Q Is there an ecological way to heat a small greenhouse?
Mark, Cardiff
A BOB SAYS Yes, hotbeds of fermenting manure in the well of a glasshouse can provide heat. These have a lead-in time as they warm up. They also cool down, so the problem (other than the smell and noxious gases) is they need remaking every month or so or the warmth simply peters out.
Another drawback is hotbeds can only provide background heat. You still need a (preferably automatic) back-up method to keep the temperature up over a series of freezing cold nights. There is no point heating at all if one frost gets in and causes damage.
It will take least total energy to well insulate the greenhouse with layers of bubble plastic, so as to need minimal heating (ideally using electric fans) and then to compensate for the low light with LED grow-lights, which use relatively little electricity.