In addtion to possible cold damage later the roots on the south side of the pot may have been killed by direct sun exposure during the summer. So either way it probably needs a different situation.

OK, I will put the pot down on the ground in a fence corner, the pot will be on the SW side of the fence corner.

I don't have a good planting site for the juniper in the pot right now, any further suggestion on how to improve its survival chances welcome!

Can you find any space to dig a hole and sink the pot into the ground for the winter and also give the juniper a good drink if need be.

Yes, just sink it into the ground pot and all. This insulates it against the cold yet leaves it easy to pick up and carry on next year.

or.. when the ground freezes.. just tip the pot over and lay it on the ground...

the biggest issue .... imo.. is late winter thaw.. and rain.. accumulating in a pot and refreezing the roots into an ice cube ...

stand the pot up when the ground starts to thaw..

all the better if it ends up under the snow.. which is a great insulator ...

you can sink the pot.. IF the native soil has full drainage... even worse if a clay soil ends up being an in ground cauldron ...

no black pot in winter sun.. if some bizarre warm spell hits in mid winter.. and the pot gets hot in sun.. you might have the roots come out of dormancy.. but not the rest of the plant..ergo.. under snow.. or in full shade ...

the key is.. get it dormant.. and keep it dormant ...

ken

btw.. no special rule for juniper.. mint julep juniper or topiary ...

its all the same for ANY PLANT in a pot over a harsh winter ...

and to repeat.. dont rot the roots off ...

ken

Well, I put the black 7 gallon nursery pot upright on the ground in a fence corner facing SW, and surrounded it with leaf litter and mulch. That fence corner does get snow, although I am not sure how complete and consistent the snow cover will be.

Hoping for the best, it is an expensive topiary!

Woody plants in pots are susceptible to cold damage at about 20 degrees F. above where the same kinds would be hurt when established in the ground* - make a mound of mulch (such as bark or arborist wood chips) or sand and stick the bottom of your potted juniper in that. So that frost has to penetrate a significant thickness of mulch or sand to get at the roots from the sides. I would also pile some of this material over the top of the potting soil, so that the roots have at least some protection from that direction also. And get the plant out of the pot and in the ground ASAP next year.

Also a sunny nook is actually a bum spot to place your tree for the winter as sun hitting frozen plant parts is detrimental rather than beneficial. So better to put it in a location that is shaded during winter - in my USDA 8 climate I put potted outdoor plants in the garage during otherwise damaging cold periods. With the main thing to watch with that being how quickly these need to be watered under such circumstances.

*Apparent introducer of the variety Monrovia nursery (link below) has it zoned down to USDA 3. So if accurate you are close to the edge for this plant if you are in USDA 4 - if it was mine I definitely would take a serious approach to getting a potted specimen of this kind adequately protected where you are. All the more so if I was concerned about how much it cost, wanted to be sure it was not damaged or killed (during exposure of potted woody stock to damaging cold the roots near pot walls die first, then older roots farther back in and then finally tops. So an affected specimen can come through cold exposure with a live top and still have a partly dead root system). Mint Julep® Juniper, Juniperus chinensis 'Monlep', Monrovia Plant

How does the nursery retail trade (small mom and pop shops) ensure the survival of woody plants in black nursery pots?

It may be several winters and several summers before a buyer puts the plant in the ground!

The nursery retail trade may be doing the most economical and yet effective precautions that a homeowner like me can emulate.

How does the nursery retail trade (small mom and pop shops) ensure the survival of woody plants in black nursery pots?

Depends on the operation - some do a better job than others. With it not being hard in my region to find stock on display that is not in good condition at all. Also "several winters and several summers" is way too long for any plant to be kept in the same nursery production container, except when particular techniques like root pruning and repotting in fresh media are employed in order to compensate. Techniques that are much more likely to be in use by a bonsai specialist than a general retailer.