[B-Chan]

Bamboo is indeed a grass — a member of the true grass family Poaceae. It's related to rice, believe it or not, which is also a grass.

[em2vn]

It is a grass in the sense that, as you said, rice is, plus wheat, oats, millet and other food grains. But it is not designed to be cut to a specific length any more than are food grains.