Bamboo?

I don't think so, the stems don't have the segmented appearance I associate with bamboo.

Here's a closer view of one stem-head & stem:

Looks like Cypriol.

could also be Cyperus odoratus / cyperus haspan.

juggalo28: Nut grass kill with weedkiller

Fortunately, it seems too tall for nut grass - aka Cyperus rotundus / purple nutsedge - which is said to have "flowering stems are erect, up to 60 cm tall" [https://www.cabi.org/isc/datasheet/17506]
My plant is growing more than twice that size.

pih: Google Lens on your first close-up seems to think it's a Cyperus Alternifolius, but I'm no botanist.

Thanks @pih, this looks like the right answer