How invasive are willow? I'm planning on using them as a fast growing hedge this year. Do I need to cut them back every season or can I just let them go?
From my experience with willow in Oklahoma, they are moderately invasive, but not all that bad. The main problem is when they wash up on a shoreline and take root where it is difficult or impossible to mow. They spread very slowly unless they have a very wet environment.
All the varieties I know of would make a lousy hedge. They don't leaf densely, they don't grow very straight, they don't root deeply (and thus after a certain height you risk them falling over)... the only things they have going for them is that they are easy to establish if given enough water and they do well in poor soils.
I'd encourage you to examine other options, unless there's some reason or variety of willow that suits your situation really well.
It's more a problem of getting to big rather then being invasive. Once establiched they will need regular pruning - either coppicing or removing 1/3 of the largest canes.
you can learn to make stick furniture and sell them for big $$$$- and then teach classes on how to do it- and let the students "prune" them for you.....
That would be fun!