Candytuft, shown here in front of azalea, is an evergreen perennial ground cover that blooms in spring to early summer.
Reaching up to 4 feet tall, Shasta daisy (Leucanthemum x superbum) is suitable for cutting. Plant in full sun. Joe-pye weed (Eupatorium purpureum), which can grow 3 to 6 feet tall in full sun or partial shade, displays reddish blooms. Both plants bloom all summer long. Grayhead coneflower (Ratibida pinnata) produces large yellow daisy-like blooms with drooping petals and grows up to 5 feet tall in full sun to part shade. Plant these tall perennials along a fence or at the corners of the house behind a grouping of shorter plants.
Medium height flowers are good under a window or in front of taller flowers. Perennial plants for sun to partial shade in USDA zone 4 include heart-leafed Bergenia (Bergenia cordifolia) or lilac cranesbill (Geranium himalayense); each grows to about 18 inches tall. Bergenia produces pink to red blooms in spring and lilac cranesbill sends out bluish blooms in early summer. Bulbs like daffodil (Daffodil spp.) that bloom in spring in colors of yellow, orange or white can also survive winters in USDA zone 4.
Short perennial flowers are useful for borders. Short flowers that thrive in partial shade in USDA zone 4 include spotted deadnettle (Lamium maculatum) and primrose (Primula x polyanthus), each growing to about 10 inches tall. Deadnettle produces small white, purple or pink blooms in summer while primrose blooms in spring with red, orange, yellow, white, pink or bi-colors.
Perennial ground covers will gradually spread during the growing season to cover openings in the flower garden and are typically less than 12 inches tall. Candytuft (Iberis sempervirens) is an evergreen ground cover, producing white flowers in the spring. A fragrant option is lily-of-the-valley (Convallaria majalis) with its tiny white bell-shaped springtime blooms. Other options for USDA zone 4 include pussytoes (Antennaria species), which presents white or pink blooms in late spring to early summer, and purple poppy mallow (Callirhoe involucrata) with red or purple blooms.