Posted: 16/04/2015 at 22:00
I would dig them out and plant them in deep pots so their roots can run. If they're big enough, plant them a couple of inches deeper than they were as this encourages the formation of extra shoots which will bear more flowers. Grow them on till they're big enough to cope in the hurly burly of life in the border with competition, wind, rain and so on.
Use the best quality compost and add some proprietary clematis feed as these plants are very hungry and need lots of building up to get the best flowering display. Keep them watered but not drowning and give them occasional liquid feeds of tomato or rose food.
When you do finally plant them in the border, bury them a few inches deeper than they were in the pot to encourage more new shoots - also helps protect against clematis wilt . They can take a year or two to get going once planted out and will need annual spring top ups of clematis food but are worth the wait and the bother. Use organic slug pellets to protect the juicy new shoots in spring as slugs love them.