Juncus gerardii forms loose swards of erect tufts from dense and far-reaching matrix of black rhizomes.

Juncus gerardii occurs on coastal sites and intertidal zones, in salt marshes, wetland margins, disturbed habitats and wastelands.

[5] In North America, it has spread to some unwanted locations, such as the Great Lakes region, where it causes several adverse environmental impacts, such as threatening the survival of native vegetation and hosting insects that can carry diseases.

[2] Juncus gerardii has been introduced to a number of countries, including Greenland, New Zealand, Australia (Tasmania and Victoria), and Asia (Primorye and Magadan).

[1] It is considered invasive, having been recorded in coastal wetlands and pastures,[6] where it can form large swards that exclude native vegetation and reduce grazing potential.