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I live in zone six. The coldest it gets in the winter here is something like -10 degrees.

I have a couple plants in pots that will need to make it through the winter. They're all cold hardy plants, so they should be fine outside. But I was thinking that since they're in pots, their roots will be more exposed to cold, as opposed to if they were in the ground.

Is this something I should worry about? Do I need to bring potted plants inside during the winter to keep their roots from getting cold? Or is there something I can do to protect them outside?

  • Oh, there are some things you should know about container gardening and winter. Definitely keep them outside. – J. Musser Sep 9 '14 at 18:53
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The general rule is that if the plants are hardy two zones colder than where you live, they'll be fine outside in pots. So for you that would mean they'll be fine if the plants are hardy to zone 4.

If they are not hardy down to zone 4 or you just want a little extra insurance there are still a few things you could try:

  • Clustering the pots and then covering them with mulch. Strawbales or fallen leaves held in by chicken wire both work for this.
  • Heeling the pots in. I sometimes do this in my vegetable garden, where I don't mind digging a big hole to bury a pot or two.
  • Overwintering in the garage. If you have space, this will give them some protection from the cold.
  • digging them in to the ground works great for me for bonsai – kevinsky Sep 9 '14 at 20:40
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    Heeling in is a good one, so long as decorative pots are covered to keep from being stained. Overwintering in the garage is alright, as long as the temperature is consistently cold (not heated). – J. Musser Sep 9 '14 at 22:18
  • Good points, J. Musser. Here in zone 4, the temperature is pretty sure to be consistently cold in the garage. ;) – michelle Sep 9 '14 at 22:44
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As long as the pots aren't overly wet when they freeze, they should be fine. Shallow rooted plants like blueberries are used to their entire root system freezing anyway. If they're hardy in the ground, then with proper care, they'll be hardy in pots as well.

The biggest problem I've seen with these is that the bottom of the pot will be the last thing to thaw, and this leads to moisture buildup in the pot. Say the pot is frozen, and you get 8" of snow. The next day, the sun comes out and the snow melts, and drains into the top of the potting mix, which has thawed, and fills the side of the container that the sun is shining on. That night, it refreezes. If this repeats all winter, the results won't be good. I always put a waterproof light-colored covering over the top of soil before winter, to keep that from happening. There won't be much evaporation in winter, so it's better to err on the side of too dry than too wet.

Another thing to keep in mind is that dark pots, such as black or dark green (very common) heat up a lot in sunlight, so that in winter, the roots at the edges of the pots may warm up to over 70°F., while the core of the pot is still frozen solid. Not good for the plant at all. So I often insulate the outsides of the pots, and cover that with something light in color, like canvas.

On bringing the pots inside, that is usually a bad idea. Most cold hardy plants use winter as a dormant rest period. Having it be room temperature, or even 50°, will be confusing to the plant. It may try to put out growth too early, and harm itself that way, not having fully recuperated from the previous season's growth. It's also possible, if you are dealing with fruiting plants, that they require the cold period to form flower buds. When a flowering plant is labeled 'zones 4-8', chances are they will survive in higher zones, but won't flower and fruit properly, or may not go completely dormant, and lose vigor that way.

So I recommend protecting them outside, and not trying to overwinter them indoors.

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