Posted: 20/06/2016 at 17:45
I buy my clematis from a specialist who sells them in 9"/23cm deep pots and with about 3'/90cms of growth above. Even so, I pot them on into bigger pots and grow them on for a season before planting them out in the borders as this gives them the chance to grow a stronger root system and thus have a better start. I find clems take a season or 3 to get established and really start growing above ground and this system cuts that time to 2 years.
If yours are very small they are all better off going into bigger pots now and again at the end of summer as their roots will continue to develop over winter. Each time you pot them on, bury them an inch or two deeper in the pots as this encourages extra shoots to grow and thus more flowers later on.
Once they are fully established in 9"/23cm deep pots they should be big enough to bury 4" deeper in the border or in a 60cm/2' pot. Clematis are very hungry, thirsty plants so paint the inside of your terracotta pot with PVA glue or acrylic varnish to make it less porous. Give them plenty of slow release food every spring and never let them get thirsty and they'll repay you with glorious growth and blooms.