Ah yes, the orchard in a hoop house trick. When I worked at Red Jacket in Geneva NY we had 2 long Haygrove high tunnels that had cherries, apricots and a few plums in them. There were a lot of issues with them principally that they were not installed on the right site (not the tunnel's problem) and the taking down/putting up of the plastic, plus wind, caused huge expenses and labor problems. As well, the plastic filtered out certain wavelengths of light that didn't allow the fruit to color properly. There were also pollination problems. And they did nothing to protect from cold temperatures per se, only frost. The tunnel were 26 x 200' long I believe, 18' high at the peak, and so there also issues with the trees hitting the top of the tunnel even though they were on the most dwarfing rootstocks we could find and pruned to short squat stature.
That said, I was always in love with the hoop house idea, it just needed to be executed properly.
- put it on a good site with well-drained soils.
- make sure the tunnels are sized to what you want to grow. With many more dwarf rootstock options for apples, I could see apples making a better fit, or cherries (but even on G5 roots were too big), but not anything like 'cots or plums on myrobolan.
- optimize the production space - the tunnels aren't cheap and even with a wide spacing you have to account for the ability to work in them, allow for air flow, light penetration, and production. What you lose in vertical capacity needs to be made up for in horizontal capacity, and even in a seemingly spacious tunnel can make for a tight fit.
- irrigation, esp in warm dry climates. What you gain in frost and rain protection, you lose in natural rainfall.
- the plastic should only be on from bud break to fruit set - after that you run the risk of increasing certain fungal diseases (powdery mildew) and not allowing for good air flow or light penetration. **note: there are plastics that do not reduce full spectrum light penetration, but then they also increase shortwave radiation from the sun causing the temperature in the tunnel to skyrocket, so roll up sides or fans can also be a nice addition.
- do not leave the plastic down all winter, wind, snow, rain, etc can really do damage and reduce the overall life span. As well, many plastic do break down in UV and so should be rolled and stored in black plastic.
- Pollination - honeybees and native pollinators do not like to go inside. Using boxed Bumblebees was an option we used, but now I don't recommend that except in closed greenhouses since they can spread viruses and interfere with natural pollination.
- Winter protection: personally I don't think tunnels provide much protection in very cold climates unless there is some modicum of heat applied; and for this reason, in cold climates achieving enough chill units isn't an issue regardless of how you manage the covering.
- Cornell - had a bunch of experiments going to moderate climate issues, some were true tunnels, other were fabric coverings, etc. all interesting, all with some merit, none ever implemented to a great extent in NY.
Anyway, I was very optimistic about using tunnels for tree fruit production, but not so much anymore - at least not from my experiences. They are expensive, time consuming, and add management challenges. Plus I think trees like being outside. Those are my experiences.
Mike Biltonen, Know Your RootsZone 5b in New York