Growing in containers

The container

Give clematis a deep root run by planting in containers which are at least 46cm in diameter and depth. Plants from garden centres are generally in 2-litre pots which are about 19cm deep. Plants will not thrive unless they have a cool root run so buy the thickest-sided container your purse will allow. If it only extends to a plastic one, line the inside with bubble wrap or polystyrene to insulate the soil inside. In a south-facing position the sides of the container will get quite warm and the root system will not give of its best, hence the insulation.

Make sure there is adequate drainage: clematis like moisture but don’t like sitting in water. Raise the container above ground level on terracotta feet or bricks so that when watered, the water comes out of the bottom. If necessary, put broken crocks or large gravel in the bottom so that the drainage holes remain open.

The compost

Any brand of compost will do although there is a move away from the peat-based. John Innes No. 3 is good but added grit will create a more open structure for drainage. Being heavy, it helps keep the pot stable in windy conditions. To prevent moisture loss put a layer of grit or small stones on the surface after planting.

Selecting clematis for containers

Generally, lower-growing plants will fare better in containers as very vigorous types will exhaust nutrients and moisture more quickly and will require more TLC. Check the label to see what height the plant is likely to reach and whether the support you will be using is adequate. Some varieties of clematis montana will reach 6m or more whilst herbaceous varieties such as clematis heracleifolia will only reach 1m. Some half-hardy types are ideal for containers as they can be moved to frost-free places, such as the garage or conservatory, in winter.

In the garden centre you may find very small clematis plants for sale at very attractive prices. However, they are babies and should be treated as such. They are not big enough to plant into the ground or into a large container. Pot them on into a 2-litre pot and rear them to adulthood, planting them out the following year.

Planting

The planting technique is similar to that of planting in the ground. Put a small amount of compost into the container and mix in some bone meal or blood, fish and bone.

Tip the plant out of its pot (hopefully it has not just been re-potted as it will break up and cover your shoes), tease out the root system at the base and sides if it is packed closely, place in the pot and back-fill to within 5cm of the top rim.
Once planted, if in the spring, prune the whole plant by half just above a leaf joint. This will encourage more stems to form. To make the plant more compact and to produce more stems, pinch out the growing shoots of each vine at least once when new growth reaches about 46cm. If the plant is a climber it will use its leaf stalks to hang on to its support until it reaches its stated height.

Some varieties do not cling to their support. These will require tying in or be allowed to flop. Clematis Durandii and Clematis Alionushka are examples of this group.

Herbaceous varieties such as tubulosa and heracleifolia are self-standing and should not require any support as they make thick woody stems.

Feeding

Large-flowered clematis are greedy feeders. During the growing season, spring to late summer, a high-potash liquid fertiliser delivered at half-strength fortnightly will be beneficial, helping to ripen the flowering wood and giving good flower colour.

Smaller-flowered clematis need less feeding.

In late autumn some bone meal and a thin layer of mulch will replenish the root system for the following year.

Pruning

When first planted, reduce the plant by half its height. When some growth has been made in late spring, pinch out the growing shoots above a leaf joint, making each vine divide into two.

For established early large-flowered clematis, go to the top of the plant in February to March, and prune downwards any dead growth until live buds are met. Stop here and leave the main body of the plant as it is. This will be the area where flowers will be formed. After first flowering, dead-head old flowers to encourage a second flush in late summer.

Plants that flower later in the season, June or July depending on the situation and location, need pruning hard at the same time of the year as early large-flowered clematis. Come up from the bottom of the plant and prune above the first leaf joint. Take all the top away every year. Flowers will be made on the new vines emanating from where it was pruned.

Herbaceous plants can be pruned in late November or into December when herbaceous beds are pruned and cleared. Prune down to about 15cm from ground level.