Great article from Garden Forum on pruning Clematis.
Pruning is one garden job that confuses many gardeners. And pruning clematis/how to prune clematis is certainly one area that has heads scratching all over the world.
You don't have to prune clematis. Pruning can be restricted to: 1) the plants are out-growing their allotted space; 2) the flowers are produced too high up to enjoy.
Pruning in the latter case can even be avoided by retraining and tying shoots lower down on the support - say just above ground level. These shoots produce their flowers much lower down.
Training shoots horizontally not only keeps the flowers within eye level, but provides better coverage of the support, and the reduced flow of sap encourages even more flowers.
Clematis can be divided into three groups when it comes to the subject of pruning, depending on when they flower.
Early-flowering species clematis (such as Clematis montana, C. macropetala and C. alpina),
These plants flower on growth produced the previous year and, if needed, are just given a light pruning to remove spent flowers immediately after flowering.
Old, woody plants can be pruned very hard - down to a couple of feet if necessary, preferably to just above some strong, healthy growth - after flowering to remove lots of the old, non-flowering growth. Plants may take a year or so to fully recover and start flowering properly again. Old plants may not recover.
Large-flowered hybrids that start flowering in May/June.
These cultivars can be cut back fairly hard - say to between 90cm to 1.2m (3-4ft) high - to just above a fat pair of buds/single bud; the former is better.
Start pruning at the top of the plant and work your way down.
Pruning is carried out at the end of February/early March.Group 3
These can be cut back very hard - say to 23-45cm (9-18in) high - again just above a pair of buds/single bud - removing nearly all of the previous year's growth.Start pruning at the bottom of the plant.
Pruning is carried out at the end of February/early March.Even though some years can encourage a lot of early growth on clematis, don't be tempted to prune Group 2 and Group 3 cultivars any earlier, as frosts can still kill this early growth leaving none to grow away on hard-pruned plants.
If you want to retain some tall growth on plants to retain cover of the supporting structure, you could cut down just half the shoots as described above. This will give slightly earlier but higher flowers on the un-pruned growth. The pruned shoots will flower a little later and produce flowers lower down the plant. This gives a better and slightly longer display.
As with any plants, pruning should be followed by a good feed with a granular fertiliser. Rose food is excellent for this.