Overwintering perennial plants outside

I have two blueberries and two Mars seedless grapes in Earthboxes on my front porch. They each hold two cubic feet of soil and a gallon or so of water.  They are very heavy when the soil is moist. Still heavy when it’s dry.  The porch faces west. It’s concrete and the house wall is brick. I’m in zone 6b but it was zone 5 a few years ago. I would like to leave them on the porch over the winter.  I’m wondering if all that concrete will conduct the cold and kill them? I think they’d be OK on the ground. I have a lifting restriction for now. The pots are about triple what I should lift. I know I have to keep the water reservoir empty so it that doesn’t crack when the water freezes. Maybe just some scrap lumber under the pots would make a difference a keep my plants alive? Does anyone have a suggestion.

in northern climates the ground can freeze a couple feet deep or more, high bush blueberries and grapes survive for decades in such conditions

Thanks Bruce! I was hoping that was the case. I don’t think I’ve ever left potted plants out over the winter.

We mostly buried our containers with perennial shrubs in the ground with the plan to pile up the snow.  

It is better that perennial plant soil is moist as the freezing of the soil helps to insulate the roots compared to dry soil when frozen provide not much insulation.  I also piled some dead branches around my perennial shrubs to trap the snow.  The thicker the snow over the plants and the longer we can keep snow in the late winter the better especially if you have plants that might be less tolerant of the cold winters.

Some people will take out their dormant plants from the container and heed them (bury) into the ground for the winter and put them back in their pots in the springtime when they are still dormant.

Another possible solution is the put straw bales around your container and make sure that the snow is piled around the pot and above.  (But the ground is always the best) Some will put their pots in an unheated garage that keeps just below freezing all winter.

I work at a wholesale nursery (zone 5b)

All our container material is packed in plastic covered huts for the winter

Ideally, you might want to try and bury them in the pot or at least provide some sort of insulation around the pots.

Straw bales, mulch, anything you could use to insulate would help.

However, I've had luck with keeping things uncovered myself - but it's often times a crapshoot

Best of luck healing and towards a speedy recovery!