Answer: Heat cables are the best solution but you may need to improvise by setting the trays on insulating styrofoam and building sides around the trays. I usually purchase a sheet of styrofoam (generally sold in one inch thickness to insulate walls) and cut it into pieces to build boxes for my seedlings. I just tape the pieces together to make four sided boxes. Then set an incandescent light bulb in the vicinity (a small lamp will do - just remove the shade). There is usually enough heat given off from the bulb to keep the air in the immediate vicinity warm. The insulating boxes will help hold the heat in. Let's hope spring arrives soon so you can take your seedlings outdoors!
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