This is just how your black bamboo, Phyllostachys nigra should look. I would leave it alone. Just a regular tidy of the discarded leaves. One word of warning; don't let it loose in your garden as it is very invasive.

Thanks Jimmyttheone your so right about the invasion as this is a clump I saved and potted up, we had a whole bed of it and was so disappointed to have to take it out but it had gone everywhere, the chap who did the garden should have told us about a non invasive one as I'd no clue I had only wanted the black for against the walls, live and learn.