You will have to dig a hole not only big enough to accommodate the root ball, but it must also be deep enough to allow one or, better still, two sets of leaf joints to be buried below the surface of the soil. It is believe that this will stop Clematis wilt. Do make sure to fork over the bottom of the hole to loosen the soil. This will ensure the plant is able to send its roots down and helps to ensure good drainage.
As a final preparation before planting give your clematis a thorough watering. If it is very dry it would be a good idea to soak it in a bucket of water for about two hours. Water the plant in well, as this will make sure the soil has settled around the root ball and that there is no air pockets left.
Some people sink a pipe, or perhaps a bottle with the bottom cut out beside the clematis and then water into this so that moisture gets down to the roots. You will find that a newly planted clematis will require fairly regular watering if planted during late spring or summer, especially if the soil is free draining. Make absolutely sure you water a new clematis if there is a prolonged period of drought.
It is very important to feed your Clematis with a liquid feed. Where time is limited you can use a handful of bonemeal worked in around the base of the clematis. This is a slow acting feed and will provide nourishment for your plant over a longer period than liquid food. You can also use Sulphate of Potash which improves the flowers, not only in terms of quantity, but also the size and depth of colour. This can be used in addition to bonemeal but we would suggest about one month later during mid spring. Like bonemeal do water it in if there is no rain forecast. Liquid food should only be applied from late spring onwards, tomato food is excellent. A fortnightly liquid feed from late spring onwards is adequate for most clematis.
We have found one of the best ways of feeding your Clematis is with the amazing Gee Up sterilised horse manure. A good mulch of about 12cm will really help your plant.