Plant of the month: Clematis (April)

Clematis is one of the most popular garden plants for good reason; it is a wonderfully versatile plant can be grown on walls, pergolas, frames, in containers, or left to scramble through trees and shrubs. There are varieties for every aspect, and for every season.

Cultivation notes

Clematis need moisture-retentive, but well-drained soil. The herbaceous species prefer full sun, but most climbers and shrub species will thrive in full sun or partial shade. Keep the base of the plant and the roots cool and shaded by carefully positioning other plants, or put a layer of pebbles or flat stones at the base.

Planting

Plant large-flowered cultivars that bloom in May to June with the tops of their root balls 5-8cm (2-3in)  below the soil surface. This will encourage shoots to grow from below soil level and also helps the plant to recover if affected by clematis wilt. Herbaceous and evergreen species such as Clematis armandii and C. cirrhosa should be planted with the crown at soil level.

Maintenance

  • Each year, in late winter or early spring, apply a potassium-rich fertiliser (such as Vitax Q4 or rose fertiliser), according to the manufacturer’s instructions
  • Mulch immediately afterwards with organic matter such as well-rotted manure, leafmould or garden compost
  • Water regularly during periods of dry weather in the first few seasons after planting. Watering to soak the root zone requires at least the equivalent of four watering cans per square metre

Pruning and training

Clematis climb by twisting their leaf stalks around supports so it is necessary to provide some form of support when growing against walls and solid fences.

Plant positioning and training

  • Grow clematis through, or over, trees and shrubs to extend their seasonal interest. Choose the clematis carefully, as vigorous types can be smothering. Avoid using clematis to cover dead trees or tree stumps, for although these may provide a useful support they may soon become hosts to certain diseases, in particular honey fungus. Plant the clematis on the windward side of the tree so that as its stems extend they are blown onto the tree where they can obtain a hold. Position it at least 1.8m (6ft) away from the tree trunk to reduce competition with the tree for space, water, and nutrients. Use a bamboo cane or wire to aid the clematis reach the tree trunk
  • When used to cover walls, fences or pergolas, provide a form of support, such as trellis or mesh, for the clematis to twine around. Ensure the support protrudes from the wall by at least 2.5cm (1in), to enable the climber to scramble over it. This can be done by fixing trellis to 2cm (1in) deep battens at either end
  • Those clematis that are not true climbers (shrubby and herbaceous types) require tying in to their supports

Initial pruning for newly planted clematis

  • To avoid the development of a straggly single stem and to encourage branching lower down, cut back all newly planted clematis to 15-30cm (6in-1ft) from ground level in February or March, cutting just above a bud
  • Even if planting later in the spring, hard pruning is still advisable. Delay pruning of autumn-planted clematis until the following spring
  • Pinch out developing young shoots once or twice to promote further branching

Pruning established clematis

Clematis has a reputation for being difficult to prune, but it isn’t, as long as you know when your clematis flowers, as pruning groups are based on flowering times;

Propagation

Clematis can be propagated by double leaf bud cuttings taken from spring to late summer, or by layering from late winter to spring.

Species clematis can be propagated by seed;

  • Ideally sow seeds fresh in the autumn and cover them with a fine layer of compost and grit
  • Keep the pots in a cold frame or unheated greenhouse, as they need a cold period to germinate well
  • They should germinate the following spring

Cultivar Selection

There is a clematis for every situations, as can be found in the RHS Find a Plant. Here are a few options…