Cold tolerance for tropical species

The very cold weather we have had recently has coincided with the temporary failure of heating in the tropical greenhouse over the weekend (3 nights, 2 days).  The greenhouse is heated with a gas boiler supplying hot water to large iron pipes.  This traditional system generally keeps the greenhouse at about 10-15 degrees above ambient temperature however it does sometimes fail.  With this in mind the redesign included two large water tanks/ponds each holding about 7000 litres of rainwater to act as storage heaters in case of heating failure.  I’m pleased to record that the water in these tanks has not dropped below 12C even when the heating had failed and the air temperaturekept 3-5C  above freezing on a night when we suffered -4C outside.  While this chance experiment has shown the value of that design element I’m hoping we don’t get a repeat again this winter.