The rule of thumb is that you lose 2 zones of hardiness when you keep plants in pots outdoors in the winter.

Keeping them in the garage will help a lot. Keep them on the floor, since the ground will act as a source of heat during the coldest days and keep them a few degrees warmer.

During the winter I keep several potted plants in the garage. They don't need much water, but may need a little, so when it snows and I shovel the driveway, I put some snow in each pot.

Some plants will come out of dormancy a little too soon if I leave them in the garage too long, I assume because they are too warm. To avoid this, I bring them out around late February of early March, once the really cold part of winter is past.

Alex

Any plant in a pot is much more vulnerable to cold because the relatively tiny thermal mass provided by a pot allows the roots to freeze thereby killing them. There are a couple of things you can do if mulching the pots is not an option. 1) build an insulated cold frame and use it for over wintering 2)sink the pots in the ground and protect the above ground parts with microfoam or heavy duty row cover. 3) Put them in the garage if the garage floor is in contact with the ground, but be sure to cover them with a cardboard box or something else to trap the ground heat.

Voles and mice can be a problem with any of the above methods, so try to protect the plants with hardware cloth, etc.

presuming they are zone appropriate plants.. cold is irrelevant ...

the problem is either black pots in sun thawing and freezing REPEATEDLY over the winter [plants really have no sense of humor about this] ....

or accumulated water freezing the plant into an ice cube.. roots need air as much as moisture ... potting media is requisite here ... dirt is simply a problem ...

my attached garage is at least 2 zones warmer .. and the problem is usually.. the plants sprouting 2 or 3 months before last freeze ... its a nice place for the real winter.. but can become a big problem in late winter /early spring .... in fact.. by late feb.. simply take the dormant plants out of the garage.. IF still fully dormant.. and put them on the north side of the house.. to hold dormancy for as long as possible ..

my pole barn ... is only one zone higher ... so the early spring flush is somewhat slower.. and if i leave the north door open.. i can keep the plants dormant longer ... but it can still be a problem ...

the KEY is.. get them dormant.. and KEEP them dormant .... and if you can do that.. i bet you will increase your success rate easily past 90% ...

your options are:

unpot and plant in the ground until late spring .. and repot .. perhaps the veggie garden ... avoiding the whole pot thawing and rotting roots issue ...

if your soil has good drainage.. simply sink pot and all ... using mother earth to temper the root mass ...

place them all on the north side of the house or garage... no sun ... and when fully dormant [mid december?] .. lay them on their sides... avoiding the water accumulation issue ...

buy 4 bales of straw ... make a square.. and place the pots in the center ... avoiding the sun ....

or anything mentioned above ...

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winter wind can also desiccate plant tissue ... think frozen chicken in the freezer too long.. so any protection from the blistering winds.. is also good ... the side of the house opposite the prevailing winter winds ...

9 year old is bugging me.. gotta go

ken

Can we assume that you are using a container appropriate potting mix for your plants, and not soil from the garden?