[5] Leaves are opposite and divided into leaflets and leafstalks that twist and curl around supporting structures to anchor the plant as it climbs.

Clematis leaves are food for the caterpillars of some Lepidoptera species, including the willow beauty (Peribatodes rhomboidaria).

[25] Christopher Grey-Wilson divided the genus into 9 subgenera (Clematis, Cheiropsis, Flammula, Archiclematis, Campanella, Atragene, Tubulosae, Pseudanemone, Viorna), several with sections and subsections within them.

Many of the most popular garden forms are cultivars belonging to the Viticella section of the subgenus Flammula as defined by Grey-Wilson.

These larger-flowered cultivars are often used within garden designs to climb archways, pergolas, or wall-mounted trellises, or to grow through companion plants.

These forms normally have large 12–15 cm diameter upward-facing flowers and are believed to involve crosses of C. patens, C. lanuginosa, and C.

[30] Japanese garden selections, mostly cultivated in Edo Period using species that are native to Japan or China,[28] were the first exotic clematises to reach European gardens, in the 18th century, long before the Chinese species were identified in their native habitat at the end of the 19th century.

[31] The climbing varieties are valued for their ability to scramble up walls, fences, and other structures, and also to grow through other plants, such as shrubs and trees.

[33] The pruning regime for a cultivated clematis falls into three categories: Over 80 varieties and cultivars have gained the Royal Horticultural Society's Award of Garden Merit.

[36] In the Atragene group are woody deciduous climbers with bell-shaped flowers in spring, produced on the last year's growth.

This follows the classification adopted by V. Matthews in The International Clematis Register and Checklist 2002, except that C. ispahanica, now considered to have been included in error, has been omitted from the list of parent species in Tangutica Group.

[61] The entire genus contains essential oils and compounds which are extremely irritating to the skin and mucous membranes.

Unlike black pepper or Capsicum, however, the compounds in clematis cause internal bleeding of the digestive tract if ingested in large amounts.

Despite its toxicity, Native Americans used very small amounts of clematis as an effective treatment for migraine headaches and nervous disorders.

[64] Clematis has been listed as one of the 38 plants used to prepare Bach flower remedies,[65] a kind of alternative medicine promoted for its effect on health.

[68]: 3  C. mandshurica specifically is known to provide inoculum transferrable to wheat in the former eastern Soviet territories, and several of this genus are hosts for several other P. recondita strains[68]: 8  and other Puccinia.