The first thing
that comes to mind are some hardy vines. A zone 4 climate will be
fine for many blooming vines provided you have healthy soil and
apply water as needed. Vines to consider would include the smaller
types of clematis, perennial sweet peas (they tend to run rampant
in all directions unless you are willing to tidy them frequently),
silver lace vine (Polygonum auberti), and hardy kiwis, to
name a few. If the retaining wall is low and you'd like smaller
plants, consider groundcovers such as creeping phlox (light shade,
flowers in spring) and vinca minor (common name is periwinkle, part
shade), or mat-forming perennials like evergreen candytuft and rock
soapwort (Saponaria ocymoides) You might also consider evergreen
creeping juniper.
