This subject has been surrounded with instructions that you have to do if you want to keep your Clematis under control and flowering from year to year. Here I intend to break down the instructions to a few easily remembered rules.
The first thing to think about is when does your Clematis flower, this then determines when you can prune the plant. When you look at plant labels on Clematis in garden centres they mention a pruning group, these are:
Group 1: These plants flower early in the year (spring) on shoots produced in the previous summer. Typical in this group are Clematis alpine, C. armandii, C. cirrhosa, C. macropetala and C. montana.
For this group no regular pruning is required, but if necessary, prune immediately after flowering in mid to late spring when the danger of frost has passed. To keep the plant flowering on the plant at eye level you will need to prune the top back to the height required or the flowers will be produced where you cannot see them up the top of the plant. I suggest that you cut back a third of the stems once every two or three years. Always cut back to pair of healthy buds so that the plant can grow new shoots from this cut.
Group 2: This group are the large-flowered hybrids that flower in May to June (Summer). Typical in this group are: Clematis florida var. sieboldiana, Barabara Jackman, Beauty of Worcester, Dr Ruppel, Jackmanii Alba, Jackmanii Rubra and Royal Velvet.
This group should be pruned in late Winter (February) or early Spring and after the first flush of flowers in Summer, this Summer light prune is to encourage a second flush of flowers in the late Summer early Autumn. The aim with pruning this group is to retain a framework of old wood and also to stimulate new shoots in order to maximise flowering throughout the season.
Group 3: The clematis in this group flower in late summer on growth made that season. Typical in this group are: Clematis viticella, C. x triternata and the following hybrids: Abundance, Etiole Violette, Duchess of Albany, Gravetye Beauty and Royal Velours.
This is the easiest group to prune, you cut back in late Winter or early Spring when buds show signs of life. To prune you cut back to about 1ft (30cm) above ground level, if you do not prune back the new growth and flowers will start from where the old growth stopped last year and soon the plant will be flowering out of site and all you will see is bare stems.
At the start of this section I mentioned that I would give simple rules to remind us when to prune, so all the above can be broken down to these 3 rules:
Spring flowering Clematis you do not prune regularly but prune only when necessary to keep the plant under control.
Summer large flowering Clematis is pruned in late Winter or early Spring and again if you want a second flush of flowers (in the late Summer or early Autumn) you can light prune after flowering in Summer.
Late Summer flowering Clematis you cut back hard in late Winter or early Spring.
Lastly, if you like Clematis flowers but do not want a climber there are a little known group of herbaceous Clematis plants that behave the same as all border herbaceous plants in that they die back in Winter and sprout again from the crown each Spring. If you want to know more about this group then do not hesitate to contact me and I will give you more details.