Is it practical to grow mangosteen, cherimoya and sapote in a greenhouse in Z6? Or is it going to be hard to do?
Very hard unless you have a heater in there somehow.
I had to run my heaters nonstop last year, and i had multiple heaters on in zone 8b for my mangos.
If i had to do it again, i would get a solar generator and run heat through there instead of paying 900$ a month for electricity when my average was 200$.
Also my greenhouse was 10 x 20 x 12ft tall. I had a heater at each end facing inward.
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Cultivating worthwhile mangosteen fruit in a USDA Z10 greenhouse is a very expensive endeavor.
Cultivating worthwhile fruit of cherimoya and sapotes (there are multiple species) in a Z6 greenhouse will cost you about $250k to build the greenhouse plus $10k / year in utilities and maintenance.
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In the interest of giving you hope, Chad Midgley grows citrus in a greenhouse in zone 6 with no heaters. He has a YouTube channel (search under his name).
In a nutshell: He puts lots of compost piles in the greenhouse, and that provides all the heat those trees need. Maybe if you watch some of his videos, it’ll give you inspiration to try something similar!
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Not practical, but its possible. Cherimoya would be the most successful. If you could keep a greenhouse above freezing, they might survive, not sure how well they handle prolong cold. Mangosteen is an ultratropical, its very hard to grow even in the near tropical environment of South Florida. I’ve only ever seen it in greenhouses in the Miami area. Sapote could mean one of 100 fruits, but most of them would be tough to grow. Prolonged cold will kill most of them.
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For probably about 30 grand depending on how much work you want to do yourself you can build a geothermal greenhouse in the snow and grow whatever you want.
in a green house you could probalby with relative ease do ice cream bean, loquat, orange/other citrus, banana, cacao, and probalby white sapote. I wouldnt be surprised if you could do cherimoya. I just have personally seen the first 5 in greenhouses here in zone 7.
Consider also any humidty treatment you might need to do, and if that means youd need either multiple greenhouses or multiple rooms within the greenhouse. Our local arboretum does cacao in a greenhouse and keeps it extremely humid with like mist machines.
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30 grand maybe if you can excavate yourself. I’m in the process of building one and have spent over that just for the kit, excavation, and blocks for the retaining wall (which the hubby and I built ourselves).
Not trying to dissuade anyone; it’s been a fun project and I’m super excited to have citrus in Iowa (zone 5a). I’ve heard from others with a greenhouse in the snow that ultratropical plants will need extra heat, even with this setup.
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it’s not practical to grow any of those in a heated greenhouse in your zone. mangosteen will only fruit in the tropics.
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Since you are excavating for the ground to air geothermal system, couldn’t you add geothermal water pipes and add a furnace to the greenhouse?
Unheated, no. Heated, seems like some black sapote is doable. I know of at least a gentleman in zone 6 PA who fruits a seedling black sapote and I think a grafted variety in his greenhouse. It’s certainly a vanity project versus the lower cost of importing fruit from a warmer climate.
Cherimoya, I think you could get it to survive, fruiting could be harder, but I think I’ve seen it done as well in heated greenhouses. The heat minimum might be higher. Same caveats as sapote with regard to cost of buying pristine fruit from elsewhere vs growing a handful yourself…it has to be for the challenge, not for any sort of bounty, at least that’s how I feel.
I do have a couple of grafted white and black sapote and they seem happy in the greenhouse so far, but they are very young and we’re barely in to winter. Fruiting will be a whole other issue. I expect it will be a great learning experience.
Edited to add: so, none of this is “practical” by the strict definition.
We ended up paying more for the liquid to air system (instead of doing the air to air that normally comes with the greenhouses over 50 feet long), as our area floods occasionally. I’m sure we could have added water pipes underneath as well, although I’m certainly not a specialist in this area. The folks at Greenhouse in the Snow told us that the liquid to air system, as the pipes run under the greenhouse, may keep it somewhat warmer than the air-to-air greenhouses. There are two of those in my area, so I’ll be curious to compare my results to theirs once we get the frame up.
I am suggesting a liquid to water furnace application.
Easier to buy from sellers on TFF.
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Like a heat pump?
We have a closed loop system at 8 feet and 6 feet underground with a heat exchanger. I don’t see why it wouldn’t be possible to have piping connected to a furnace and fan system instead.
Yes, like a ground source heat pump.
Or in addition
Maybe. 
I’ll add this to my hubby’s “to research” list, haha. I would love to be able to grow cacao in there!
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There is a big difference between having a greenhouse in which sensitive plants survive, vs. cultivating worthwhile fruit.
It’s not just a matter of keeping the plants above freezing. You must consider the month in which the plants flower and set fruit, the degree-days required to ripen the fruit in an acceptable number of years, the air conditioning needed to keep the greenhouse from overheating in the summer, the tree box size to ensure long term survival, the pH of the irrigation water, the nutrient requirements of each species, and more.
A visit to these seemingly lower cost projects 12 years down the road is telling. The fruits (if the plants are still alive) is far from worthwhile.