Congratulations on your new greenhouse!
Just one note. From my past experience, please be sure to get advice from folks who do have or have previously had a greenhouse for bonsai. Experience is key.
We’ve two smallish greenhouses and work in another much larger one. From experience, there are two general options available. Heated or unheated.
From what is described inside the greenhouse - trees with little roots, likely the heated greenhouse option is best. But only heated to keep the temperature in the above 32F and below 45F temperature band on average .
To do this best the greenhouse will need some sort of temperature regulator, like a
Thermocube ….. a heater sufficient to keep the temperature from freezing. (I use a
milk house heater on the 750W side for 8x6 ft greenhouse - maybe the 1500w side would work…. but likely two might be best.)
… and of course a good reliable fan is mandatory to prevent fungus..
It is also wise to install an auto temp activated opener for both your ceiling vents. (Many sold on the internet will work) Can’t see if there are lower vent louvers… which likely would be a great idea to have… with an auto opener.
Mulching in can help if you choose not to heat the greenhouse. Wont hurt. (Won’t be my first choice due considering the roots, but your trees, your choice.)
@JudyB has a really swell set up I learned a lot from. Might be good reading.
There is an
Overwintering Bonsai resource with background, theory and a greenhouse example. Don’t be put off by the azalea focus, overwintering theory and the examples are the same for all trees.
Good Luck with your new greenhouse!
cheers
DSD sends