Juncus balticus; the Baltic Rush is often considered by specialists to be a suspecies of Juncus arcticus. As such there are three related (sub-)sp[ecies , being
J. arcticus ssp arcticus,
J. arcticus ssp intermedius and
J. balticus (= J. arcticus ssp. balticus)
The first one (ssp arcticus) has very compact inflorescence. Juncus balticus has a more spreading inflorescence with rather long inflorescence stalks. ssp intermedius - as the name suggests - holds the middle.
On Iceland the "Baltic" form only grows in wet areas near the coast. It tends to make densely covered vegetation-patches.
It is member of the rush family (Juncaceae).