Greenhouse Heating

Hi can anyone tell me the cheapest and affective greenhouse heating please.

 

The sun
No seriously, I use electric, but you must have a good thermostat one that does keep a steady temp. You can leave it to get on with it. Problem--- Power cuts. But I still think its best.

 

Putting a layer of "bubble wrap" all over the inside walls & roof is cheap and acts as excellent insulation thus keeping the cost of whatever heating source you use down. Just take it down when the weather allows and use it again next year.

 

I have built a propagator,undersoil cable with thermostatic control,with 24" high frame covered with bubble cover. And overwinter all my tender plants in there. I have had 1 in use for 25 years now,8x4 and still works perfect, just replaced some sand now and again. Hardly noticed bigger electric bill.Can set u to 70 F. but in most cases 45-55 is all they need.