Late summer to early autumn is the best time for container planting as the clematis will start to establish itself during mid- to late autumn and be in a position to produce some flowers the following summer. The choice of container is vital for the successful cultivation of clematis. Although clematis need a moist, cool root run they do not like cold wet feet in winter, so a free-draining compost is important. This rules out the use of thin plastic containers as in the summer the soil in the container would become far too hot and in the winter no protection would be given to the thick fleshy roots of the large-flowered clematis, so they would freeze and then decay. The ideal container is one made out of stone, or reconstituted stone, with a 5 cm (2 in) side wall, or an old cider or beer barrel made out of thick wood. Specially made wooden boxes that have been treated against rotting are also ideal, particularly if they have an inner skin, maybe of tin, to help preserve the wood. Thick-walled frost-resistant terracotta pots and urns and large strawberry pots look most effective with a healthy clematis growing out of them, but terracotta does not give the same amount of protection against the sun in summer or the frost in winter.
The size of the container is also important. A container with a diameter and depth of about 38cm (15 in) is the smallest size that should be used, and this will only suffice for about two years before the clematis will need to be upgraded to a larger container. Moving a clematis up in such a way involves extra work, but it is worth doing as clematis dislike being put in too large a pot at the outset. However, the larger the better because it is then possible to plant other shallow rooting plants within the container to help keep the clematis root system cool in summer - 60 x 60 x 60 cm (2 x 2 x 2 ft) is ideal.
The container must have adequate drainage holes. These depend upon the size of the container. A container measuring 45 x 45 cm (18x 18 in) should have at least three drainage holes measuring 2.5 cm (1 in) across. To ensure the drainage is effective, raise the container off the ground by placing bricks beneath it.
Before placing the compost in the container, put a layer of broken crocks, stones or pebbles over the drainage holes to stop the compost from clogging up the holes. Lay large pieces first and then place smaller stones, large pebbles and then small pebbles or pea-gravel on top of these. If available, rotted turf can be placed upside down on top of the small pebbles or pea-gravel. If not, well-rotted farmyard manure or garden compost can be used in its place but these materials must be kept away from the newly planted clematis roots. Then fill the container with John Innes Potting Compost No.2, or an equivalent. A loam based compost is best because loam-free composts sometimes dry out too quickly and can then be very difficult to re-wet even if they do have a polymer mixed with the compost. Once the compost has been placed into the container, firm it well to avoid sinkage -it should be 5 cm (2 in) below the rim of the container, allowing sufficient depth for watering.
Use a small garden trowel to dig out enough compost to make room for the clematis root ball. Planting in a container should be done under the same guidelines as planting into the garden soil, which is to say drench the root ball before planting, plant an extra 5 cm (2 in) deep and water well after planting. The aftercare for a clematis in a container is of course much more than that required for one in open ground as it will require extra training, tying and feeding and attention to watering is a must, especially in dry weather.
A clematis growing in a container obviously needs some support. The type of support very much depends upon where the container is placed. If it is alongside a wall so that the clematis will grow up and through other wall-trained shrubs, a strong bamboo cane or thick stake can be placed at an angle between the container and the wall or limb of the host tree or shrub. The clematis stems may then be tied to the cane or stake and up into the host plant.
If the container is to stand in the open, some form of more elegant support will be required -how elegant depends upon the location in the garden. A simple wigwam made out of thick bamboo, pea-sticks or thick hazel poles is an inexpensive solution. Alternatively, there are attractive iron or plastic-covered steel supports available. These are freestanding and can be placed securely within the container.
The training of a clematis in a container is important, the aim being to produce a bushy, well-furnished plant with plenty of foliage and flowers at the base of the support as well as up it, rather than a bunch of foliage and flowers at the top of the support as is so often seen. After planting, train the stems horizontally around the support as low down as possible. The aim is to produce a well furnished plant and build up the framework of stems for future years. If the clematis has been planted during the late summer or early autumn, any dead or weak flower stems should be removed to a pair of strong leaf axil buds during the following late winter/early spring. Train in the remaining stems. If these are weak or thin, they too should be reduced. As new growth appears during mid- and late spring, this too should be tied in horizontally to help cover the base of the support as well as the top.
If the planting time is mid- to late spring and the clematis has not been pruned before it was purchased from the nursery or garden centre, remove any weak stems and possibly reduce others to a pair of strong leaf axil buds. As new growth appears, gently pinch the top growth with thumb and finger after two or three nodes have been produced on each stem to encourage new growth from the leaf axil buds, which will produce a bushy plant. The best clematis for growing in containers are the small-flowered mid-spring flowering types and the late spring and early summer-flowering large-flowered clematis. These clematis produce their main crop of flowers from the growth ripened the previous season and this pruning and pinching back means that very little flower can be expected during the first season. However, some flowers can be expected later during the summer months and as these clematis are more compact in their habit and will produce more flowers in a given space than other types it is worth being patient.
Adding other plants to the containers is advisable, partly to provide shade for the clematis root system but also to give extra foliage, flowers and form. If such plants are added, they should be of a shallow rooting habit and, if possible, be allowed to flop over the sides of the container, giving a natural effect. Summer bedding plants are an easy choice in the summer; in the winter some carefully selected winter-flowering heathers would be ideal. A few spring-flowering bulbs, for example tulips, daffodils or shorter growing bulbs such as Iris reticulata will give added interest and color, perhaps with a selection of wallflowers and forget-me-nots. The bulbs will die down once they have finished flowering, and the other spring-flowering plants can be removed to allow replanting with summer bedding plants.
When growing clematis in containers you must consider the type of clematis and the winter protection it will need in your area. In mild locations, with night temperatures dropping only to about -7°C (19°F), the top growth of the small-flowered mid-spring flowering types and the late spring and early summer-flowering large-flowered clematis will not be damaged or destroyed. It is vital to retain the top growth in order to obtain the early large flowers of the large-flowered cultivars. If the night temperatures drop below those figures for two or three weeks at a time bud damage may occur, depending upon the wind chill factor and the amount of desiccation caused by the winds. The very hardy C. alpina and C. macropetala types will withstand open garden temperatures as low as -35°C (-31°F) and still flower the following year, so they will only need winter protection in very exposed cold climates. For safety, a fleece material can be wrapped around the top growth to give added protection. This can also be used in cold locations with the large-flowered cultivars, but only as a short-term protection; if the winter is expected to be severe, any large-flowered cultivars should be removed to a shed, outbuilding or garage where the temperatures are not expected to drop below -7°C (19°F) for any length of time. The fleece may be removed from outdoor clematis as soon as the weather improves and when spring has almost arrived -before the weather becomes too warm, as the extra protection may cause the plant to come into growth too early and the growth will be damaged when the fleece is removed.
Clematis containers wintered inside a building may be brought out once the worst of the cold weather is over. Again, it is important that the plants are not forced into growth prematurely by rising indoor temperatures. Any plants which have been over-wintered under cover must be well watered as soon as you have placed them in their spring and summer locations, choosing a mild day to do so. Early-morning watering is preferable so that the compost within the container can drain well before nightfall.
A clematis in its first year is usually planted in an 8 cm (3 in) pot, and in the second year should be moved to a 15 cm (6in) pot; later still, to a 30cm (12in) or larger pot. In small containers the soil should be changed once a year, either in late autumn or spring; the soil below, at the sides of the plant, and above should be replaced. If necessary the clematis can be moved to a larger pot at the same time. Clematis in large containers need not have their soil changed as often. However, the top 5-8cm (2-3in) should be replaced with compost and bone meal or slow release fertilizer once a year; every few years it may be necessary to change all the soil in a large container. If the containers are outside they may need a measure of protection in the winter by a mulch of leaf-mould or bracken, and if possible moved into a sheltered part of the garden. In severely cold areas of the world the tubs may need to be taken indoors in the winter; they should not be allowed to dry out. In the garden they should be stored on shingle.
Clematis under glass can be grown in containers or in permanent beds. The first value of glass is that it allows any clematis in a pot to be forced, that is to say, flower earlier. In this way it is possible to have early flowers -at least a month earlier - and therefore it is particularly useful with early-flowering hybrids. Secondly, growing under glass lends itself to growing clematis which are too tender for growing in the garden. It thus increases the range of clematis which can be grown. Thirdly, and this is probably the best use of glass, clematis can be brought into a conservatory in a sequence.
To make the third method possible there has to be a nursery bed in the garden. Here the plants are grown in pots, with flowering periods throughout the year. As each approaches the time of flowering, it is brought into the conservatory. Once the flowering is over, it returns to the nursery bed where it is nurtured for flowering the following year. While the nursery bed may be in a secluded part of the garden, it must be in full sun so that healthy, strong plants are produced. The nursery bed can have a floor of thick black polythene supported by a frame of bricks or breezeblocks. Size would depend on the number of plants to be grown. A layer of peat 23 cm (9 in) deep is put on the black polythene and the pots are placed in this. The peat should always be kept moist, and throughout the year the pots must be treated as for container-grown plants.
Almost any clematis can pass through the conservatory by the method mentioned above, but particular attention will be paid to the early tender evergreen clematis species, such as C. indivisa, C. cirrhosa balearica, and C. armandii. Other tender clematis include C. afoliata, up to 2m (6ft) and with a daphne scent; C x vedrariensis, up to 6m (20ft), which flowers in May; and C. florida 'Sieboldiana' which may grow up to 2.5m (8ft).
Also early in the season you can have the alpina clematis and macropetala clematis. Later there is a choice of spring-, summer- and autumn-flowering hybrids, the viticella varieties, orientalis varieties, as well as the texensis varieties. Late in the year, and where there is a great deal of room, it is possible to grow C. napaulensis, up to 5 m (30 ft).
In the conservatory, care must be taken to supply ventilation, the temperature should go no higher than 13°C (55°F) and humidity should be encouraged by syringing. Water given to clematis should be tepid, or the same temperature as the conservatory. The clematis should be regularly sprayed with insecticides and fungicides.