- 1). Place the seedlings into a tray that does not leak. Set this tray up on a warm appliance that stays on constantly. The top of a refrigerator, clothes dryer or large television should stay warm enough to keep the soil around the seedlings in a comfortable temperature range. This would only work for a small quantity of seedlings at a time, unless you own multiple refrigerators, clothes dryers and televisions.
- 2). Set the tray or trays up on metal shelving units. Use a clamp-mounted light and secure it just under the metal shelf. A 40 watt incandescent bulb would work to warm the metal shelf above it, thereby evenly warming the tray with its seedlings. The shelving should be in a room that is kept at about 70 degrees Fahrenheit. Setting this structure up in a garage or outside area will defeat the purpose of generating the heat.
- 3). Construct a light box using an old insulating foam cooler. Clamp-mounted lights or a desk lamp hanging over the edge of the cooler will help generate the heat. Use a 40 watt incandescent light bulb as the light-emitting diode (LED) light bulbs do not generate heat. A variation on this would involve using an old refrigerator with lights at the top and bottom of the inside. Set the top light on the top shelf and set the tray or trays on the shelves beneath it. Cut a small notch in the refrigerator seal to allow the electrical cords for the lights to fit into it.
- 4). Bury a length of heat cables in a flat of clean sand that is the size of the trays of seedlings you wish to keep warm. Set the trays on top of the sand once the heat reaches between 75 and 80 degrees Fahrenheit. If the heat cables are a little out of your price range, another alternative is to use Christmas rope lights that don't use LED lights. These decorations can be found discounted in late December and early January once the holiday has passed.
- 5). Set a thermometer into the soil of one of the seedling cups regardless of which method of heating you use. Adjust the heat depending on whether the temperature is too cold or too hot by adding or removing lights. Many of the DIY methods do not allow for automatic control of temperatures, but steady temperatures can be maintained through the adjustments of adding or removing lights or adjusting the location of the seedling trays.
- 6). Set up an old water bed frame and place it somewhere that it won't be disturbed or need to be moved. Put in a 2-inch layer of play sand in the frame. Set the water bed heater in the center of the frame and add another layer of play sand on top. Turn the water heater on high for two days. Take the temperature in the center of the frame to make sure it falls within the 75 to 80 degree range and adjust the heater accordingly.
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