The propagator base is made up of 4 watering trays lined with metallised bubble insulating foil. The heating cable lies on the foil and the trays are placed on top capturing the warm air. The heating cable can be seen in the base of the watering tray at the top of the picture. The white cable is a temperature probe used to monitor the compost temperature.
The cells of the trays allow the warm air to circulate around the compost and roots. The 60 cell trays were covered with plastic propagator covers for the first few days then these were removed to allow the compost to dry out and to avoid rotting of the seedlings. The heated sand bed in the greenhouse is currently being adapted to take these seed trays.