Flower:

A single spike at the top of the stem, oval to narrowly lance-elliptic in outline, mostly pointed at the tip, 5 to 17 mm (to ~2/3 inch) long, with 10 to 40 florets tightly packed and spirally arranged, each floret subtended by a single scale. Scales are 2.5 to 6 mm long, 2 to 3 mm wide, blunt to pointed at the tip, straw-colored to brown with a green midrib that dries pale to brown. Florets have a 3-parted style and 3 stamens. The lowest scale in the spike is broader than the rest, surrounds the entire stem or nearly so, and lacks a flower. The second lowest scale is similar to the lowest and has a flower.

Leaves and stems:

The 2 leaves are bladeless and reduced to sheaths on the lower stem. The upper sheath is firm, usually reddish, concave on the back, the front bluntly pointed to nearly straight across at the tip, with an obscure tooth or slight swelling at the apex.

Stems are wiry, slightly flattened, .3 to 2 mm diameter, initially erect to ascending. Not all stems produce flowering spikes, the vegetative shoots becoming arching and taking root where the tip touches the ground. Plants form dense clumps and form mats.

Fruit:

Each flower produces a single achene (seed), that drops off independently of the scale, the achene often with a cap-like appendage (tubercle) at the tip that appears to be a continuation of the achene, though may be a different color. Achenes are 1.5 to 2.5 mm long, 1 to 1.2 mm wide, olive to brown, smooth and shiny across the surface, 3-sided in cross-section with rounded angles, oval to urn-shaped in outline, rounded at the tip end and tapering at the base. Tubercles are sometimes absent; when present are pale to dark brown, pyramidal, as long as or longer than wide. There is no neck or constriction between the top of the achene and the base of the tubercle. Surrounding the achene are usually 4 to 6 (sometimes 0) barbed, brown bristles, about as long as the achene.

Notes:

Eleocharis rostetella is one of the rare Spikerushes in Minnesota, most often found in calcareous fens or spring fens in acid peatlands, habitats that are rare in themselves. According to the DNR, Minnesota populations are disjunct from other populations possibly due to its specialized habitat requirements. It was listed as a Threatened species in 1984. E. rostetella should not be confused with any other Spikerush in Minnesota, distinguished by the combination of: clump-forming and colony-forming, long, wiry, slightly compressed stems that become arching and root at the tips, plus shiny, olive to brown achenes that may lack a tubercle, or when one is present appears to be a continuation of the achene. While there are other Eleocharis species that tip-root, this is unique to E. rostetella in Minnesota.