Not ready for the season to be over and don’t have the money for a greenhouse? Think small and build a hotbed. A hotbed is a 3’ x 6’ box with a clear top and heat cables in the bottom. With one, cool season vegetables can be grown through the winter.
The first thing to decide is the location. A southern exposure will ensure the maximum amount of light. Ideally the back of the box should sit up against a building to block the cold north wind and to provide insulation. If a building is not available, then bales of straw will provide the insulation needed. Make sure the site has good drainage, otherwise, the bed will need to be raised which will result in more heat loss. Overall place the bed near the house or a convenient location near a water spigot, otherwise when it gets cold outside the bed will be forgotten and the plants will dry out which can result in the cables overheating and failing.
Now it is time to build the structure. There are two options: buy one or build one. Hummert’s commercial cold framecomes assembled and ready to use. Manufactured with aluminum extrusions and 8mm polycarbonate twin wall panels, and come in two sizes: 4’ x 4’ and 4’ x 8’. The other option is to build one using wood or concrete block. This option allows you to build whatever size works best. The first thing to consider when choosing a size is what will be used to cover the top. An old window sash or a wood frame covered with polycarbonate, greenhouse plastic, or clear fiberglass are some options. A good size for average home use is 3’ x 6’ while 6’ x 6’ is a good size to start for commercial use. Don’t use treated wood as fumes from the preservatives may damage plants. Redwood, cedar or untreated wood painted with white latex-based paint or white greenhouse paint are good materials to use.
To build the structure out of wood, use 2 x 4 for the corner post and should extend to the bottom of the excavation which should be about 14” deep. For the back use 2 pieces of 1 x 10, the sides will also use 1 x 10 but the top piece should be cut from corner to corner down its length and the front will use just 1 piece of 1 x 10. For more insulation from the surrounding ground the side panels can be extended down to the heat cables.
The excavation should be filled with 6” of gravel then covered with a layer of weed fabric. On top of this will go 1” of soil or soil mix. Stay away from mixes heavy in peat moss (flammable), vermiculite or sand (both insulating). Now comes the time to install the cable, but first the size of the cable needs to be discussed.
Using 12-14 watts per square foot should provide enough heat to grow cool season plants through the winter. At this rate a 3’ x 6’ bed would need a 40’ 200w cable will be ideal, which is a good thing since Hummert has the last stock of automatic 40’ cables available. After these are gone only non-automatic cables will be available, which is not a bad thing. If the bed is bigger, a thermostat will be needed and the desired temperature can be selected. The 40’ automatic cables are set to 74°F. The cable should be attached to ½” hardware cloth using zip ties, with 3” from the sides and 4-5” away from other portions of the cable. Do not allow the cable to crossover or otherwise touch itself. To do so could result in overheating, failure or other hazardous conditions and the warranty will be voided. Also do not attempt to modify the cable to lengthen, splice or shorten it, as this is also dangerous and will void the warranty.
Place the hardware cloth and cable assembly into the bed on top of the initial soil layer, cable side down. This will prevent damage to the cable if it needs to be dug up. Now a 6” layer of soil can be placed on top of the cable. The instructions that come with the cables give additional ideas if using pots or overwintering desert plants. If using a thermostat, place the capillary bulb or sensor halfway between the center and the edge of the bed, about 1” below the surface. Set the thermostat at 65oF, this temperature is ideal for seed germination and growth for cool season plants.
The hotbed is now ready to plant. Remember to keep the bed watered as the heat cables and the normally dry winter air will dry the soil out quickly. Now you will be able to enjoy those cool season vegetables in the late fall and winter months!!!