I am not sure what your questionis, exactly, but here is an answer.

The ground freezes slower than things that are above ground. If you leave a dish of water out in the earliest freezing temps, it will freeze while the gound will not be frozen, at least not down to 6-8 inches.

I live in zone 5 around albany, ny. Professional nurseries here bury pots down to the rim in the soil, and they make it through zero degree winters with lots of snow. The trick is to get them out early in the spring and start watering them, before you really start thinking about gardening in the spring.

Anyway that you think about it, plants are always safer in the ground. Get them in the ground anywhere, and then place them where you want next spring. Above ground, they are subject to lots of freezing and thawing which hurts the root system. if you don't have space, find a neighbor who shuts down their vegetable garden for the winter, and bury your pots there.

Hope this answer helps.

I have a follow up question. I had a conversation with a tree guy at Countryside Farm and Garden at Route 176. He told me that the coldest temp the ground can get is only 28F. Is that true?

You can put lots of leaves around your pot above ground. I have some grasses in big pots that hold over 2 cubic feet of potting mix. I get them together and dump bags of leaves over them. Make sure leaves are between the pots as well as on top. Also these are hardy grasses. I have no luck with the purple fountain grass.

Things in a pot or out of a pot will freeze the same.

A pot on top of the ground will freeze sooner and longer than one in the ground. The soil does provide heat, which comes from well below the surface.

The ground will get much colder than 28F.