Different types of clematis need different treatment for pruning. All the types fall into one of three groups. An easy way to remember is Group A should be pruned after flowering, Group B should be pruned before flowering and Group C needs major cut back.
Group One or Group A
This group comprises the early-flowering species, such as Clematis alpina, C. macro-petuhi and C. montana. and their varieties. Prune them immediately after flowering to remove dead shoots and to keep the plants within their allotted space.
Group Two or Group B
These are the early large-flowered hybrids such as ‘Nelly Moser’ and ‘The President’, together with mid-season large-flowered forms such as ‘Marie Boisselot’. These should have dead or weak shoots cut away in early March. Flowers in these varieties are produced on wood which grew the previous year, so aim to keep as many year-old shoots as you can. Cut out shoots which have flowered after the blooms fade.
Group Three or Group C
These include C. viticella and its varieties, C. x ‘jackrnanii Superba’, and the late-flowering species such as C. rehderiana, C. tangutica, C. campaniflora and C. texensis. These should also be pruned in March, but harder, with the previous year’s growth being taken back to a strong pair of buds just above the base. On these varieties, the flowers are produced on new shoots each year.