Planting Clematis plants in your containers

• Select a container that is at least 18 in. deep and wide (45 cm) and has good drainage holes. If you opt for a large or vigorous clematis that grows over 8 ft. tall (2 m), select a larger container.
• Be mindful of the material your container is made of. Ceramic or terra cotta containers do not stand freezing temperatures without cracking. Resin containers may not provide sufficient insulation to protect the roots from the summer heat. Stone containers are heavy and cannot be easily moved around. Wood containers are a good option in terms of weight, drainage, or durability.
• Place a layer of coarse gravel or pebbles in the bottom of the container, to add extra drainage.
• Add some organic compost or composted manure into the bottom, along with a handful of bone meal or organic rose and flower food.
• Fill the rest of the container with a loam-based potting compost. 
• If you're planting a bare-root clematis, soak its root ball in water for 10 or 20 minutes. Your new clematis needs to be well moistened at planting time.
• If you are planting a large-flowered clematis, bury its base such as the lowest nodes are buried by a couple of inches of soil, or mulch them afterward. Plant any other clematis vines at the same level in the ground as they were in the pot in which you bought them. 
• Water thoroughly after planting.

Caring for Clematis plants in your containers

• Make sure you water regularly during the growing season and apply a general-purpose liquid fertilizer monthly during spring and summer. Avoid using fertilizers high in nitrogen, because these may reduce flower growth and encourage foliage growth. A tomato or rose fertilizer is suitable.
• Replace the top 1–2 in. (2.5-5 cm) layer of compost each spring with fresh potting compost.
• The roots of clematis in containers should be protected both from freezing in winter and baking in summer. Use bubble polythene for winter insulation and shade the pot in summer, perhaps by placing pots of taller plants on the sunward side.
•  Concerning pruning, Clematis fall into three categories:
  Group 1 - Early-flowering clematis which flower on shoots produced in the previous summer: No regular pruning required. Just clean them up after flowering.
  Group 2 - Large-flowering Clematis which flower on short shoots developing from the previous year's growth: Prune in late winter or early spring and after the first flush of flowers in early summer.
  Group 3 - Late-flowering clematis which flower on growth made in the current year: Prune in late winter or early spring and cut back the stems to a pair of strong buds about 1 ft. (30 cm) above ground level before growth begins in early spring.