Container gardening is tricky for me but I still keep doing it. I read somewhere that what ever plants you put in containers need to be hardy to a couple of zones hardier than the zone you are in.

For instance in my case, I'm suppose to be a zone 6, so I should plant zone 4's in containers. Unless you can wheel them in to a storage area in your garage for the winter. I might do that this season.

Potting soils in nursery containers can be excessively moisture retentive. In our winters, that's not a good thing, especially with plants that prefer it on the very well-drained, drier side. And the last two winters offered some pretty cold temperatures that could have easily destroyed vulnerable containerized plants. Cold and wet in combo typically lead to root rot. If purchasing in fall, it is probably a good idea to at least heel the plants into the ground rather than leaving them exposed in nursery cans to the weather and temperature. If you do choose to leave them in their pots, provide winter protection by grouping them closely together in a protected location or sinking them into the ground.

And in our area, I'd just avoid purchasing NZ flax in the fall :-) With the last few winters, it is just not reliably hardy - even more so in a container - to risk overwintering before establishment.

I think it's time to give up on container plants that look (and especially smell) dead. I gave up on my potted NZ flaxes a couple weeks ago. They'd been in pots for 5 years but this winter's single digit temps (two nights at 9 degrees) did them in. They were in plain thin plastic pots. And, even though they are considered wetland plants in NZ, they really do need perfect drainage here - we're much colder in winter than NZ is.

On the other hand I have many plants in plain plastic pots, cheap pots or nursery containers, and it appears that only the phormiums died. I have a hinoki cypress in a plastic pot that is left out totally exposed and it's fine, ditto a japanese maple. I have a bonsaid pieris that was left out totally exposed, albeit in a very large ceramic pot, and it's fine. Too large a pot to move or pack away like the rest of the bonsai, which spend the winters with the pots sunk in wood shavings in plastic crates. All the smaller containers, like 1g cans and 6" pots, spend the winters under the deck or up against the house where they get a bit of shelter. Larger containers are left where they are.

You can get extruded plastic pots, kind of like styrafoam, that are very thick but light and those insulate the roots a bit from cold. I've never yet lost a plant in one of those. They're expensive but very nice.

I yanked the dead phormiums last week and replanted the pots (new soil) with a combo of small hinokis, delphinium, snapdragon, heuchera, dianthus and white alyssum. Time for something very different!

I do usually leave on dead foliage til spring, as it insulates the plants from further freeze damage. But, if the plant smells rotten, then just yank it. And it can take til April for a plant to show any signs of life. With container plants you can pull it out of the pot to see if there's any sign of life in the roots, which could send out new top growth even if the top is totally dead.

If I understand the poster correctly, these were plants that were overwintered in their original nursery containers, not a planted-up container. There is - or should be - a great deal of difference bewteen the two. One is the quality of the potting soil -- nursery soils are not intended for long term container culture but rather to support the growth of the plant only as long as it takes to sell. They tend to be too finely textured and too moisture retentive for long term container culture. And nursery containers are often smaller than ideal, given the plant size. This offers much greater exposure to root damage from cold with unprotected containers.

If late season purchases are left to their own devices over winter without protection or shelter from our worst weather, I would not be at all surprised to see a fairly high rate of failure over the winter months, depending on the type of plant. If taking advantage of late season sales, plant immediately or at the very least, provide some sort of appropriate winter care.

Knock them out of the pots, shake the potting medium off and look at the whole plant to see if it is still viable.

A pattern that has been seen is for plants in pots to be about 20 degrees F. less hardy than the same kinds would be in the ground.

update: Out of the goner list there is actually one survivor -- blue sedge has been sprouting back up.