One variable is the hardiness of the trees you will be storing. What kind of trees are they?

Burying the pots in the ground in the shed is a good option. The dirt will keep the containers warmer and at a more consistent temperature. For water, I would put a couple scoops of snow around them when it snows.

Alex

You are in zone 5 Iowa. Your waterlines are buried, what, about 4 feet down to be lower than the frost line? A newly planted tree will have frozen roots anyway, so frozen roots is not a factor. Your run-in shed should be fine. Thow some snow on them once in a while and as it melts they'll have enough moisture.

tj

Thanks for your replies, Alex and Tj. I like the suggestion of throwing snow on them. The trees are all hardy to my zone- 3 Japanese maples and a Harry Lauder's walking stick I picked up for a song. I know the Japanese maples will appreciate spending their first winter out of the high winds. They may have been coddled too much in a greenhouse previously.

jap maples are not all that hardy in my z5 .... good luck with that .., and HLWS is marginal ...

the issues you need to address are ..

sun on black pots ...

desication ...

and media drying ...

so no sun on the black pots.. EVER ... shed sounds good ...

dessication from freeze drying winter winds.. especially on sunny winter days ... so again.. the shed stops that ..

and finally .... put a snowball or ice cubes on the media.. and on warmish days.. they will melt and provide whatever moisture the plant might need ...

come about march ... open the shed door ... and let the warm air out.. or you z7 shed [that is at least what a closed shed will be in my experience] .. will make them sprout a month or two too early.. subjecting them to frost/freeze problems ...

in fact.. by march.. put them on the north side of the house.. out of sun and wind.. and let nature moderate it all ...

so.. sounds like the shed is my recommendation ...

ken

ps: if you have a good draining soil ... you could probably sink pot and all .. in shade and with some wind protection.. and just leave them outside ...

Protect from wind and sun...place containers butt-up against each other and mulch around sides and tops of containers heavily. You're basically providing insulation. You also don't want the drying effects of winter winds or the premature thawing effect of the sun on the roots...black plastic containers can get warm on cold winter days when the sun is shining on them, and this will lead to thawing of the roots, which you don't want to happen when warm, frost-free season is still weeks or months away.

shast said: You're basically providing insulation.

====>>>> but it might not be what you think ...

its insulation to KEEP THEM COLD!!!! ... not warm ....

the key .. simply stated is: GET THEM DORMANT ...KEEP THEM DORMANT .... as late into spring as possible.. to avoid frost freeze issues ....

potted stock has a nasty tendency of going in and out of dormancy .. and up here in the great white north.. that kills plants ...

ken

hmmmm...more to contemplate, thank you Ken and Shastensis. The black pot variable hadn't occurred to me. The 3-sided run-in shed where I was planning to sink the trees faces South and has a slanted roof so lots of sun does get in and the space gets rather cozy on a sunny day. I imagine there might be a fair amount of thawing on sunny days and refreezing at night. So if stored them in a regular old unheated shed, the trees would be protected from sun, wind and zone 5 temps as low as -20. The roots wouldn't be insulated, but that's ok since the shed would be more or less like a zone 7. Good to know, Ken. I think I'll stick them in my shed.
Shastensis, it's good to know containerized trees can be stored outside, as well. I would not have thought they would need to be kept out of the sun, but this makes sense.
It seems like I have plenty of south-facing spots that are protected from wind, but they are in full sun.

Sink 'em in the ground. Frozen roots in the ground are much different from frozen above ground. If they're not winter-hardy frozen in the ground, so be it, but frozen in pots above ground, you give up the equivalent of approximately two(2) cold-hardiness zones. I've had trees that would have been fully winter-hardy here, in the ground, die overwinter when I didn't get around to getting them sunk in the ground, or at least getting a big pile of mulch piled up around them.
In the shed, they'll be protected from dessicating winds, but I would anticipate that the root systems will be prone to excessive drying(unless the roof is really leaky), so if you put them in that setting, you may need to water periodically so that they don't 'wake up dead' next spring.

You can sink them in the ground, but it's not necessary if the pots are butt up against each other and you mulch heavily around them. This is the "insulation" you want and which the ground provides - yes, from boti hh heat AND cold. I have ovedr wintered trees this way in zone 5a many times and never lost a single one. Dig all those holes if you want, but there's easier ways.

Also, you would want the potted stock to be on the NORTH side of a fence or wall, to protect the plants from the sun. You would also want the stock on the EAST side of a wall, to protect from the wind (assuming your wind comes from the WEST. This is all actually pretty simple, it's just hard to trust it the first time when your plants are on the line. Protection from sun and wind, and insulation on the roots/pots.