I have a worn-out clematis that I would like to repace which is planted beneath an obelisk. I know from experience that there will be a large amount of root growth so what's the best way to tackle this?
Prune it, feed it and revive it? What kind is it?
Failing that you're going to have to dig out all that root and replenish the soil then plant something new.
Just dig all the root out, then fill the hole with water and let it drain away overnight, backfill with some multi purpose compost and plant your new clematis. The planting depth will depend on which variety you choose.
Variety is Doctor Ruppel. Suggestions please - Is it too late for pruning now? What would be the best thing to feed and revive it?
I think Dr Ruppel is worth trying to save. - http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=143
He's a group 2 for pruning but I always did him as a group 3 in my last garden as the top growth always died back in harsh winters. Cut him back low to about 9" or to the lowest pair of buds if he has any, then pour on a generous drink of liquid tomato food and a generous handful of slow release specialist clematis food or else rose or tomato food if you can't find that.
Give him a couple of months at least to recover and re-grow or all summer if possible.
Or just do as Richard advises and get a new one but do look after it well with pruning and feeding.
Excellent advice from obelixx as usual, if you want to save the plant, well worth the effort.
But, the Group 2's are difficult and Dr Ruppel is rather old hat. There are much better clematis, easier, longer flowering, than good old Dr Ruppel.