Although we are in Cornwall, we are 500' elevation and East facing and get
some pretty severe frosts. Our Wisteria gets morning sun, but seems fairly
frost hardy. It's also about 7 years old, but used to produce very few
blooms and about 75% of them, fell off.
I got hold of the RHS pruning book and pruned to 4 buds in early Winter and
then 2 buds in late Winter. After pruning, they suggest you aply a mulch to
the base. For the last 2 years there has been very little 'drop off' and
most blooms have reached full size. I don't know if it's the pruning and/or
mulch that has caused this dramatic improvement. Here's some photos I took
this time last year -

http://s222.photobucket.com/albums/d...t=91beb42d.pbw