Plant folk apparently refer to that as a cloche... The idea is to mulch, tarp and pray.

The mulch provides depth so the soil may avoid freezing solid underneath, the tarp keeps water from rotting the tubers, and then you pray that your new $25 intros from Swan Island has durable enough tubers to last the winter.

Some varieties have amazing storage abilities, some rot if you look at them wrong. Most are somewhere in-between. If you just grew the same varieties, the heartier of them might make it, while the others are rotted and forgotten.

One grower in 5/6 PA has overwintered Romance for several years in a semi-protected area with nothing added... He didn't plan on keeping it, but it kept coming back! But most folk that overwinter in the ground do that with varieties that they don't mind losing. They dig those intros.

I have accidentally left Dahlia tubers in the ground in an area with good drainage, and none of them came back even though the beds are mulched and we always have a good mulch of snow that prevents deep freezes.

Soggy soil with dormant tubers are the kiss of death, from Florida to Ontario and everywhere between. That snow has to melt sometime!

gotta dig em, here in iowa. plant them in a pot and bury the pot to the lip. makes fall digging a breeze. pot comes right up and you don't have to cut the tuber

Commercial growers encourage this practice as it means more sales as the people usually lose them. There are so many ways besides freezing too: rodents especially gophers, rot, insects, slugs, snails and probably more.

I discovered a couple of dahlia tubers, fairly dried up, that I neglected to plant last spring. Is there any way of wintering them - in ground outside or pot in basement - for best chance of survival in zone 7?

Since they seem to have made it an extra year wherever they were, leave them there and plant in a pot in March. I have found that about 25% of tubers stored for that second year will grow. But not a very good practice.

I tried to leave my Dahias tubers in the ground over winter "for science". At this time one of four is coming up. Better than my canna and caladium experiments (none of those returned).

Editing to add that another one is coming back.

I live in northern Delaware. Last spring I realized I'd left a Crazy Legs dahlia tuber cluster in the ground over that harsh winter, but the only reason I realized that I'd left them in the ground was because of how they shot up in April! I think they survived because it's a sunny area covered with black mulch that absorbs heat from the sun, which probably kept it from freezing. But I also left a few tubers and an entire clump in the ground this past winter, and they all rotted due to excessive moisture(and probably cold too).

I've always dug mine up after first frost here in Chicago. Last fall I neglected to do so with three potted tubers. They came up and are growing like gangbusters. Go figure:) Two are at the bottom of the stairs in the photo.

@billiejoebob

That sounds like a recipe for success: mulch really deep in a very well draining area. My friend who I gave dahlias to a couple of years ago had success with this here in North Carolina did it. I like to dig mine up, but his came back earlier and bigger than mine so go figure.

I leave mine in the ground here in western Oregon zone 8b and they have made it thru the coldest winter in 40 years last year(-10) , they actually multiplied, here there is more problem with saturated ground in winter than the cold and I did not cover it with anything
Im going to cover them this winter , I had a bed that was unused and covered with plastic and when I uncovered it this spring it was bone dry .
this winter I will cover my new tubers with a tarp and then 6-10 inches of hay and leaves
don't put the mulch under the tarp or mice will move in and eat your tubers
I also plant my tubers a little deep 5-6" , to protect them from the ground freezing it never freezes deeper than that here