The usual method recommended for overwintering nursery stock is to place the plants on their sides once they are fully dormant.. Then cover with microfoam which provides important insulation. Then cover with white poly plastic to protect the microfoam and reflect heat. Since you lost plants last winter, I think the car shelter allowed for too much head room above the trees which in turn allowed temperatures to fall too low. The microfoam plus plastic method relies on trapping ground heat to keep temperatures from dropping too low, so having the cover(s) tucked around the plants is important. Many universities and agriculture departments have more detailed information about this online. Covering the car shelter with white poly would help to shed snow and would certainly do no harm. There are also various methods of insuring that you do not provide a cozy winter home - complete with a ready food source - for mice and voles. This is an important step.

i dont know how zone appropriate plants can get too cold????

and we have no info about how bad last winter is ... my z5 dipped into z4 ...

but if you have repeated losses ... for trees in pots... the first thing i would look to.. is your media.. of which you dont talk about ... including fert in said media... how wet they stay late into fall ... etc ....

if you are using a standard high peat media favorable for annuals/perennials ... i bet most of your problems start right there ...

ken

Ken, there is really very little relationship with either the presumed hardiness of the trees in question OR the pottiing media in successfully overwintering containerized or potted trees.

Container soils can freeze very easily. And since tree roots are far more vulnerable to freezing temperatures than the top growth, those overwintered in a container are at high risk for fatal cold damage. This is even a concern in my very mild zone with Japanese maples. JM's are very sensitive to winter root damage at relatively high tempertures -- about 25F, which I assume is well above what the OP might expect in zone 5. In the ground they are fine, but without the insulating factor of that soil mass around the roots they are extremely vulnerable.

Different trees have different trigger points at which damage could occur but few could tolerate temps into the single digits for any length of time. btw, hydrated or properly moistened potting soil freezes solid at a colder temp than bone dry soil, so some moisture retention in the potting media is desirable. It's the same concept as making sure your garden is well watered before a deep freeze.

Subconciously, I know you understand this, as I often see you responding to questions like this about heeling the pots in for winter or sinking them into the ground for over-winter storage :-) This is the same concept but on a larger scale.

Here, nurseries that are overwintering any remaining container or B&B stock heel them into sawdust beds. Or sometimes keep them in greenhouses, depending on the stock. But we don't have the same kind of winters by any stretch! I would think mainegrower's suggestions very appropriate for colder climates.

overwintering pots in your z8.. has little of nothing to do with the same.. in z5 ...

if you freeze the roots into an ice cube... because your media does not drain properly .. even when frozen ... then trees in pots die ... roots need air.. as much as moisture.. especially in winter ....

IMHO .. proper media is imperative.. in the great white north ..

it really doesnt matter what he does ... if his media is improper ...

and yes.. i have inserted pots into banks of wood chips ... for winter protection.. works rather well ... but my pots had proper media ... which was 50% wood chips and 50% promix ...

another thing about his tenting method ... i like to say.. get them dormant.. keep them dormant ... and you usually win ...

but when i tried putting them in the pole barn ... in my z5 MI .. i found out.. the hard way.. two things ...

first ... the barn was at least a z7 ... accumulating heat.. in sun in mid winter ... and no winds.. etc ... meaning things came out of dormancy a month to 6 weeks early .... and then died getting hit with repeated frosts and freezes ....

but also... unlike last winter.. if we have some bizarre mid winter warm spell ... again... the pots MIGHT come out of dormancy ... in the middle of winter ... many plants have little sense of humor about doing such.. repeatedly .... especially in a zone.. where 8 hours later.. it can be back below zero ...

i just keep wondering.. in his need to baby them.. he isnt really adding so many variables .... he is actually making it harder than it should be ... i mean.. i would agree.. that keeping them out of winter wind is a plus... but hermetically sealing them in a small hut.. might be problematic ... and if he had problems before.. why is he doubling down.. and increasing his variables????

he might have better luck.. if he simply tipped the pots on their sides... after dormancy.. and letting a snow bank cover them ... and be done with it ... presuming it snows in canada.. lol ..

or better yet.. do half one way.. and half another... but dont do what failed last time ... eh???

ken

It's not a media thing, it's not a zonal thing, it's not even much of a species thing...........it's a temperature thing. Containerized tree roots must be protected from excessive cold.

There are numerous references online that address this issue - I've linked to one that is appropriate to the area.

Here is a link that might be useful: overwintering containerized nursery stock

What Gardengal said about roots being more temperature sensitive is completely correct and a majorly important factor, BUT, Ken brought up some good points! Potting medium and temperature fluctuation factors are indeed important in keeping nursery stock alive in winter. Often, trees with roots in heavy waterlogged medium fail.

To quote a section of Gardengal's link...
"In addition to root hardiness, factors such as soil moisture (which is mainly a function of the type of media used), temperature fluctuations (part of what Ken was talking about), and root development in the container also affect plant survival. The type of overwintering technique used is determined by the plant species and the ability of their roots to withstand cold temperatures during winter."

BTW, there's lots of other good info in that link, including stuff that could be very relevant to this particular situation.

I'm sure there is more then one way to mess this up.

One problem is simple physics. A tree in the ground has it's roots surrounded by dirt. That's insulation. It may get covered with snow, which is great insulation. If you dig and bury a thermometer you'll find the soil temperature is often a lot warmer then the air.

If a tree is in a pot the roots don't have as much insulation and air can circulate around the root ball.

Sticking the pot in wood chips is actually an elegant solution.

From experience, I've noticed that Alberta Spruce survive the winter in pots on the porch just fine, but Rose of Sharon (which grow like weeds in the ground here) die in pots over the winter.

Not as sure about this, but...do dormant plants have to be watered? Does every plant go COMPLETELY? Never been clear on that point with dormant plants in pots.

This post was edited by edlincoln on Mon, Oct 13, 14 at 11:16

I used to overwinter perennial stock (both woody and herbaceous) and young trees and did it in the same manner as mainegrower suggested, with foam insulating blankets under white poly (overwintering film) in a dedicated hoophouse. We used the foam blankets not to keep them so much from dropping below a certain temperature but to keep them COOL on sunny winter days to maintain dormancy. That is why overwintering film is cloudy and WHITE, to reflect sunlight. We also occasionally had to water the stock to keep the rootballs from drying. Not often in that situation, but it is needed when it is needed. It's a PITA and means removing the blankets, uprighting the stock, venting the structure and then putting them back to bed. I lost very little stock btw. Keeping the temps moderated by ventilating on warm days is also part of it and I'm not so sure how you can do that in the situation you used with the tarps. My hoophouse had roll-up sides and when the temps started to climb uphill, so did the sides. I have seen other nurseries use their own ideas, like hay bale surrounds for the containers. I have no idea how much stock you are handling. Is heeling them in (container and all) an option? BTW, I had a nursery stock growers license and some of the things we do to nursery stock is not the same as one would do as a landscaper or end user. If you are serious about eventually doing this as an income source you might want to start thinking about a proper structure. They are not a large expense for the purpose you will use them for. One only slightly crappy winter experience will rob you of any hope to make it profit making and believe me, profit margins on stock are slim. LOL You can't afford to loose stock.

This post was edited by calliope on Mon, Oct 13, 14 at 1:31

With due respect to Mr. Adian, it is simply not true that " if you freeze roots because the medium doesn't drain properly, the plant will die." Once a plant is fully dormant, temperatures somewhat below freezing will do no harm. In Zones 4, 5, and 6 the frost line is anywhere from 2 to 5 feet+. Roots within this area are subject to freezing temperatures nearly every winter. What does not happen is extremes far below 32 degrees because of the thermal mass of the open ground. Potted plants have a very limited thermal mass, so temperatures can drop into the 20's and below. These temperatures can, indeed, kill a large number of plants which are perfectly hardy when planted inthe ground. A well dained medium is important for a number of reasons, but avoiding freezing just isn't one of them.

In our region many nurseries use hoop houses to store their stock in. They use wood chips/mulch to completely cover the pots to keep them from a freeze/thaw situation all winter long. I do not know what they do about mice/voles. They likely need to allow some air movement during warm spells.