Answer: Unfortunately, a tall thin shape will not allow for much insulation around the roots. Most plants have a root system in proportion to the above ground part of the plant, so a small root area limits your choices somewhat. The hardiness issue is also difficult, since the less insulation the less cold protection. A northern exposure is also tough -- most plants require more light to stay healthy. You might try an experiment with an English ivy topiary on a wire frame in a geometric shape such as cone, pyramid, diamond on its tip, ball on a stick, etc. Another option might be a "Sky Pencil" holly -- the name is descriptive of its shape. But given the parameters, you should probably be prepared to replace the plants periodically no matter what you use. I would also be a little concerned about freeze/thaw damage to the glaze and/or container -- if moisture enters through an imperfection in the glaze and freezes, it can pop it off and/or crack the pot. And damp soil will of course expand and contract as it freezes and thaws. Good luck with your project!
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