Answer: How about wooden tulips? No seriously, talk about a tough spot! The problem is not only the freeze thaw but also the fact that many plants are not root hardy at such cold soil temperatures even though their tops may be hardy in your zone. It might pay to think laterally and plant the (real) ivy or some other vine in the soil directly in front of or next to the planter so that it can crawl upward to soften the look, and then try a plant with a winter "presence" such as one of the ornamental grasses in the planter itself, knowing that those in the planter may have to be considered annuals.
Judging from the suggestions made by your local nursery I would guess the planter to be hot, sunny and very well-drained. Other plants to experiment with might be basket of gold (aurinia saxatilis), baptisia, catmint, peachleaf bellflower, bloody cranesbill (geranium sanguineum), "Cerise Queen" yarrow, purple coneflower, amsonia, and sedum "Autumn Joy". According to the book "Perennials for Dummies" by Marcia Tatroe these are able to withstand temperatures to 40 below zero and they all do well in a hot sunny well drained spot.
Finally, you might think about either widening the planter or possibly insulating it somehow. Good luck with your project!
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