These thick plastic Seedling Warmers (heat mats) have an embedded heating wire in it. Because the thermostat is outside the mat, the heat given off by the mat is related to the temperature of the surrounding air. So, when the room is cool, the mat is 5-11°C (10-20°F) warmer than that. And when the room or cold frame is warm, the mat is much warmer. In late spring when the surrounding temperature (not soil temperature) is over 24°C (75°F), it is best to unplug the mat so that the seedlings do not become too hot. Direct sunlight will overheat the air surrounding your flats and cause overheating as well.
ZHG126 – 10 x 20″ (fits one tray)
ZHG127 – 20 x 20″ (fits two trays)
ZHG128 – 20 x 48″ (fits four trays)
ZHG622 – 4 x 24″ (windowsill warmer)