Seasonal tree-covering idea

I'm in zone 4a but I'd love to grow fruit trees that prefer zone 5 or 6 (peaches, figs, etc).  I think they would be happy living here most of the year, it's just from mid Nov to mid Feb that it gets a bit too cold for them.

I think a year round hoop wouldn't help them since it gets nearly as cold at night in a hoop house as it does outside.  And pollination may be more of an issue.  

So what about having a hoop structure up over a row of trees and leaving it uncovered in the spring/summer/fall.  Then once they go dormant in the late fall, cover it with something that is moderately insulating and doesn't necessarily let much light through.  Perhaps a swimming pool cover or 4 layers of tarps?  Then they can spend the winter in dormancy with protection from the radiant cold of the winter night.  As a hoop, it would hold snow so that might be enough insulation by itself.

For the plants that tend to flower too early in spring, the covering can just be removed at a time that lets them wake up a little bit later.

Is this the best idea ever or pure folly?  

Why not install a waste oil heater inside for use in winter.
They can be burned clean

I'm looking for a simple solution to get perennial trees to get through the winter.  I'm thinking this would do the trick and I'd definitely like to avoid burning oil (waste or otherwise) to support my trees.  Or fiddling with anything throughout the winter.  Kind of a "set it and forget it" solution.

It seems like opaque or semi-translucent would be better than transparent so that it doesn't heat up too much on sunny winter days.

I can see it.
I think I would go for a strong enough structure that you could climb on it.
That would make harvest and changing the covering easier.
You could use netting to protect the fruit from birds and squirrels

I imagine a dome more than a tunnel.

Oh, netting would be a great option as well.  

I was thinking of a tunnel so that you could have a row of 5+ trees and cover them with one structure.  I'm thinking it would be higher than the trees so I'm not sure harvesting from the structure would be a viable option...

If there was some terrain that you could use to your advantage, other types of support structures could be possible.  In my field it's flat as a pancake so I think I'd have to build the structure.  

For the tunnel, just how tall are you thinking?
I suppose that depends on the size you want the trees to be.
I think a semicircle made of two 10 pipes would be less than 7 feet tall at the apex.
A Gothic arch will be higher but more narrow.

My idea for covering a row of trees is basically a dinning fly with roll down walls, crafted from translucent tarp.
5 guy lines stretched between 3 pairs of poles and parallel to one another, would form the ridge and edges of the roof plus the foundation of the walls
Ratchet straps would be used to create easily tightened and adjustible lines.

I don’t see it working without a source of heat.  Some kind of solar arrangement might help.  I know of a farmer in Kansas that kept Peach trees over winter by wrapping them in insulation and canvas tarps.

Yes William, other arrangements of support for the winter insulation would be possible.

John, the main idea is that the winter covering isn't necessarily transparent, it's a moderately insulated covering.  Maybe like a concrete curing blanket...   Once it's in place in November (after the trees are dormant), it just protects them from the cold snaps and radiant night-time cooling.  With some snow on top it might make it 20 degrees warmer in there.  I think that's all we need here to get peaches to live.  So it's kinda like the insulation idea of your Kansas farmer, just more conducive to a row of trees and it takes advantage of the warmer earth and thermal mass under the footprint of the hoop house.

If it was just a layer or two of poly as in a normal hoop house, yes, you'd need heat to fight off the nighttime lows.  

It seems like a good idea. It would have to be strong enough to support the weight of snow and secure enough to hold up to high winds. Well, you know that already. I'm amazed how much warmer it is inside my uninsulated, not-air-tight, not-translucent garage. It has a dirt floor and we leave the door open 6-8" for a stray cat that's been coming around our place. (I'm trying to get him to stick around and kill mice.) I haven't measured the temp inside but it is noticeably warmer than outside!

I think it's an excellent idea.  Maybe consider putting blue board or some kind of insulation out to the sides a couple feet to keep frost from creeping in from the sides and to help retain the ground heat.  A couple feet of wood chips would work as well.

Good point Trace!  Might the frozen ground be a benefit come spring so that they don't flower too soon?  

One thing I keep thinking about.  Trees in central Illinois probably have bare frozen ground around them in the dead of winter.  Up here they have 1-2' of snow around them and the ground doesn't really freeze under that snow.  So trees that can handle southern IL just fine can also likely handle colder ground than we get up where I live.   That's rattled around in my brain from time to time the last few years...

Marisa, any chance you could measure the temperature inside and outside your garage really early in the morning after a cold night sometime soon?  It would be neat to have a data point.  And a pic of the garage if possible :)

Mike Haasl wrote:Good point Trace!  Might the frozen ground be a benefit come spring so that they don't flower too soon?  

One thing I keep thinking about.  Trees in central Illinois probably have bare frozen ground around them in the dead of winter.  Up here they have 1-2' of snow around them and the ground doesn't really freeze under that snow.  So trees that can handle southern IL just fine can also likely handle colder ground than we get up where I live.   That's rattled around in my brain from time to time the last few years...

I've never been really clear on that.  If a tree can survive -10F, but dies at -30F, what actually kills it?  If it's frost depth, ours is much lower than central IL, but it may very well be something different entirely.  I'd like to see some data on the actual ground temperature at the surface.  My gut feeling is that our ground temperature is colder than IL, even with some amount of snow cover, but how much?  I know we lose far more trees in really cold winters without as much snow for insulation, but I don't know where the "sweet spot" would be.  As I said, I've never really heard an explanation, so I don't know what mechanism actually kills trees at a certain temperature.

I'm not certain about flowering either, you be very well be right that it would be an advantage to have the ground stay frozen longer, but I guess that leads back to my earlier question about what actually determines a tree's zone and what kills them in a colder one.  After all, your insulated hoop house has the sole purpose of keeping the tree from getting as cold, so wouldn't that in itself lead to the tree blooming sooner?  I really don't know, but it's a great discussion.

Yeah, I agree on the concept and have the same curiosity.  I have heard that it's the radiant night cooling from a clear dark sky when it's -30 that kills many trees or at least causes tip die back.  

Not that it is dealing with your situation exactly, my experience with Wisconsin is that it's wicking cold for a long time. But my experience with USDA zones is that they don't tell you the whole picture. I'm in zone 8a but we are likely to only have sub-freezing temperatures for a couple of hours a day during those periods. My non-tender vegetable plants recover fairly well. But I feel like you could extrapolate to greenhouses that are kept at or near freezing over the winter. Not everything is going to thrive, but most will survive. Honestly, think the greatest issue for northern areas during winter is lack of sunlight rather than temperatures.

From what I've seen here, it's important that the trees stay dormant. Too much cold will kill. But thaw-freeze is equally deadly -- if they start to awaken during a warm spell in winter (or too early in spring) and then re-freeze. I'm not sure of a hands-off solution, but it's something to consider.

I would agree with that. It's just as important to ventilate a greenhouse so as to prevent overheating as it is to prevent overcooling.

Some of those issues should be resolved by this not being a greenhouse.  I think I'll edit the title of the thread a bit.  I'm thinking more like this in the winter.  Insulated and covered but not picking up a lot of solar gain: