It somewhat depends on the size of the plant and what zone it is rated down to. Keep in mind that roots tend to be a zone or two higher than the plant itself when they don't have the benefit of the ground. You also have to keep in mind that water can be complicated with potted plants in the winter. I had a poor little Top Hat blueberry bush that didn't make it through the winter in a pot in my screen porch (tucked against the house), though we're not sure whether it was killed by lack of water or the cold. I've seen suggestions of putting some snow into the top of the pot to help with the water issue, you just need to make sure that it doesn't get too wet and rot either. I think some people will also pile leaves up around the pot or wrap something else around the pot to improve insulation and get closer to the situation in the ground.

I do have several small hostas that live in pots, and I actually sink the pots into the ground for the winter. Basically, before the ground freezes and temps get really cold, I plop the plastic pots the hostas live in into the ground, and then I just pull them out come spring. Mine have a winter home under a forsythia bush near my house. For larger bushes, that might not be as practical.

it can be as simple as tipping over a pot ... so water doesnt accumulate in a frozen pot over winter .. even dormant roots need air ... and prefer not to deal with being in an ice cube ...

even better if the pots end up under snow .. you would place them so black pots never warm due to sunshine ... and warm during some mid winter relative heat wave ... under a bush or conifer.. or whatever is a good idea ...

this topic is much discussed.. every fall.. in the hosta forum ... many regional experts ... too bad you didnt link that forum ... repeat the topic there ...

in ground freeze.. i would never leave them in fancy pots that might crack ... so plastic is preferred ...

water plant late into fall ... often.. fall is a drought.. though we dont tend to notice since its cool and dewy ... but you want the plants to go into winter fully turgid.. full of water .. but you want the media sopping wet at freezing solid ... too many peeps put the hose away in october .. but pots might need a drink or two.. as late as thxgvng ....

sometimes the most important thing in a pot is the media ... rather than the pots ...

air temps are not really an issue ... with zone appropriate plants ... dormant plants dont care how cold you think it is ... air temps isnt the important variable ....

healing in pots/plants ..... is approved ... per above ... it reduces a lot of the variables you need to deal with .. especially the potential for a pot to thaw in say. mid feb.. as compared to mother earth keeping that plant dormant until almost april ....

frankly .. imo.. dragging them in a structure just complicates it all .. unless its a zone pushing issue ...

as noted.. this is highly regional .... you can take hints and clues and ideas form anywhere ... but hone in on peeps in a similar area .... peeps who dont know ground freeze .... are weird.. lol ... and though many of the variables are the same ... there are subtle differences ...

see you in the hosta forum????

ken

ps: two asides:

1) never use plastic is the garden.. for anything .. imo.. its against nature .. jsut dont....

2) my attached unheated garage in ground freeze z5 is at least z7 ... which will entirely complicate your life.. e.g. when the plants come out of dormancy in early march... and you have a last frost freeze date in mid may .... or that mid winter thaw in feb.. is even worse ... and my pole barn is about a z6.. same deal ... its not a big deal with a few plants ... but is you have hundreds.. it can become a nightmare .... the problem is the sun on the garage will heat it.. and hold the heat all day ... etc ...

In your zone you will need to winterize or winter protect any planted containers. And it doesn't matter much how hardy the plants it contains may be. If the soil in the container is allowed to freeze solid, which it will if left outdoors, you risk the roots freezing and that will kill the plant. Container soils can get far colder than inground soil will.....even frozen inground soil :-)

Many do bring planted containers into an unheated garage or other protected but cold area. Or you can also sink or heel in the pots in the ground outside, as already noted.

Inside a garage, you will need to check periodically to make sure soil is barely moist - not bone dry but not wet either. Light is not an issue and neither is wrapping, unless your garage gets very cold. You want the temps to stay just around freezing or slightly above....not more than 40F.....to maintain dormancy.

btw, any comments about not using plastic in the garden are just personal opinions :-) There is no valid reason not to use if that is what you have and prefer. Plastic plant containers are exceedingly common, from very inexpensive nursery or box store ones to high end plastic resin ones that sell for hundreds of dollars. They all work perfectly well...........

So, I am gonna give some hostas a try this year and we will see how they do.. Is one type of pot better than the other? I had a porcelain one that I forgot to pick up from the garden and it was cracked the following spring :(

Over years, I've overwintered various herbaceous perennials in plant containers (usually the one gallon containers larger perennials are purchased in) in a protected location (between two houses) in the garden. The perennials have usually been from divisions of garden plants; though sometimes they were recent purchases. However, in all cases, they were (re)potted in our garden soil.

Overwinter, these plants are frozen solid and also get covered by snow. I do water them in fall before their soil begins to freeze. The only losses I normally get are from rotting in spring; the way to avoid that seems to be to get the pots out into the sun in spring. Sometimes, however, I've delayed acting, hence the rotting.

Last winter, I only overwintered irises this way, but, as said, it works here with a wide variety of herbaceous perennials. Haven't tried hosta yet (but likely, as some shade spreads, will do so soon). Suspect this would also work for small shrubs.

I've assumed this strategy mimics what happens naturally to our perennials over winter, but it still surprises me that it actually work (at least here).

soil that freezes expands ... and can crack a clay pot.. of any kind .... especially porous clay pots ... so i find a plastic pot that fits in the pretty pot ... and take the plant out in fall.. and store the empty pot in the pole barn or garage ... fully glazed pots should not retain water ... but why risk it ...

and easy way to water pots in a garage.. in winter.. is to put an ice cube or two on each pot ... and let them melt.. on warmer days ... just that little bit of water to keep roots turgid ... full of water.. without the mess of trying to put a little water on with a watering can ... small snowballs also work ....

ken

That is a great idea actually Ken, I have PLENTY of heavy duty plastic buckets in my garage, what sort of soil should be used for this? Regular potting soil?

It would be interesting to know from other zone 5 gardeners who've tried overwintered herbaceous perennials (or shrubs) outside in plastic pots above ground level in a protected location.

Why not overwinter dormant perennials in their natural winter state? The intent is presumably for the plants to resume growth in spring. During winter the perennials here, at least, are frozen solid in frozen ground.

I would certainly not use the type of potting soil that works well for growers who raise perennials in greenhouses.

There is a significant difference in temperature between frozen ground soil and the temperature an above ground container is exposed to. One barely drops much below freezing (32F) while the other can be as low as sub-zero. One WILL kill plant roots and therefore the plant; the other will not. That is why containerized plants require some sort of winter protection in colder zones (heeling in/sinking in the ground or stored somewhere temperatures do not drop that low) while inground plantings do not.

And the only soil mix/potting medium that should be used in a container is one that is formulated for that specific usage. Good drainage and proper aeration has nothing to do with winter temperatures.

Yes, the general rule applies that plants do much better in the ground than in containers, but I presume some reasons require the OP to pot grow.

Whatever theoretical consideration are raised, I garden in zone 5 and have successfully overwintered hundreds of herbaceous perennials, winter after winter, both potted and outside, in the manner which I outlined above.

The one source of failure (rotting) seems to me to be that after the pots had thawed, in their protected and largely shaded above-ground location, I was slow in moving them out into the spring sunlight. That tended to happen when I had more plants than I was going to need for that spring.

I don't see grounds for dispute. I did it and still do it. It works, at least here. Obviously everyone should approach overwintering perennials in pots the way they want, but I've always been under the impression that it's useful to take the experience of other gardeners, gardening in our sort of climate, into consideration.

It works (at least here). It would be just plane silly to give reasons why it should not have worked, when my experience clearly shows that it did.

Sunnyborders, I don't think anyone said it wouldn't work..just that care requires more steps and attention than simply planting them would in regions that freeze.

Dbarron, I can only read what people write; namely, "This is why containerized plants require some sort of winter protection in colder zones (healing-in/sinking in the ground or stored somewhere temperatures do not drop that low) while inground plantings do not" (gardengal48 above).

My experience shows that containerized perennials do NOT require " --- (healing in/sinking in the ground or stored somewhere temperatures do not drop that low) ---". Perhaps other gardeners gardening in zone 5 have a different experience; in which case, it would be informative to look for differences in more detailed aspects of how the overwintering was done.

Dbarron, I'd say that my approach has, if not less "steps and attention", at least no more. It a very simple approach to the problem, but it works for me.

I'm zone 6 PA and I have never had a problem. Most of my pots are at least 5 gal plastic or galvanized tubs. I grow many of my herbs this way.

sunnyborders, I think the protected area between two houses you're using might qualify as 'somewhere the temperatures do not drop that low.'

Interesting point, BlueberyBuntcake. True it does cut down on the wind exposure, but the temperatures are still low enough to freeze the pots solid before standing snow falls.

Another point would be that the plants end up by being covered by snow; on the other hand, by that time they and their contents are already frozen solid; also they can be temporarily exposed during a mid January thaw.

I've wonder whether the size of the pots/containers is a relevant factor. When I initially started, overwintering didn't work for me with small perennials in small plastic containers; perhaps there's also a relationship between the size of the container and the size of the plant it contains. Still, Cathy mentions large containers, though not necessarily for large plants (viz. her herbs).

It certainly never worked for me when perennials were not repotted from the soil they were originally purchased in.

I am at the northern edge of zone 5, and have found that of the various plants I have tried, mostly hardy to zone 3, 4, or colder, the only one that has been hardy in an unsheltered container has been a variegated ribbon grass, a plant I would never let run loose in a garden, but it does fine in a whiskey barrel.. Everything else has been dead in the spring ranging from bulbs such as hyacinths, grape hyacinths, and daffodils, to hosta to violas to hardy chrysanthemums etc. If I put the containers into a small south-facing porch, I can overwinter zone 6 plants in containers such as Dahlias and Salvia Black and Blue. I have buried pots/heeled in plants I wasn’t able to plant before it got cold and they have done fine, and I have overwintered a potted lavender in an unusually warm winter by tipping the pot on on its side, tucking it against a building, and burying it in snow.

I’m in 5a about 50miles west of Chicago and have overwintered hosta for several years, here is what works for me. My plants get put in an unheated shed after foliage has died back and been trimmed away. They get put in the shed slightly damp. Ken is right you have to plant containers with loose potting soil, most important. Never cover pots with anything. I do not add any water or ice cubes over the winter, I ignore them. Personally, I have not had any luck tipping pots outside in the shade to keep them out of the sunlight. I haven’t buried pots in the ground, makes no sense to me here. When it gets warm in the spring, anywhere from March to May, the pots come out and get watered. I have had to put them back in the shed for an night to protect from a late freeze on occasion. I haven’t lost any potted hosta

Sherry

Perennials I remember overwintering here (repeated over more than one winter) include: a New England aster, white wood aster, a culver's root, daylilies, a globe thistle, irises, lungworts, peonies and a rudbeckia).

Even in zone 7, I've lost perennials that had pots freeze solid because I was an idiot and let them.

Perhaps freezing and thawing cycles are relevant in more southern locations. Once frozen, our potted perennials stay frozen until spring.

That's a very good point SunnyBorders. I think it's probably multiple factors, but we certainly get cycles here.

Great advice here!
@Sherrygirl I am about 3 hours south of you (Indy) What potting soil do you use?

I use a good commercial potting soil, a light mix without those rehydration granules. Some gardens make their own mix. Good luck with your potted plants!

Sherry

I'd also assume that multiple factors are involved, as dbarron says.

I feel that that is a major reason why following general gardening guidelines, though one requirement for perennial gardening success, is in itself not enough. Success (however individually defined) also takes individual gardeners, trying out the possibilities, to determine what combination of choices will work in their own situation.

Roxana..I'm glad that your potted plants survived..I have 1 macrophylla hydrangea, 3 hostas in large pots, 1 shallow bowl shaped pot and 2 very small pots all in my shed..I should've checked them yesterday or today but was busy and didn't..will report back as soon as I take a look..just as Sherry does no water since last fall..typing this now makes me super curious but it's dark and will have to wait..

Obviously, being in a Z8, I have no real concept of over-wintering plants...but I do go through winters with ALL my plants in pots (at least in my home garden)...and what a ride it has been, with multiple considerations. What struck me immediately, Sunny, was your solution of sheltering between 2 houses...which is a complete winner when the houses have great thermal mass (ie brick or stone built). My tiny garden is surrounded by brick walls and acts as a sort of heat sink which lifts my zone allowance sufficiently to grow tender South Africans and Mediterraneans with nu further protection other than moving them nearer to the walls. I also use terracotta for all my containers, but there is terracotta and terracotta, not all being equal, (porosity, thickness, clay composition, glazing temps play a part...but again, I may be talking out of my arse here since mine are not enduring a snowless icy Z4 freeze). Some will flake and crack during the first freeze and thaws...but most are thick, hand-thrown and some are ancient (well over 50years old).

So, the point, I think, to my rambling and probably irrelevant post (as least as far as my experience of freezy Canada) is that there are many variables, from rainfall (I am very dry), to storage, garden aspect (that early morning eastern sun has done untold damage), soil type, container size, shape and strength, plants (pf course) and modes of shelter. Horses for courses.

My soil (in pots) can, and will freeze solidly, for days on end...yet many plants seem untroubled by this (auriculas, for example)...although it is fair to say that solid freezes last days rather than weeks or months.

Oh yeah, just remembered - the shape of the clay container is important - all my losses seem to be pots which narrow towards the top or deviate in other ways from the standard narrow at the base, widening near the rim, type of pots.

We have an unheated zone 6 attached garage. We keep our spring water there and it has never frozen....until this year. So, In my opinion, an attached garage is somewhat warmer than a detached garage. That being said, generally, perennials make it in my garage--even some tender annuals have survived, but this winter, likely not. I usually water only once or twice in the winter--usually as an afterthought. I also keep the pots up against the wall of the house and not over by the garage door.