Start with tough perennials
The first secret to gardening in a cold climate is to grow really hardy perennials, and there are a number of plants that are troopers no matter how harsh the winter or spring. As a general rule, those that die back to the ground each fall do better than evergreen perennials, although I have successfully overwintered both lavender (Lavandula angustifolia ‘Munstead’) and sage (Salvia officinalis).
The first plant to force its way through the last of the snow is perennial cornflower (Centaurea montana). It often blooms by the beginning of May, when I am in desperate need of some color. After it flowers, I cut it back by two-thirds and it blooms again by the end of summer. Another early riser is oregano (Origanum vulgare), which I grow not only for its use in the kitchen, but also for the flowers it produces. Artemisias (Artemisia ludoviciana ‘Silver King’ and ‘Silver Queen’) poke through the soil much later, but are consistent every year, along with other herbs like valerian (Valeriana officinalis), bee balm (Monarda didyma), and anise hyssop (Agastache foeniculum). These tough plants will continue blooming through an unexpected mid-summer frost (photo, above).
Domesticated varieties of wildflowers (which I prefer for their showier blossoms) also do well. Lupines (Lupinus cvs.) are early summer bloomers that generously reseed themselves. Delphinium cultivars—6-foot giants compared to their cousin, the native larkspur—love the cool temperatures and pull through most winters without a problem (photo, above). Their biggest enemies are the wind and hail that often accompany summer thunderstorms. To keep these delphiniums from being blown over, I stake them, plant them near buildings, and grow the shorter varieties.
Peonies (Paeonia cvs.), bearded irises (Iris cvs.), sneezewort (Achillea ptarmica ‘The Pearl’), sea holly (Eryngium maritimum), and tansy (Tanacetum vulgare) are also sound choices for tough climates. I never worry about their surviving, and they produce fantastic flowers every year.