which the heatingcable is placed. Side walls usually slope toward the front about 1inch per foot of width. A 6 foot wide bed will be 12 inches high infront. The footing for concrete or block walls must be placed belowthe frost line. Nail a 1 x 4 inch board to the outside top edge ofthe back and side walls. (The sashes extend over the edge of thefront wall to shed water.) The boards serve as weather stripping andreduce heat loss between walls and sashes. Bank soil against theoutside of the walls to prevent air leakage. The sash orplastic-covered frames, sometimes hinged at the back, are lifted infront and braced open for ventilation.

Above-Ground Plastic Covered Type

Units of this nature may be of most any form, from arch, A-frame toquonset, with the structure of wood or thin-wall electrical conduit.These frames are inexpensive to build and easy to construct. Theframes are covered with 4-mil clear polyethylene plastic filmdesigned to be rolled down the ridge or up the sides to allow foradequate ventilation.


Although steam, hot water and even manure have been used to heat hotbeds, most home gardeners use electric cables. A thermostat is neededto control the temperature in the bed. Although heating cablesoperate on either 240 or 120 volts, most small beds of 10 feet orless can be satisfactorily operated on a 120 volt system. One 60 footcable is required for a 6 x 6 foot bed and two 60 foot cables for a 6x 12 foot bed. The cables should be arranged in the beds. Heatingcables and thermostats are available from mail order and gardensupply centers.


A soil temperature of 70 to 75 degrees F is ideal for germination ofmost seeds. Following germination, adjust the temperature to suit theparticular plant. Cool-season crops such as lettuce, cabbage andcauliflower require an air temperature during the day of 60 to 65degrees F. Warm-season crops such as tomatoes, peppers, and melonsrequire an air temperature of 65 to 75 degrees F. Night temperaturesare usually 5 to 10 degrees F lower than day air temperatures. If theair temperature in the bed goes above 85 degrees F, ventilation willbe necessary. The beds usually require ventilation on all mild, sunnydays. Electrically-heated beds tend to dry the medium rapidly, andattention to watering is a must. The soil should be kept moist at alltimes but not wet. Apply water in the morning so the plant foliagewill dry before evening.

NOTE: Disclaimer - This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.


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