Answer: There are lots and lots of choices for your Zone 3 garden. In no particular order, these perennials bloom from early summer to frost: yellow perennial flax (Linum flavum compactum), Veronica whitleyi (a groundcover that starts blooming with the tulips), Delphinium grandiflorum 'Blue Elf', astrantia (even after the flower has dried up in very late summer, it still has the appearance of a fresh bloom), western Canada violet (Viola rugulosa), Corydalis lutea and also the blue-flowered types, Symphyandra hoffmannii, lady's mantle, Aquilegia vulgaris (they bloom throughout the summer here), Clematis integrifolia, fern-leafed bleeding hearts, tiarella's, lamium, repeat-blooming shrub roses.
Those with very long blooms (though not as long as those above because they start a bit later): Primula florindae and alpicola (both from midsummer to fall), Campanula punctata, Campanula persicifolia, Digitalis grandiflora (perennial yellow foxglove), Jerusalem sage (Phlomis tuberosa), Heliopsis helianthoides 'Summer Sun', repeat-blooming daylilies (e.g. Stella D'Oro), potentilla's (the shrubs), and sea holly. Most will need adequate water to become established, but then should get along with only occasional water.
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