Use redwood or cedar for wood frames. Do not use creosote or pentachlorophenol to treat wood because they are harmful to plants. Although any reasonable size can be made, a 6' x 6' size is common with 3' x 6' as a minimum. Lengths are usually in multiples of 3'. The width and length also depend on the size of the sash or top panels to be used.
Construction begins with excavation to the desired depth.
Most frames are built of wood. Two-inch lumber may give more strength. Some frames have insulation boards inside to keep the structure warmer. Some home gardeners use a portable frame which is not buried so it can be moved or stored. If the sides are not dug into the ground, bank soil around the frame or add insulation boards to conserve heat.