When first choosing clematis, look for types hardy for your area. The species types tend to be the ruggedest clematis, while the large-flowered hybrids may need a bit more pampering. Then select for flower color and bloom time. If you select wisely, you can have clematis blooming in your yard from spring until frost.
Consider the types of flowers you like when buying your clematis vine. The flowers can be star-like and either small or large. Colors range from pure white to deep purple. There are also species, such as 'Betty Corning', that has nodding, bell-shaped flowers. Some clematis have lantern shaped flowers, such as the yellow flowered 'Bill MacKenzie'. Others have small, dogwood-shaped flowers, such as the creamy pink 'Little Nell'.
Bloom time is important as well. Some early-blooming clematis include purple-striped 'The President' and 'Multi Blue', with its bold stamens and sepals that leave behind visual interest after the bloom itself has faded. The Armandii types are evergreen and good choices for warmer climates. For early to midsummer blooms, look for the large flowered hybrids. This large group includes 'Niobe' (red), 'Jackmanii' (bluish-purple), 'Nelly Moser' (bicolor mauve and pink), and 'Snow Queen' (white). An advantage of these hybrids is if you're diligent about deadheading the blossoms after the first flush of flowers, you'll get a second flush later in summer.
Come fall, look for clematis varieties that provide colorful flowers, but also interesting seed heads. These can be left on the vine to add winter interest to your garden.
Some good choices include the native Virgin's bower (C. virginiana), C. jouiniana 'Mrs. Robert Brydon' (blue), and C. tangutica 'Helios' (yellow). The common Sweet Autumn Clematis ternifolia (paniculata), has very fragrant, small white flowers in profusion in fall. As it can self-sow rampantly, you'll need to be diligent about deadheading and pruning so that you can keep it in bounds.
There are also shorter, bushy types of clematis that grow only a few feet tall. These are excellent in small-space gardens or even containers. Look for shorter clematis varieties such as 'Bushy Blue Bell', 'Rosea Bush', and 'Mongolian Gold'.
A good evaluation of many northern clematis varieties for their vigor, flowering quality and disease resistance was undertaken by the Chicago Botanic Garden.