Carex rostrata; the Bottle Sedge is one of the larger sedges on Iceland. It is about 60 cm tall, sometimes even more. It usually grows in or along the edges of lakes. It also grows in bogs where it is less tall (30 cm or so), probably due to to less favourable nutritional soil conditions. The leaves are 3 to 5 mm broad and are greyish to bluishly colored on the bottom (more so as water plants, less so when growing in dryer conditions). The female inflorescences are placed under the male inflorescence. They are pointed stiffly upwards. This characteristic differentiates it easily from the other larger sedge species on Iceland being Carex lyngbyei (Lyngbye's Sedge). This sedge species has drooping female inflorescence. Another characteristic feature of the Bottle Sedge are the long pointed beaks of the utricles of the female inflorescences - see photo female spike.
It is a member of the Sedge family (Cyperaceae). The Icelandic name of this species is Tjarnastör.