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Northern panhandle of Idaho. Suppose to be zone 5, although we plan for Zone 4 due to cold air coming down slope from the local mountain.

That is difficult. Not getting enough heat units to ripen the fruits..
Can you reccomend varieties for those conditions?

I grow most of my tomatoes in a hot house (hoop style greenhouse) to get around our cold nights. We can go from a high of 99 degrees Fahrenheit (37 Celsius) to the around 38 (3.33 Celsius) at night. Growing under plastic has worked well for me.

That is rather extreme! i wonder of theres a way to balance out your temperatures using thermal mass.
Using the mass a flywheel to moderate the extremes
have you heard of the Walipini system?


https://thehomestead.guru/whats-a-walipini-underground-greenhouses-101/ https://au.pinterest.com/yolimk/pit-greenhouse/?lp=true

Years ago I looked at underground greenhouses like in your link. I forgot about it. Thanks for the links. I will investigate. This could help.

Actinidia arguta may be ok there
And its delicious.
Cv Issai is self fertile