Joe

Aside from aquatic plants, I've never run across a plant that likes to remain saturated on a constant basis. Watering properly takes place as needed, and not on a schedule, like once a week whether they need it or not.

I do have the storage, so I bring all my clay pots inside a garage for the winter, but I suppose if you emptied them and placed them upside down, they'd tend to crack less than if allowed to remain filled with soil... which will expand and contract with freezing and thawing and moisture levels, thus causing the pots to crack.

Even plastic will wear, fade, and become brittle if left exposed to the elements, though... so I think it's going to depend on what your personal preference or budget is. I prefer unglazed clay.

My brother has had a regular terra cotta pot on his waterfront home's upper deck for about 10 years, zone 9 I think. But he showed it to me this spring, and it was turning into a pile of shale-like thin wafers. All those years exposed to heat and cold and north winds took its toll. I replaced it for him too.

And I had some ceramic pots (not terra cotta)which had a glaze fired on their outsides, but apparently not their insides. I left them outdoors with mulch around them to see if my pelargoniums would survive the cold temps in Alabama last winter. The colored glaze fractured and big chunks of it came loose, although the basic pot remained unbroken. They hold about 2 gallons of soil.

Now I'm trying some really high fired pots which hold about 25 gallons, they are colored by reduction firing to a higher temp than the ordinary terra cotta pots. I'm hoping the two I leave outdoors this winter will NOT split. The pots are not glazed, but I think there is pigment mixed into the clay for their deep oxblood matte finish. Not super expensive, but not cheapies either.

I'm wondering if a pot within a pot can withstand the cold weather that gets low and stays there for months at a time. Until last winter, we never had more than a couple of days below freezing, and now we've had two winters with up to two weeks below freezing. In a row, I mean. That is a whole new ballgame for this south Alabama gal.

One plant I've found that survived in a container,outdoors and unprotected, for two winters in 5b conditions, was the heuchera, and then some chives. I was very surprised.