Posted: 20/09/2013 at 09:06
I'm not sure hydrangea petiolaris will cope with windy exposure as it's a woodland plant from Japan and Siberia, so, while it can cope with cold, I suspect strong winds in spring will damage new young shoots.
On the other hand, it will cope very well with the cool, shady side of a north east facing wall so if you can give it plenty of organic matter to retain moisture and stop it drying out it is probably worth a try.
Other than that, I would suggest a group 3 clematis which is pruned back hard in March and then flowers in summer on new season's growth. Have a look at http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=30 which has small flowers less likely to be damaged by wind but with good colour.
Another to consider is http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=365 or maybe http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=398 or http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=550 or another small flowerd one with a long flowering period - http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=3206
All are suitable for semi shade but be aware that clematis can take a couple of years to settle in before they really perform well. They are also very hungry, thirsty plants so need a deep hole back-filled with plenty of organic matter to feed them and retain moisture for their thick, fleshy roots.
Clematis need to be planted 4 to 6" deeper than they were in their pot to encourage more shoots to from and protect against clematis wilt. Make the hole at least 2 feet away from the base of the wall so they don't sit in a rain shadow or dry spot and feed anually with a good mulch of garden compost in spring and a generous handful of clematis offd at pruning time. A liquid feed of rose or tomato food every couple of weeks from pruning to flowering time will help too and make sure you have wires or a trellis on battens attached to the wall for it to cling to.
I also find dead heading helps prolong flowering in the first couple of years and then they look after themselves apart from training the new stems out along the supports for maximum coverage.