I have a small heater on a timer in my greenhouse, which I use when the forecast is for exceptionally cold weather. It usually keeps the temp there at around 40 degrees. I was wondering the other day what is the optimum temp to heat a greenhouse to in the winter. Not trying to grow or germinate, just keep plants alive and healthy.

Check the cold hardiness of the plants you are overwintering but 40 degrees is probably fine for most plants.

Sumire's greenhouse is full of citrus and a few hardy orchids. She keeps hers at 40 but it often dips to close to freezing in really cold weather. Her plants always do great but are all hardy to zone 8.

I grow tropical orchids so try to keep temperatures between 55 and 60.

I grow tropicals so I keep mine at 60. It does dip to 57 when it gets very cold like it is right now. We are 32 and heading lower by the hour. We may get snow tonight.

My greenhouse is pretty bad. It gets up to 100 during the day (assuming there's sun). But it goes down to 37 at night. It's just not very helpful.

Mine has been up to 106, with no damage to the plants. I do have the vents and a big fan that goes 24/7.

I keep mainly tropical Orchids and Succulents in my greenhouse from ca September to May. I set the Thermostat to minimum 60 dgr F and on sunny days it ramps up into the seventies.

Mine has reached 110 with no ill affects but fans (lots of fans), open doors and vents and misters. The only thing I have noticed is some of the Cattleyas do some weird blooms things because of the wild swing from day to night - sometimes between 40 and 50 degrees.