I plan to do this for my leftover potted perennials. After the poly cover, I'm going to spread at least another 6" of straw over top. I want to keep the sun out to keep things from heating up during sunny days.

Wintering potted perennials in a covered cold frame is a good way to have them get so dried out, they will die. Rather, set well drained pots into the ground, or just set up some type of frame on the ground, as high as the tops of the pots, then fill in around the pots with soil. Pack it well, mulch, and leave Nature to carry them through to spring.

Then, when the ground thaws in spring, just lift them out and set them into cold frames to jump start spring growth. If you have horses or have any nearby, you can start even earlier, by lining the bottoms of cold frames with fresh, straw covered manure, for hot beds.

My poly won't go on until the pots are frozen. Everything will be kept watered until then. I've plunged pots into the ground in the past...it is very labor intense. There is a risk of the top portion of the pots thawing and then if it rains, the pots can't drain...certain death for most plants.

I'm also in zone 7 but much further south (Raleigh). I use straw bales to make a "bunker" or corral where I store plants. I make them small, only one or two bales across the back and one or two bales down the sides. I leave the front open. I shove all the pots up into the bunker with the most sensitive plants towards the back. I do this so that I can work in and around the plants off and on throughout the winter. If I have a lot of plants to store I just make more bunkers. If the weather calls for hard freezing I rake a huge pile of leaves up onto the whole bunker, burying it a couple of feet deep. After the storm has passed I uncover the pots. In the spring when I want to speed things up I usually rebuild one bunker with fresh straw bales (more rigid) and place the pots of my first crop inside and cover the whole top with large plastic (lexan) bubble top domes used as skylight covers in large industrial buildings (5' by 5', with an 18" dome).

I only use wheat straw because I have access to plenty for free. They do not break down by spring unless you water them a lot. Voles do find their way into the bunkers some times.

I have done it for several years and it works fine. I sprinkle several shovelfuls of partially composted horse manure/bedding on top, then cover with a poly roof. I water a few times through the winter if it is warm and the plants look dried out (some were broadleaf evergreens like abelia).