For long term plantings, the gritty mix is hard to beat. Search this forum for more info or look here.

Also, there is a Thuja occidentalis v. 'Bobazam' ( nicknamed Mr. Bowling ball for its natural tendency to grow in the form of a perfect sphere) you might find interesting.

Al

I'd question the advisability of leaving it outside year round. Unless in a very large container, the soil will freeze solid and very few plants will tolerate that sort of intense cold with regards to their root system - that is often the portion of the plant that is the most vulnerable to cold. Containers experience much colder temperatures than would the same plant growing in the ground.

I have no personal knowledge of growing in that cold of a climate but would recommend either heeling the pot into the ground for winter or moving it to an area where it receives some cold protection......a garage or cool basement, etc. Somewhere where it is unlikely to freeze.

@tapla: Oh wow, this ignoramus (i.e. me) asked an honest question and I discovered dozens and dozens of religious wars about potting mixes, they confuse me. All I need is a one paragraph recipe for a potting mix that I can use.

Thanks for the Bowling Ball suggestion. Not sure if that is the same or similar to the various species of Globe Cedars sold in local garden centers around here. If the container is a semi-globe, then the plant will have to be pruned as the upper semi-globe (dome-shaped).

@gardengal: Thanks for cautioning me. I asked the question because I have a difficult spot to dig in, which is where I wanted to put the (mostly) above ground container. Bringing the container in-house in the cold season is not an attractive option either. At the moment, I am not sure what I will do next, but I like to keep my options open for now.

Most buxus are listed as hardy o z5; in container it should be hardier plant (it is suggested to grow plants 2 zones hardier in containers).

Buxus microphylla koreana 'Winter Gem' is supposedly hardy to z4, so it could work.

Healing in container for the winter as gardengall suggested would probably work, or some other kind of protection: you could try to pile up lots of leaves around the container, making sure they don't get blown away (using a wire mesh surround or something similar.

Once again I feel compelled to point out that hardiness zone has nothing to do with a plant's ability to survive deep cold in a pot. The "2 zones hardier" advice is repeated over and over again even though it is entirely a myth. Hardiness zone indicates the minimum temperature a plant is likely to survive when planted in the ground. The thermal mass of the ground protects the roots from damage. In the much smaller pot, there is no such protection. Many plants will die if the roots are subjected to temperatures in the teens even though the plant is rated as hardy to -15 or -20.

If not buxus, what evergreen shrub would survive in an outdoor pot all year round? Blue spruce? Other?

Sinking the pot into the ground as gardengal mentioned is one possibilty. Storing in a cool cellar, garage or outbuilding well wrapped with insulation (except for the cellar) is another. There really is no plant I can think of that would survive a Zone 5 winter just sitting outside. A week or so of near or below zero temperatures will kill just about any plant including those that are rated hardy to Z3.

FWIW - I leave several species of junipers, pines, and native larch, out on the benches all winter z5b and they don't complain about it in spring when it's time to wake up and start pushing. MANY plants, even in bonsai pots will survive our 5b winters if all I do is protect them from sun and wind (a dessication thing).

Wrapping a plant with insulation isn't particularly helpful unless you're arranging the insulation so it's trapping heat from an extraneous source. If you put a Dixie Cup of water in a freezer set at 0* F vs putting a Dixie Cup of water in a cooler and putting the cooler in a freezer at 0*, both are going to yield water frozen at a temp of 0* in time. The only difference is, the cup of water in the cooler just takes a few hours longer to reach lowest temp. If you put a cup of water in a cooler in a garage that stays at 25*, eventually the water in the cup reaches 25* because there is no extraneous heat to trap; however, if you set the water on the garage floor and overturn the cooler so it's opening is down and it surrounds the cup of water, it will trap geothermal heat rising from the floor and almost certainly keep the temperature in the overturned cooler well above freezing, while it's counterpart that's isolated from the floor is at 25*. Put a plant in a closed cardboard box and set it on a milk crate and it soon equalizes with ambient temps; but, leave one side of the box open and set the plant on the ground with the box covering it; or set the plant in the box and push the open end against an exterior wall on your balcony, and you'll be trapping heat from an extraneous source.

As a rule of thumb, I've found that plants listed as hardy to 2 zones colder than they find themselves growing in will survive outdoor chill w/o protection. W/o protection means not in contact with the ground. They might suffer from dessication, but they don't die because root cells burst due to freezing of intracellular water. I've also found that plants listed as hardy to one zone colder than the zone they find themselves in will do just fine on the ground out of wind with some dead leaf mulch around the pots - though I can't predict what toll mice/voles/rabbits might exact on the plants. Most plants listed as hardy to the zone you live in will survive if you bury the pot in the garden, or in beds, and you can actually pick up a full zone by burying or mulching plants on the north foundational wall of a heated building.

This isn't something I'm repeating because I've read it somewhere. It's something I have done and continue to do.

Ontario - do you realize that a good % of the plants you might consider lose their vibrant green color when it gets cold? Many evergreens 'bronze' when freezing temps arrive, turning them quite brown. It's a temporary thing, but something you might not have considered. They green up again and look like proper plants in spring.

Al

Perhaps I should consider evergreens with needles then, not broadleaf evergreens. Evergreens with needles don't change color AFAIK. Something like this dwarf globe blue spruce which is hardy to zone 2. It requires full sun, whereas the intended space has dappled shade in the morning hours in summer, I hope it is still OK.

What potting mix would be best for it?

Thanks! This 'Approximate Root Killing Temperature' table provides useful and critical info. Unfortunately, the lowest temperature in that table is -11 F. Whereas in my USDA zone 4b, the average annual minimum temperature ranges from -30 F to -20 F.

I understand it to mean that ANY (ornamental) plant left outside in an above ground container will be killed by the cold (in my zone) unless (elaborate) precautions are taken. It looks like I have to abandon my idea, but it is nonetheless useful insight.

Thanks to all who contributed to this thread!

Not necessarily "will be killed". "Will always be at risk for being killed" is more accurate. For the internal temperature of a container to reach -10 there would have to be an extended period of very cold weather, not just one or two mornings of -25 which moderates considerably during the day. Likewise, a large container would be less vulnerable than a small one because it would give up its heat more slowly. Sinking the pot into the ground or otherwise insulating it would also help a great deal and is not really overly elaborate, but that determination can only be made by the person undertaking the precautions.

Well, will be killed sooner or later under the given circumstances, not really a practical proposition for a permanent garden feature.

I have overwintered many perennials in containers for years. The plants that didn't survive are the plants that were in garden soil - I would just scoop out the plant with some soil into container thinking about planting it somewhere else and never got to do it. Regardless hardiness zone, if the drainage is not good, they didn't make it - I believe that it was thaw & freeze cycle much worse in badly draining soil, and worse in small containers.

I had buxus in plastic pot for about 9 years - someone bought it at the garage sale & it was not for sale! But they wanted it so badly, and the plant was getting big and very heavy to move so I gave in :) That was in Toronto, zone 6 (?)

I have couple of small ones that I grew from clippings, started 4 years ago. They are in gallon plastic pots. I have been living in z5a now for almost 4 years. They are in 511 type potting mix. Here is the photo just taken:

Things aren't all black/white. For instance, killing lows for individual species varies considerably by the plant's state of health, nutritional reserves, and how much intracellular water there is in the plant. In addition, not all roots on a plant succumb to the same 'killing low' temps. Fine roots succumb to freezing temps much higher than required to kill more lignified roots, so roots are killed over a range of temperatures until finally the stake in the heart is the temperature at which tissues in the oldest roots die.

Too, the example of Picea glauca as hardy in z3 to -40* F you used while additionally noting roots could ultimately be expected to succumb at a balmy -10 seems more to support, rather than refute the rule of thumb that a plant should be listed as hardy to 2 zones colder than that in which they finds themselves growing in a pot if they are to remain viable w/o protection ....... so long as the pot is in contact with the ground. -20 is listed as the low temp extreme for zone 5, and it would be expected that plants in contact with the earth would be at least 10* warmer than ambient air temps at those extreme lows.

It will always behoove growers to keep their temperate plants cold enough so they're exposed to the chilling units necessary for the plant to enter a deep dormancy and ultimately pass from that dormancy into a quiescent state by virtue of the same chilling units, while at the same time protecting their plants from temperatures low enough to kill even a small fraction of the roots. Shooting for root temps between 28-40* is a good way to ensure a snug winter's rest and a plant eager to show its stuff when spring temps start flirting with numbers above the 42* mark.

Al

I think you're overlooking a very important issue in the p. glauca example. If "contact with the ground" means only sitting on the ground, the walls of the container will lose heat at a much higher rate than if the pot were sunk into ground to its rim or insulated in some other way. Even a relatively large 5 gallon pot would drop to the -10 temperature pretty quickly with several days of -20 temperatures with highs only in the single numbers or low teens. Such temperatures accompanied by winds accelerates the process. Not too many years ago my area of Maine experienced around 8 days of sub-zero (-10 to -18) temperatures with highs barely reaching +10 and high winds nearly every day. There was no snow cover at all. Code requires water and septic pipes to be 4 feet down, but many people experienced frozen septic lines and water lines. In some cases, these refroze after an initial thawing. Many plants in the ground listed as Z4 hardy were killed outright, hellebores being a notable example. After a few days of this the ground surface temperature was certainly not 10 degrees higher than the air temperature. It was well below 0 at 18" and would have been supplying no heat to the bottom of a container sitting on the ground. One nursery lost a sizable number of Alberta spruces (Z3) even though they were under opaque plastic and microfoam. Those at the center of the pile were damaged but survived. Those on the perimeter were killed. The p. glauca example is really coincidental. Of course you are correct that no plant keels over dead the second the magic root killing temperature is reached. Once it is reached for a sufficient amount of time, however, the plant will die. Hardiness zone is irrelevant.

Interesting. FWIW - I don't overlook much (especially not intentionally) when it comes to weather and other cultural factors that affect a plant's viability as it relates to chill resistance. That I might not mention something like wind doesn't mean I've not considered its effect. Again, though, wind doesn't ultimately affect what low temperature the soil/roots in a planting will be, only how long it takes to arrive at that low temp. Unless the ambient temperature reverses itself and starts to rise before the pot equalizes with ambient low temps, the temperature of the soil and roots is destined to be the same as ambient temps unless there is an extraneous heat source involved.

It's said that even in the middle of a plowed field with no snow cover and a week or more of sub 0* temps that the 6" ground temp never gets below about 25*, no matter how deep the frost runs, here in z5 MI. A MI State study in Saginaw county (z5) found that the lowest 5 cm soil temp of tilled land w/o snow cover for winters 1967-69 was -2.7* C or about 27* F. Thousands upon thousands of acres of Thuja swamps in Michigan's Upper Peninsula have never seen frost penetrate the ground in modern times. Interesting stuff - nutriment to feed the nerd gene.

Al

I don't doubt the MI State findings. Remember though that the soil at 5 cm depth continues to receive warmth from the sun even in the coldest weather. It is not uncommon to find a layer of relatively warm soil near the surface, then deeply frozen soil below it. This phenomenon is a major contributor to "mud season" in ME and no doubt MI. It also contributes to in-ground plant death. Water can find it difficult to drain away because of the frozen layer. Roots can rot in this case even though the temperatures are not all that cold.

As for heat transfer and wind, consider the ordinary 3 gallon (nominal) nursery can. The bottom is 7" in diameter or 22 sq. inches. (All figures approximate) We have 200 sq. inches of sidewall plus 25 sq. inches of upper surface. In other words, we have a total area of 22 sq. inches in contact with the ground and absorbing some heat vs. 225 square inches radiating heat into the air. We have more than ten times as much surface losing heat as we do capable of absorbing any. Air turbulence of any kind will increase the rate of heat transfer from the pot to the air. There's probably a neat formula involving ground temperature, air temperature and wind velocity that would provide loss per minute, etc., but I have no idea what it is.

BTW: I have no idea how complete the archives are or if you would be interested, but back in the 1970's the Journal of the North American Rock Garden Society published a number of articles by Jim Boland (Borland?) about container soils and root death temperatures. These created something of a stir as they challenged conventional wisdom. For example, for decades, sharp sand or grit was advocated for the best drainage. Boland pointed out that spherical grit would be far less likely to pack tightly than angular stuff. Lots of follow-ups and disagreements. Plant nerd paradise. The Journal also published a table of plant root death temperatures during the same era. I believe these were among the very first systematic experiments on the subject.