Answer: There are so many perennials you could use but the selection really would depend on which way the fence is facing and whether the area is sunny or shady and at what time of day. For instance, if you are planting on the north side where it is shady all day then you could use hosta or ferns, on the hot west side with afternoon only sun you could use sedum, on the all day sunny south side you could use daylilies or purple coneflowers and on the east side you could use Siberian iris. Hydrangeas ultimately grow to be large shrubs.
If you have an eastern exposure you could try Hydrangea arborescens such as Annabelle, these have white flowers. Most of the Hydrangea macrophylla cultivars will not bloom reliably in your area due to excessively cold winters. But, you could try one of the new varieties of bigleaf hydrangea that are able to bloom on both old and new wood such as "Endless Summer" if you have enough space for it to grow.
I hope this gives you some ideas.
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