At one time, almost every Mum was winter hardy. I'm sure many of us northerners remember our Grandmothers growing them in the garden, and not doing anything overly special to ensure their return in the spring.

Then, hybridizers began breeding them toward flower size, shape, and color, and for plant size and shape... and not so much for hardiness. Actually, the more plants that die, the more that will be purchased to replace those that didn't make it. It's the same with almost every industry out there... with the bottom line being happy stockholders and corporate management, never mind the consumer.

But I digress...

I actually have some pink Mums with smallish flowers that come back every spring. They're planted in a mixed perennial border that faces west, and gets wicked frigid winds and snow during winter.

If you really want to keep them potted, you could try planting them in the pots... burying the pots up to the rims in the garden... in a protected spot where they won't get windblown and have more of a chance at survival. A good, heavy layer of mulch will be in order, as well.

There are no guarantees, but they may fare okay planted in this manner.

I'm not sure I would go with a greenhouse, mainly because the temperature will fluctuate due to sunshine on the glass or plastic. You could be looking at quite a difference in temperature between day and night.

Perhaps someone else has a better idea... anyone?

I actually keep all my potted roses and perennials stored in an otherwise unused garage over winter. It stays closed from the first really hard freeze we get, until spring. The plants are grouped together in plastic baby pools, just to keep the pots off the concrete... and the only time I open a door is if I notice the pots are dry, and I throw a few shovels of snow over them just for moisture. The plants don't need any light because they're dormant, so that's not an issue. As spring comes, I begin opening the doors, which face east, for a few hours every day... making sure to close them before the sun goes down.

I've had very good luck wintering everything over this way... though you couldn't use a heated garage, or one that was used for the family cars. It's important that the doors remain closed... unless it's just to add a little snow for occasional moisture.

I'm not sure if I've helped you, at all... but I'm sure someone else will be along shortly to offer more advice. Good luck! :-)

Here is a post with a lot of talk about overwintering mums:

Here is a link that might be useful: overwintering mums