So I’ve been thinking about the best way to grow figs in the ground in zone 7b, where it gets down to 5-10°F, during January and February.
My first idea involved rolled insulation and a pipe heating cable hooked up to a temperature controller for each tree. But I was thinking that a better option might be to just prune the trees down to 5 ft tall and then use bent 1/2" electrical conduit and 6 mil clear poly sheets to create a low tunnel over the row of 4 trees. One sheet of poly won’t do much so I was also planning on upping the insulation by layering on some bubble wrap and then another sheet of poly to create a kind of insulating poly-bubblewrap-poly sandwich.
For temperature management I would use a temperature controller and either a very small space heater or pipe heating cables for heating and maybe a small PC fan for cooling (sucking in outside air). The goal would be to keep the temperature between 20°F and 35°F and then start bringing it out of dormancy 2-3 weeks before last frost by setting the target temp to just above 45°F.
This is all just brainstorming so I would love some feedback on anything I might be missing.
The heating and cooling is going to take a lot more inputs than you seem to have in mind. The double layer poly with bubble wrap still only has a very low R value. It reduces heating needs a little but not that much. Some real insulation like 2 inches of foam is much more effective. On the other side in Z7 it will get very hot under clear poly anytime the sun is out even in mid winter. Down here in Texas it will get 50F and more above outside temperature. You can’t keep it cooler than outside during sunny weather with just fans. I have a very elaborate system of shades and evaporative cooling to hold down temp during my chill cycle inside the greenhouse.
If you built something covered with real insulation like the afore mentioned foam you’d have a much easier time keeping it warm at night and cool by day during winter. Then in March or April, about 4-6 weeks before last freeze, go to the clear poly to warm it up during the day.
Yeah I think you are right about foam insulation being better than bubble wrap, especially considering it will reduce or even eliminate the need to do cooling. I might still do the whole insulation sandwich but use roll out insulation vs bubble wrap.
Bending the trees over and covering with 3 layers of the thinnest type or Agribon row cover worked great last winter. It is on some trees right now to extend the season, picked about 20# from 15 3rd year MBVS today.
When the leaves drop I will bend them over to about 3-4 ft, tie them to their neighbor in the row so they stay down, and then toss half full bags of leaves on the highest points to keep the row cover from touching the branches. You could run a wire over them to suspend the row cover instead, but I think the bags of leaves will result in a lower profile that catches less wind. They do wake up early so need to be kept covered till threat of frost is over.
