I'm in zone 7b (Raleigh NC) and have all of my citrus in pots in a 20 by 30 high tunnel that I seal up and heat during the winter. I heat it with an old wood stove (firewood is free for me) and only try to keep it above freezing. The benefits of high tunnels isn't only the temperature, there is something going on by being in a less windy with less extreme shifts in temp and humidity environment (even though the side walls and doors are open). I can't explain it but my plants have never looked better. I also think that certain insect pests can't figure out how to get in. I still have pests but more greenhouse style of pests instead of field types.

The only reason my citrus are in pots is because I built the high tunnel on top of an old basketball court that came with the property. If the trees get big enough I will eventually chip holes through the asphalt and plant them in the ground.

I'm in North Carolina 7B also (orange county). Just like John I have all mine in pots and have constructed a greenhouse for winter. I also keep the temp just above freezing.

I think you will find many like us who grow citrus on the edge of the normal citrus production areas. I know high tunnels are also very popular in Japan for citrus production. Southern Japan is right on the edge of citrus zone as well.

I wouldnt call mine a high tunnel although it is a hoop house. Only 10x12 with a 8 foot peak. The benefit of doing a hoop house is if you construct it right you could take the plastic off during the summer and let the inground plants have full access tot the sun then put it back in fall. I plan to do that with the next one i build next summer.

Mike