I hope that you have lots of cold sinks in the form of pails of water in your
greenhouse to take up the cold?

Last year I installed an old furnace I inherited in my greenhouse and during the last week, the furnace has been running a lot at nite even though I have it turned on to +3C in the greenhouse for overnite. Supposed to warm up this weekend so that will be nice. Stan

Mine maintains about 5 C warmer than ambient on calm, clear nights, 2-3 C warmer on windy nights. Did you find the heat differential more favorable in the fall? Just curious.

I only have frost-tolerant crops (peas, spinach, brassicas) planted in the greenhouse but they still show some freeze damage from the hard frosts earlier this month. I have eggplant seedlings in my heated sunroom ready to replace them in June.

I didn't check in the fall. I took the water out in the summer. This year I'll leave most of the water in (my rain barrel will be a rain barrel again) and try growing some plants in the greenhouse (sweet potato, tomatoes).

I built my greenhouse last year and I have 6 - 25 gallon drums that are on the north
wall , painted black and full of water,I made my potting bench over the top of it , There is about a 4 deg c difference right now , I also have tried this year- built a 6' x 2' x 12" high box. installed a 100 watt light bulb inside , and use a piece of plexiglass to cover, Yesterday it was 0 outside and it was 18 c in the box, which is in the greenhouse, Cheap heat.

I really like your idea about the box with the light bulb inside and I think I will build one tonite because it is certainly cheaper to run a 100 watt bulb than a furnace overnite. I am doing containers for some customers, only 12 customers but about 30 hanging baskets and about 30 containers so in a small 10' by 20' greenhouse, I use every inch of space available. I already have 21 hanging baskets built and about 12 different size pots, from 12" to 18" so this weekend will be very busy. Thanks for the idea. Stan