Posted: 16/04/2015 at 10:51
Clematis are very hungry plants so I suggest you go and buy some proper clematis food and give it a generous top dressing of this every spring as well as regular liquid feeds of tomato or rose fertiliser between waterings from March up to mid July.
If it flowers in May/June it is a group 2 and you can trim it back after the first flush of flowers finishes, dead heading and pruning back to a pair of buds. This will encourage fresh new growth that should flower at the end of summer.
You can also cut one or two stems at the base and let them wilt before pulling them out. This will encourage new shoots to grow - as long as you're feeding it well - and provide flowers lower down the following year. Doing this each year will keep your plant invigorated for better flowering.
If it flowers in July/August and later then it's a group 3 and all its stems ca be cut back to the lowest pair of buds in March every year when frost is not forecast for the following couple of days.
If it flowers in April, it is most likely to be a Group 1 which just need pruning back to maintain form and size after flowering finishes and you can cut out one or two stems as explained above to re-invigorate it.