Timber Press Pocket Guide to Clematis

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Introduction

The genus Clematis boasts more than 1000 species and cultivars. Many produce large, striking, and colorful semi-double and double flowers; others distinguish themselves with small, single, dainty, and charming displays of flowers.

Clematis in the Landscape

Clematis can be grown in many different ways to add to the design, elegance, and beauty of any garden. Typically, they are grown on supports. Many trees, shrubs, roses, and conifers make excellent living supports for clematis. Before planting clematis with such living hosts, however, consider the vigour, flowering times, and pruning requirements of both plants. Some pairings are not advisable: a very vigorous clematis with a compact, less vigorous supporting plant, and vice versa; an evergreen clematis with a shrub or a climber that requires annual pruning; and any clematis with a hedge that needs regular clipping and maintenance. Roses are natural companions of clematis and have similar cultural requirements.

Walls, fences, and various free-standing artificial supports also enable clematis to make their vertical journey with ease and to show off their flowers. Attach plastic-coated wires or wooden trellises to walls and fences so that the modified leaf stalks of clematis can wrap themselves and grow away. When hanging a trellis on a wall, leave at least 2.5 cm (1 in.) between the wall and trellis by attaching blocks of wood to the wall and fastening the trellis to the blocks with rustproof screws. This space ensures enough room for air circulation, thus preventing the plants from being attacked by mildew. Passing showers of rain seldom reach the base of a wall, where the soil can be very dry, so plant clematis at least 30–45 cm (12–18 in.) away from the base and enrich the planting hole with organic matter.

Many clematis may also be grown as groundcover plants or trained along the ground. Plant these in the usual way and route the stems horizontally. Use hairpins made from sturdy wire to hold the stems in place on the ground. Strong-growing clematis, for example, Clematis montana and its early-flowering cultivars, and the late-flowering hybrid C. 'Praecox', may also be grown as groundcover plants, provided adequate garden space is available.

The not-too-vigorous and compact species and cultivars are excellent container plants. Although many clematis come with labels that read "Suitable for containers," gardeners should beware.

Planting Clematis in the Garden

When purchasing a clematis, select a good, strong-growing, healthy plant. Look for two- or three-year-old bushy plants well established in good-sized pots, preferably about 2 litres (approximately 1⁄2 gallon), with two or three strong basal stems. The more basal stems per plant, the quicker it is to establish a handsome framework. Potted clematis can be planted any time of year. Early to midautumn is ideal. The upper layers of soil are usually warm and moist, and enable the roots to grow quickly and establish themselves before the onset of winter. The next ideal season in which to plant clematis, especially the tender or evergreen varieties, is early to midspring. As soil and air temperatures begin to rise, new plants establish quickly.

Evergreen and tender varieties of clematis need a warm, sheltered position in the garden. Some forms can only be grown under glass. Pale-colored flowers tend to be bleached in strong sunlight and are best suited for growing in shade or part shade. Large-flowered clematis require protection from wind damage, and scented clematis perform better in sunny sites, although they demand a regular water supply.

Clematis thrive in fertile, loamy soil with balanced nutrients and adequate moisture. To improve clay soil, add coarse horticultural grit or sharp sand. To improve sandy soil, add humus (organic matter). Well-rotted farmyard or horse manure, garden compost, leaf-mould, mushroom compost, or good-quality, proprietary, soil-based potting compost are excellent materials for enriching the soil.

Although clematis are known to thrive in alkaline soils (pH value more than 7), they also grow satisfactorily in neutral to acid soil (pH value of 7 or below). To increase soil alkalinity, add lime (calcium carbonate) to the soil far in advance of planting. Do not add lime with farmyard manure, as it reacts with the manure to release ammonia, which may damage the plants.

When planting a clematis in the garden, dig a hole at least twice as wide as the pot in which the plant is growing and at least twice as deep again. A hole 45 by 45 cm (18 by 18 in.) or larger is ideal for accommodating organic matter below and around the root ball, giving the clematis a good start for healthy growth. Loosen the base and sides of the hole gently with a garden fork. If the soil is heavy clay, place some coarse grit or sharp sand mixed with the soil at the bottom of the hole before placing any organic material in it. This improves drainage and prevents water-logging. Place some well-rotted manure, leaf mould, or compost at the base of the planting hole to a depth of at least 10 cm (4 in.). To prevent the plant roots from becoming burned or damaged through direct contact with the manure or compost, cover the manure or compost with top soil and peat or peat substitute.

Immerse the container in which the plant is growing in a bucket of water for 10 to 15 minutes to thoroughly wet the compost and allow the roots to absorb water. Ease the plant with its cane support out of the container and gently loosen the roots at the bottom of the root ball to encourage quick growth into the surrounding soil. Place the root ball in the prepared planting hole.

If the plant produces large flowers, ensure that the surface of the root ball is at least 6 cm (2.25 in.) below the rim of the hole. Deep planting encourages large-flowered cultivars to develop a healthy basal root crown of buds below the soil level as a precaution against sudden wilting. Such planting is unnecessary for Clematis alpina, C. tangutica, C. viticella, and their cultivars because they generally do not succumb to wilt. Similarly, evergreen and herbaceous species such as C. armandii, C. cirrhosa, C. heracleifolia, C. integrifolia, and C. recta should be planted with the crown of the plant level with the soil.

When the plant is in position, fill the area around the root ball with equal parts of good soil and potting compost mixed with the recommended amount of any general-purpose fertilizer. Gently firm the mixture around the root ball. Cover the base of the plant with additional organic matter, taking care to keep it away from the stems or vines. Water the plant well, allowing at least 4 litres (about 1 gallon) of water per plant. Finally, attach a permanent label to the plant with the name of the clematis, pruning group, and date of planting.

To provide some shade for the plant's root system and to prevent excessive loss of moisture, plant a low-growing perennial or shrub close to the newly planted clematis. Avoid using slates, slabs, or tiles to shade the clematis roots as these also provide hiding places for slugs, snails, woodlice, and other insect pests.

Pruning

The main reasons for pruning clematis are to establish a neat and tidy framework, to encourage vigorous growth, and to stimulate the development of buds and flowers. Most newly planted clematis need to be pruned back to at least 30 cm (12 in.) from the ground in the spring following the initial planting. This pruning encourages plants to produce new shoots from lower down the stems and from below the soil surface; however, certain evergreen and tender varieties, such as Clematis armandii, C. forsteri, C. paniculata, and their cultivars, should not be severely pruned provided the plants are strong, bushy, and healthy.

In areas with harsh winters, especially when snow cover is absent and temperatures fluctuate unpredictably, pruning is best done in midspring. Gardeners in coastal and temperate regions can begin pruning in midwinter or even late autumn.

Established clematis plants fall into three major pruning categories. Whether plants are grown in a container or in the garden, the pruning procedures are the same.

Pruning Group 1

Evergreen and deciduous clematis that flower on old ripened wood during winter or early to late spring do not require any major pruning. A simple rule for these is this: If a clematis flowers before early summer, do not prune it. All winter and spring-flowering Clematis species and their cultivars need little or no pruning, including C. alpina, C. armandii, C. cirrhosa, C. forsteri, C. macropetala, C. napaulensis, and C. paniculata. If, however, the plants have outgrown their allocated space or become overgrown and untidy, a certain amount of cutting back and tidying up may be undertaken after the flowering period has ended. Ideally, pruning to maintain a plant's handsome framework should be done annually.

Pruning Group 2

Some clematis produce two flushes of flowers. The first display appears before early summer on old growths made during the previous year(s); the second display appears during late summer on new growths made in the current year. Most clematis with double or semi-double flowers belong in this group. These plants do not require major pruning, but all dead and weak stems should be removed in late spring. If a certain amount of selective pruning is necessary, it may be undertaken immediately after the early flowering period is over, starting from the top of the plant and working downwards. It is also desirable to prune back the flowered shoots to encourage a second display of flowers. The general rule for pruning clematis in group 2 is this: To avoid loss of early flowers, do not indulge in large-scale pruning of old wood made during the previous season(s).

Pruning Group 3

Clematis species and cultivars that flower on the current year's new growth after early summer are commonly referred to as midsummer- to late-summer-flowering clematis. These plants need annual pruning in late winter or early spring, or even later in the season, depending on when spring arrives. Among this group are Clematis integrifolia, C. tangutica, C. viorna, and C. viticella, as well as some of the large-flowering cultivars. These clematis must be pruned very hard.

A rapid and easy method of pruning these clematis is to start at the base of the stems and work upwards to the first pair of healthy, plump buds. Prune the stems just above these buds, and remove all old growths above the cuts. At times such buds may not be easily visible, so look for the nodal points of the leaves and cut just above those to remove all old growths. Such severe pruning encourages plants to produce strong new shoots and to flower very well. The general rule for pruning clematis in group 3 to cut back all the old stems to the lowest pair of live buds.

Partial and Optional Prunings

Certain well-established Clematis species and cultivars, including C. 'Helios' (Aztek), C. 'Huldine', C. 'Kugotia' (Golden Tiara), and C. tangutica, may be partially pruned to produce a continual display of flowers. To achieve this, hard prune only half of the old stems or vines (from the previous years' growths), allowing the unpruned half to come into flower early in the season. When the old growths have stopped flowering, new growths will come into flower, thus ensuring a continuous display of flowers on a single plant.

Nursery catalogues and plant labels often indicate that pruning of a particular clematis may be optional or done as for pruning groups 2 or 3. Examples of such plants are Clematis 'Carnaby', C. 'Ernest Markham', C. 'Huldine', and C. 'Lady Betty Balfour', which flower on both old and new wood. If these clematis are not pruned or are only lightly pruned, they will produce early flowers on old wood. If, however, they are pruned very hard, the flowering period will be delayed by at least six weeks, and the plants will remain somewhat compact.

Pruning Herbaceous, Semiherbaceous, and Woody Clematis

Herbaceous clematis, such as Clematis aromatica, C. heracleifolia, and C. integrifolia, can be treated just like other herbaceous perennials in the garden. With the onset of very cold winter, almost all the soft top-growth of these clematis dies back to ground level. Where winters are mild, the process of dying back may not be complete. In this case, cut the top-growth down to ground level towards the end of winter or beginning of spring. Semi-herbaceous and woody subshrubs, such as C. durandii and C. recta, can be pruned just like herbaceous clematis by cutting the old semi-woody and woody top-growth to ground level towards the end of winter or beginning of spring.

Pruning Companion Clematis

Clematis grown with shrubs, trees, roses, conifers, heathers, and other climbers should not be hard pruned if they belong to pruning group 1 or 2. If they belong to pruning group 3, however, they can be pruned in two easy steps. Cut away all the top-growth of deciduous climbing clematis after the leaves have fallen, leaving about 2 m (6.5 ft.) of stems so that the natural supports can come into their own during winter. Complete the final pruning in early spring.

Pruning Unidentified Clematis

Occasionally, the identity of a clematis plant is unknown, making it difficult to determine its pruning category. The label may have been lost or perhaps the plant is an established one in a recently acquired garden. Where the name of the plant is unknown, especially in an old garden, do not rush the pruning. Seek help from an experienced clematis gardener or assign a number to each plant and observe its flowering pattern over 12 months, recording the details of the flowering period. Using the pruning guide in this chapter, assign the plants to their respective pruning groups and maintain them accordingly.