A perennial herb, mainly confined to areas within reach of sea water or spray. It is found on muddy or sandy estuarine flats, at the uppermost levels of saltmarshes and the edges of brackish ditches, and on moist coastal rocks and low cliffs. It often grows with C. distans. Lowland.
There has been little change in the distribution of C. extensa since the 1962 Atlas.
European Southern-temperate element.