Miscanthus? Pampas grass?
Have you got a photo you can put on here?
Andy - What time of the year do they flower?
How tall are they?
What are the leaves like?
Annual or Perennial?
Is it the flowers that look like reeds or just the foliage
it may be the dreaded pampus or a reed that grows in very damp soil or ditches, use to cut it for displays(cant think what the name is) or it could be a kind of grass, if you have photo it would help.