Clematis: Queen of the Vines

Among ornamental vines, few bear flowers possessing the majestic splendor of the clematis. More than 150 years of hybridization has been devoted to the improvement of this noteworthy ornamental plant resulting in the availability of dozens and dozens of cultivars bearing large flowers of purple, blue, mauve, pink, red and white color. Given the exquisite nature of clematis in full bloom, there is little wonder why it often is referred to as the "Queen of the Vines."

Although we use it as a common name, Clematis actually refers to a genus of plants belonging to the Ranunculaceae or buttercup family. There are over 300 species in the genus Clematis and some of these individual species have enjoyed some interesting uses over time. In Europe, the stems of C. vitalba were used for making fish traps, baskets and wicker-work; in Africa the dried roots of C. scabiosifolia are still used as tinder for starting a fire. As do other members of the Ranunculaceae family, certain species of Clematis contain toxic compounds. Protoanemonin is a acrid substance found in the likes of C. vitalba, C. recta and C. flammula that causes skin irritation. It is said that beggars in ancient Rome rubbed their hands with juices of these Clematis species to cause blisters to form on their hands in order to evoke more compassion as they reached out to ask for money. These same species were used in medieval European medicine as topical blistering plasters and, by some, as diuretics. In other parts of the world species of Clematis have been used as stimulants and to "clear the brain."

A few species of Clematis are prized for their ornamental value. For example, Clematis ternifolia is the scientific name of the popular sweet autumn clematis with its billowy white blooms produced late in the growing season that fills neighborhoods with a delightful fragrance. The Chinese clematis (Clematis tanutica) is a species that produces bright yellow flowers. However, most of the modern cultivars grown today represent hybrids between various species of Clematis and usually are placed into one of five different groups according to the species of the dominant parent in the hybrid cross.

• The Jackman group, which probably is the oldest, flowers later than most groups but flowering often continues throughout the summer.
• The Languinosa group contains some spectacular hybrids which flower earlier than the Jackman group.
• The Patens group flowers in late summer on previous year’s wood; thus severe pruning should be avoided in spring.
• The Viticelli group is primarily summer flowering but has a modest repeat bloom.
• The Florida group contains some of the most spectacular double flowers found among clematis. It, too, blooms on previous year’s wood so pruning should not be done until after flowering has occurred.

Another method of clematis classification is according to the pruning they require. Proper pruning is important in producing attractive clematis in that it encourages branching and helps develop a greater display of blooms. The timing and severity of pruning required depends the vigor of the cultivar as well as whether or not it blooms on current year’s or previous year’s wood. The following grouping is provided by the International Clematis Society:

Group I. These cultivars are the first to flower in the spring, produce flowers on growth produced the previous year and require little if any pruning. If pruning is required, it should be done after flowering has occurred.

Group II. This group includes the large-flowered cultivars that bloom in May/June on short new flushes of growth produced in the spring on last year’s wood, followed by a smaller flush of blooms produced later in the year. Pruning consists of removing deadwood and it is best to wait until late spring to do so.

Group III. Cultivars in this group bloom in June/July on current year’s growth and require severe pruning. Group III cultivars should be pruned back late winter or early spring to within about 12 inches of the soil. Leave at least two pair of buds on each stem of the plant.

Cultivar descriptions contained in catalogs or on point-of-sale posters or care tags usually clearly state to which pruning group the cultivar in question belongs.

The key to growing clematis is to keep their roots cool. Clematis require about six hours of direct sunlight to thrive, but their root system must be kept cool. Therefore part sun/shade settings are preferred. Root systems can further be kept cool through the use of mulch or from the shade of low-growing plants such as ground covers planted at the base of the clematis. A rich garden loam high in organic matter and well-drained is preferred by clematis.

Planting usually is done in the spring of the year after the soil becomes workable. Dig a hole 18-24 inches wide and 6-8 inches wider than the root system of the plant. Work into the soil well-rotted manure and a few rotted handfuls of bone meal to enrich it. Position the plant so that the original soil ball is about 4-6 inches below the surface of the surrounding soil. Backfill the hole with equally amended soil, and water well. If the stem of the new plant is still green and tender, wait until it becomes woody before backfilling. Clematis climb by using their petioles to "grasp" onto objects. A trellis, fence or some other means of open support will be necessary to allow it to climb. Once the new clematis is established, fertilize lightly each spring with a general purpose garden fertilizer such as 5-10-5.

Stem rot (wilt) probably is the most frequently encountered disease in clematis, especially during periods of wet weather or if planted in poorly-drained soil. The whole plant can be affected or just one or two stems might show symptoms. Pruning back the plant to well below the point where symptoms first appear can help stop the spread of wilt. Proper site selection and soil preparation can help to prevent wilt as can the timely application of fungicides. Slugs are known to feed on new growth during the spring of the year and can be controlled with baits.

Clematis have enjoyed increased popularity in recent years due to the abundance of attractive cultivars with spectacular flowers as well as their ability to add to the vertical dimension of the garden. By following a few basic guidelines, even the novice gardener can enjoy what truly is the "Queen of the Vines."

David Trinklein Associate Professor of Horticulture
573-882-9631