Posted: 20/09/2013 at 15:43
Have you considered boxwood shrubs?
A Green Velvet Boxwood grows about 4-6 feet upon maturity and also fast growing. Best for growing zones 5-8. Since the mature width is 3-5 feet, I don't think the roots can reach and damage the pipes. But i'm not sure though. It's just my guess. I never heard anyone having problems with pipes when it comes to shrubs.
Buxus is not fast growing - you can be dead before it gets to 6ft as that takes 20-50 years depending on position. It is also expensive - over £1 per 9cm plant. A bare root wip is about 60p for a native hedge tree that will reach 4-6ft in 3 years. Also the Zones system form the USA are of no use in the UK as out temperatures are milder in winter but much much wetter - it can be bone dry in a US winter and winter is longer here than in the US as we are far more northern.