Posted: 03/10/2017 at 11:07
If you want twice a year it needs pruning in between.
If you go for a viticella or other group 3 you'll get flowers most of the summer and can just prune it all to the base every March or even late autumn if you want to remove the dead and dormant stems once the leaves drop.
i suggest Huldine which, tho it may take a year or two to get its feet down and establish, will be vigorous enough to grow thru or over tall yews and the flowers - white with a vein of purple on teh back - will be perfect for your colour scheme.
You'll need to make it a decent, deep planting hole and enrich te soil with plenty r moisture retentive well-rotted manure and garden compost and then water and feed it well to establish it. Bury it a few inches lower than it was in its pot and tease out its roots gently to encourage them to seek nutrients and extra flwoering stems to grow. Feed generously with clematis food every spring and give occasional liquid feeds of rose or tomato food through till mid summer.
Clematis are hungry, thirsty plants so make sure there is some space between them and the yews to reduce competition form the yew.