James, that is a great greenhouse :)) what is the nighttime temperature inside it?

That should give you about a 2F rise. before sun rise.

I've measured a 3 or 4 degree difference at night versus the outside with this type of cover.

These coverings do not help a huge amount at night. But during the middle of the day when it is sunny, temperatures can really heat up in there (even when it's cold outside) so that can help promote growth in cool climates and extend the growing season. You'll probably want to take the cover off in about April, otherwise the plants can bake. (Even if most of the days are still cold, one or two unusually warm sunny days is all it takes)

One suggestion that might help a little bit is to surround the container with 2 or 3 clear water containers. That could help provide some additional insulation around the roots and prevent temperatures from going below freezing.

I'm growing a Satsuma mandarin outdoors in the PNW with this type of cover.

I'm also experimenting with a Bloomsweet grapefruit covered by one of those tomato growth accelerator covers.

Maybe a combination of light protection and choosing a slightly cold hardier variety can succeed in climates that are less friendly towards citrus.

I think this concept is particularly suitable to the PNW which is cold for much of the year, but almost never extremely cold. (Also, even though 90% of the days are overcast here in the Winter, it's those rare clear days that are the ones preceeding the coldest nights)

Nice! I will add to socal and say that I have used a similar quick and easy setup years ago and I filled up empty 2 liter plastic bottles with hot water and surrounded the pots/trays with them on cold days and low temp nights. It really does help insulate the plants from a light/medium frost.

Oh, I only use this during the day to extend the growing season. These two trees looked pretty weak last spring after the long winter so I wanted to come with a cheap way to shorten the winter for them this year. As others have said this won't be of any help at night.

I suspect a setup like this could work on sunny winter days that are below freezing but the problem would be getting the plants from the house to the enclosure without the cold killing them. I'll have to think about how I could make that happen.

It's actually a little bit of help at night for me, since I'm using it right on the climate zone border of where (hardy) citrus can grow. For me just 3 degrees could make all the difference.

On overcast days (that's basically all Winter), even when it's bright I notice that the temperature inside is still a maximum of only 4 degrees warmer than outside. You don't get the real heating effect until the sun actually comes out clearly through the sky. Being far North in latitude also has the disadvantage of shorter days (definitely noticeable), so that's less light and less heat. Hopefully your climate doesn't have these disadvantages.

Great quick solution! :-D The other benefit being that you can actually SEE your tree. Meanwhile mine are still outside (I guess I'm in experimentation mode here) enclosed in frost bags, wrapped in frost cloth, and propped up on plant stands -