Cyperus polystachyos (bunchy sedge, "Igagayatsuri")

Japanese name: イガガヤツリ （毬蚊帳吊）
Scientific name: Cyperus polystachyos

[Overview]
A perennial 10–50 cm tall, found on seashores.
It has a short rhizome.
The stems are tufted and bluntly triangular in cross section.
The leaves are linear, 0.1–0.3 cm wide, and shorter than the stems.
Many spikelets are densely clustered in a head at the stem tip, with long leaf-like bracts at the base.
The spikelets are partly tinged blood red, flattened and linear, about 1–2.5 cm long and 0.2 cm wide, and somewhat erect.
The fruit is about 0.1 cm long, obovate and blackish brown.

[Notes]
The name compares the way the spikelets are borne to the spiny bur of a chestnut.

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