Herbs, perennial, rhizomatous, 7–10 dm. Rhizomes 3–7 mm diam., nodes not swollen. Culms erect, terete, 2–4 mm diam. Cataphylls 2–3, straw-colored, apex rounded. Leaves: basal 0, cauline 1; auricles 0.5–1.5 mm, apex blunt, cartilaginous; blade 25–35 cm × 1–2 mm. Inflorescences terminal panicles of 24–60 heads, 5–10 cm, branches spreading; primary bract erect; heads 15–50-flowered, spheric, 4.5–6 mm diam. Flowers: tepals light brown, lanceolate to oblong, 2–2.5 mm, nearly equal; stamens 6, anthers longer than filament. Capsules included with beak slightly exserted, chestnut brown, 3-locular, ovoid, 2.5 mm, apex acute proximal to beak, valves separating at dehiscence, fertile throughout or only proximal to middle. Seeds not seen.
Phenology: Fruiting fall.
Habitat: Upper border of salt marsh
Elevation: 0 m
Discussion
All sheets that I have seen have aborted seeds. Juncus pervetus was thought to be Juncus subnodulosus, a European native (H. Weimarck 1946). It has been shown that this is a distinct species, however (S. Snogerup and B. Snogerup 1996). It would appear that this species is now extinct.