Among the most charming and varied of flowering vines, clematis plants confront the would-be grower with a stunning range of colors, heights and blooming times. Horticulturalists group clematis vines into three distinct categories: Group A early flowering, Group B large-flowered (mid season) and Group C late-flowering. Because clematis vines are relatively lightweight, gardeners often train them on flowering bushes to either coincide with that plant's bloom time or to add post-bloom color. Of course, clematis also grows happily on trellises, arbors and mailbox posts.
Early-flowering, or Group A, clematis includes Alpine Clematis, a 6 to 8 foot vine bearing small, bell-shaped flowers ranging from lavender to purple. Vanilla-scented Armand's Clematis clambers up to 30 feet tall with clusters of white flowers. Downy Clematis, which reaches 15-feet tall, prefers shadier locations, and produces on either single or double bluish-purple blossoms, depending on the variety. Anemone Clematis, which blooms about a month later than other vines in its group, bears luxurious clusters of white or pink flowers on a 20 to 30 foot vine.
Early-flowering clematis vines rely on the previous season's blooms in order to produce new growth, making careful pruning critical. Cut back soon after the vine finishes blooming, and take care not to cut into the woody trunks.
Although botanists named this group for the size of its blooms, Group B clematis also flower mostly at the same time, and can be considered mid-season (June) bloomers. Flowers range from 3 to 8 inches in diameter. Cultivars include Clematis lanuginose 'Candida' whose yellow stamens contrast cheerfully with its 8 inch white petals. Florida Clematis features green and purple centers in a white setting. Many hybrids exist in this category, including the white-flowering 'Henryi;' the lilac or lavender 'Marie Boisselot' and 'Mrs. Cholmondeley;' the dark red or mauve 'Nelly Moser' and 'Niobe' and the light blue 'Perle d'Azur.' Most of the vines in this group (with the exception of 16-footer 'Perle d'Azur') grow about 8 feet long and make suitable climbers for mailboxes, short trellises and twining companions to rose bushes.
Prune Group B clematis in late winter by first removing dead or spindly vine sections, next cutting back the healthy stems by making cuts just above the highest set of leaves. If you'd like to try for an early autumn bloom, cut back the entire vine to 18 inches tall after its June flowering season. Many clematis plants in this group will bloom a second time if they are forced to sprout new growth.
Group C clematis vines bloom anytime from late June to November, depending on the variety and the growing climate. Cultivars include Sweet Autumn Clematis, a 30 foot vine bearing small white clusters of flowers known for their heady fragrance; Orange Peel Clematis, named after its bright yellow-orange small flowers; light blue Texas Clematis, which, as its name suggests, prefers dry and hot summers and deep-purple-flowered Italian Clematis, also known for its tolerance of hot climates. Hybrids include two short vines with large flowers: pink and white 'Comtesse de Bouchard' and 'Vyvyan Pennell,' which boasts shadings of deep blues, reds and purples in each blossom. Prune the stems to 2 to 3 feet in late winter; vines blossom from the current season's growth, not the previous year's.