Posted: 07/04/2016 at 17:39
I've only ever pruned clematis at the time they need pruning. Some varieties when young can produce a lot of growth at the expense of flowers so you can cut some of that back during the growing season, but I've only done it on reasonably mature plants - ie a couple of years old.
Young plants are largely making roots in the first couple of years, and planting deeper helps to encourage that Jinxy. Once they get to three years old or so, they're producing more growth in general. It does depend on variety though. Some plants get to a decent flowering size quicker than others, and your own soil conditions and management of the plant will also dictate that. Perennials are the same really - they don't generally reach full size for at least a couple of years from being in a small pot.
Plug plants are a long way from being full sized clematis, that's why these offers are cheap. When you buy a clematis in a GC or a nursery at £10/£12 or more, what you're paying for is the time that a nursery has spent on that plant, nurturing it from tiny beginnings to a full sized specimen ready to go.
Essentially - you're doing the nursery's work by potting on and looking after a small plant for 18 months or 2 years, till it's ready to cover a wall, obelisk or fence