I would say maintaining at around 7C/45F would be optimal , you can even go slightly lower . basically a comfortable median on the lower temp range , most citrus should be fine at 3-4C as well but thats giving some stress esp for Limes .

Depends what is more important to you, low heat bills, or growth of your trees. I keep my greenhouse between 55 night to 75/80 day F, plus I light some of my tree to insure winter growth (up to 5 flushes per year). - Silica

My trees do most of their growing during the winter months. I don,t know why.

6b Steve

Vladimir..It would all depend on the amount of light your provide and how much growth you want...

Brian is the king of over wintering his citrus..I think he keeps his pretty cool, like in the 40's and 50's and 60's or more by day with heat set on low? I'm not sure..I think his trees slow down and flower and fruit and ripen. still though..

If it were me. I would set my space heater on to 60 at all times, the lowest setting, and if you find it gets too cold. like lower 40's, you could set two space heaters at 60...It is better and cheaper than trying to heat your greenhouse with one space heater set on high..

Cory, I think she keeps her temps pretty high because she has orchids that can not take temps lower than 555 degrees,..

if you are going to provide lots of sunlight. like all day sunlight then you keep them temps much warmer by night and day, like Silica, always above the low 50's and watch them continue to grow like crazy..

I hope Cory or Brian add in, since they are very successful every year and their trees are amazing..

Hey guy's . Vladimir I use two 1500 watt heaters one has a thermostat one doesn't . The heater with the thermostat I set at 57 degrees I could set it for 100 degrees wouldn't make a difference on the coldest night the temp is still going to get cold in there trust me . On the coldest nights I fire up the second heater to full.. Last yr we had a string of nights with mnus 30F night time lows with both heaters maxed out the temp in the GH was 33.5 F . So for extreme northern citrus growers with GH it really doesn't matter what temp 55 , 60 , 70 and above will make the heaters can only battle extreme low temps so much that is why I would really suggest to either bubble wrap the inside or pool cover the outside . Also have a plan for the coldest nights have another heater prepared to go in there because you might get frozen out of your greenhouse and not be able to setup another heater .. I also bank the snow around the bottom wood around the GH snow is a great insulator . Vladimir you should not run into the same problems I do with heating it is much colder here .

Hi Dev I used to leave my tree's out until mid October if I could but here in Ottawa we get very cool nights low 40's upper 30's with lot's of fog or heavy dews I find my tree's were not to fussy about that kind of weather watch out for fungus on the leaves and they really dont grow .

Mikey buddy I hope I don't over water lol I water once every 2 weeks in the winter . You definitely have to adjust your watering for the first few weeks after you bring them in to either a GH or home . My new GH is so sealed that it stays very humid new learning experience for me .

Brian

Hi Vladimir. I think that if I was setting up a greenhouse in a cold climate, I would take a couple of days to seek and visit indoor gardens, botanical gardens, and tropical forests at the zoo, etc. in the nearby areas. Surely, those places have experts on maintaining proper temps and conditions for their habitats during the winter. I would ask if they'd be willing to take a few minutes to answer a few questions.

I kept my thermostat set at 63°F for many years when I had warmer growing orchids and no citrus trees. I am planning to maybe set the temperature at 58°F this year as most of my remaining orchids are intermediate and cooler growing and the citrus trees have mostly taken over all the space. On a sunny day the heat rarely needs to run during the day and the furnaces do not start up until just before sundown at about 4 pm when the first one kicks in. I have 2 Southern Burner vented natural gas furnaces. I run my furnaces about 5 degrees apart so one backs up the other if one alone cannot maintain the temperature alone. My trees flower and grow most of the year with a slight rest from November thru the end of January. But even then they will produce new leaves and branches. I fertilize year round, but much less often and at a reduced amount in winter. Watering is less often during the winter also. There is a big blooming spurt in February after which most of the fruit for the year start to set although they will go through 4 or 5 blooming cycles a year. My kumquats bloom and set much later though. They seem to need the hot July summer to start them blooming and fruiting. I like to watch my trees grow and bloom throughout the year so I keep the roots above 55°F in the winter months in the greenhouse. I keep my trees' pots raised off the floor. I don't start bringing my trees in until night temperatures start reaching the low 40s regularly and the 15-20°F difference between summer day and night temperatures narrows. I start with bringing in the orchids first as the gap narrows (most orchids need the temperature difference and night lows of about 50°F to trigger the blooming mechanism), then the limes go in when night temperature are regularly in the low 40s, followed by the progressively hardier citrus as the temperatures drop into the low-to-mid 30s. Not as tiring that way although I still am oftening scrambling usually when the temperatures are suddenly predicted to fall. In addition I have a grow room in my basement and use high output T5s to light it. Most of my tropical plants go there now. The temperature is constantly about 60°F there. This year I bought some frost protection bags to test to see if I can leave some of the hardier ones out when the night temperatures are predicted to go in the low 30s. I am hoping that way I don't have to keep lugging them in and out as much. I try to get them out early in the spring also and the bags might help then too. I'll report how well they work for me. Scale is my worst insect problem, some years worse then others, no matter how careful I am spraying before they come in. Mites and aphids are sometimes a problem. Maybe that comes with growing in a warmer temperature. I have lights inside the greenhouse (HO t5s, mainly to see by, but I also use them to supplement the natural light the trees get during the longest dreary cloudy days of winter and the trees seem to do better than they did during a couple of years that I didn't use them. The lights are on a timer and come on at dust for a couple of hours and on again a couple hours before sunrise. The inside of my greenhouse is bubblewrapped and I put insulation over the intake vents and exhaust outlets and disconnect the automatic air ventilation system during the coldest winter months to save on heat. I crack the door open on bright sunny days when needed, and run lots of fans to circulate the air inside.

Cory

Cory, what are the frost protector bags?

Brian - yeah usually by this time I also bring in my citrus , but this year has been unseasonably warm so will keep out little longer .

Vladimir - In my own winter setup the temp is 15-16C , thats the steady temp in my basement after closing the vents . I would prefer a lower temp but its nice to see blooms n new flush in winter .

It;s nice to see what everyone does as regards to temps......It seems our trees do good whether they are kept under very cool conditions or very warm...So sunlight plays a very important role as so artificial lighting...

I guess it is all a matter of choice..

Either one can hold their trees at lower temps, into the 40's and they do fine..They still bloom and slowly grow..They will loose a bit of greener tone since they will not take up nutrients and one will have to watch that they don't water as often...The cold pretty much keeps them pest resistant and one will need to use a fan to encourage air circulation...

Or

Higher temps above 55...One would have to watch for pests, but they will get much more active growth. Depending on the amount of sunlight or light provided, they will get new strong or spindly growth. One will still have to fertilize, but lightly and will have to water more often. Will still need a fan for air circulation...

Brian and anyone else using a greenhouse setting with humidity..VERY important to keep the air movement in there strong to move humidity and prevent disease.....

Brian and Cory hugs as you continue to enjoy your huge greenhouses...

Try to never go below 55f. I don't have leaf drop and they flower like mad around February. Usually just one 1500 watt handles it but if the outside gets below mid 20's the other 1500w heater has to kick on. It's set to come on at 45.

Great info...I'm taking notes...;-)

Vladimir, these are the freeze protector bags I ordered. It is the same material as those floating row covers but made into bags to cover trees and shrubs. They only give a few degrees of protection, but that may be enough on many nights when temperatures are still bouncing around. I tried to get the 2.0 oz. with coating but they were out of them. Maybe 2 layers could help to provide a few more degrees protection? They also have shorter bags too which cost less. I figured I might as well get the larger ones. I thought they might help on those nights when I am afraid the forecast might come in lower than expected. I like to keep my trees out as long as possible.

Agfabric Warm Worth Tree/ shrub cover, Protecting bag for frost protection, 84"Hx72"W, 1.5oz (12) https://www.amazon.com/dp/B01H4WMDYW/ref=cm_sw_r_cp_apa_ghX8xbJ3VDX7E

Cory

Silica - hope this isn't a dumb question, but do you light your trees 24/7 to protect from cold and if so, do you think this helps or hurts fruiting? Suzanne

Thank you for your very helpful reply and advice! You're a treasure!

Ditto ...very helpful Silica. Thank you! I've learned so much from you!

Me too Silica. Thanks for sharing all your knowledge with us.

Cory

Silica, I just read your explanation to a fascinated class of homeschoolers who are learning about trees. Thank you!