I have a number of clematis grown in both pots and in the ground. Should I be feeding them, and if so, what do I use?
Yes, they are greedy and need feeding to do well.
You can give them special clematis food available from good garden centres and nurseries. In spring they appreciate a general purpose food such as blood, fish and bone or pelleted chicken manure. You can also give them liquid tonics of rose or tomato food as they have a high potassium ratio which encourages flowering.
How often would you give them tomato food.
In pots, once a week from new spring growth through flowering time. In the ground, it depends on how well they've been planted and in what soil but, after a hard wet winter I'd give a few drinks from spring growth to the start of the flowering period if I think they need a boost.
They sure do need feeding, ithey are one of the few plants for which I buy specialist fertiliser, available from many plant outlets or on line They have benefitted from this a good deal, they are very hungry plants indeed.