Prepare the site by digging a hole 18" square and at least 18" deep (this is also the minimum diameter of a pot/container). Break up the soil at the base of the hole and dig in peat or garden compost mixed in with a few handfuls of bonemeal. Soak your plant in a bucket of water for twenty minutes then remove the pot. Remove the lower leaves of the plant so the 1st 5 inches are bare stems then gently loosen a few of the roots (take care not to damage) place your plant in the hole so that the top of the rootball is about 5" below ground level, then backfill with a mixture of John Innes number 3 and multi purpose compost 50/50. Clematis like nice well drained positions not Heavy clay (their roots will rot off if sat in water) Do not use manure. As this can also rot the roots.
Clematis Need a cool Root Run
This is one of the most important parts of the planting process. Make sure that you cover the surface after planting with something like Stone, Slate or Woodchips etc. This is to stop the roots from getting hot through the summer. If you don't want to use things like this then you could grow some perennials around the base (that do not have invasive roots) to do the same 'shading' job.
Leave the lower 'ties' still attached to the cane (these will hold the base of the plant secure whilst the top half gets a grip to your clinbling frame) these lower ties can be released in 4-6 weeks if you wish, but still leave the cane in until the clematis is fully established. If your soil dries out quickly or your plant is in a position where it receives very little water, place an open ended pipe (About 1 1/2-2" dia.) in the planting hole, one end close to the rootball the other just above ground level. By adopting this procedure water can be conveyed to the plants roots where it is most needed in times of drought. Top dress with a slow release fertiliser eg. Osmocote. Always remember that Clematis are planted for the following season ( don't expect too much of it the same year ) cutting single stemmed plants back half way shortly after planting will encourage new fresh growth and will make for a bushier stronger plant the next season.
You don't necessarily need a wall to grow Clematis against, they are equally at home when planted amongst trees and shrubs. The montanas lend themselves very well to growing through trees - a word of advice here though if you plant montanas through apple trees you might not see the clematis flowers for apple blossom. Clematis that are allowed to scramble through shrubs and small growing conifers provide much interest - Roses and Clematis when grown together make an excellent combination. Planted into containers Clematis will enhance any patio. Herbaceous types deserve more recognition and look excellent in perennial borders. We have even got some varieties that can now be planted in window boxes and hanging baskets! they are so compact!
When growing on walls or fences, make sure that you have plenty of support for them vertically and horizontally about 4 inches to 6 inches apart in the formation a bit like a 'hash tag' ####
Leave our cane in the ground for support, until the clematis gets a good grip on whatever you choose to climb it through (maybe a year or so)
Good feeding is important. In addition to slow release fertilisers & In order to produce good flowers and good growth the plants should be fed by a good high Potash based, branded liquid fertilizer following the manufacturer's recommendations- twice a week. Start feeding early spring and continue until just before flowering. Do not feed during flowering as this will tend to reduce the flowering period. Tomato feed is fine & should give good blooms. Feed again after flowering until mid autumn to encourage further growth and more flowers.