Cyperus is a species of Cyperaceae. From the photo, we can see it is a robust herb. Cyperus has simple leave which culm triangular in shape. It is monoecious with bisexual flower. The flower is covered by glume and it has 3 stamens with 3 stigmas.

Cyperus is a common plant non-native to HK which is planted near artificial ponds. It is regarded as invasive plant to many places because it is fast-growing and stress-tolerated.

Source: https://keyserver.lucidcentral.org/weeds/data/media/Html/cyperus_involucratus.htm