Flower:

A single spike at the top of the stem, lance-elliptic in outline, usually pointed at the tip, 1.5 to 9 mm (to ~1/3 inch) long, with up to 20 florets spirally arranged, each floret subtended by a single scale. Scales are 1 to 3 mm long, blunt to pointed at the tip, reddish-brown to purple, sometimes with green streaks, and have a green midrib. Florets have 3 stamens and usually a 2-parted style, occasionally 3-parted. The lowest scale in the spike is similar to the rest but lacks a flower.

Leaves and stems:

The 2 leaves are bladeless and reduced to sheaths on the lower stem. Sheaths are fragile, thin and papery, loosely wrap the stem at the tip, whitish to translucent at the tip, straw-colored to red at the base and disintegrate with age. Stems are .3 to .6 mm diameter, straight, somewhat flattened, bright green, and usually of varying lengths. Plants form clumps and typically form colonies from creeping rhizomes.

Fruit:

Each flower produces a single achene (seed), that drops off independently of the scale, the achene with a cap-like appendage (tubercle) at the tip that is clearly distinct from the rest of the achene. Achenes are .5 to 1.1 mm long, green to dark golden brown, finely textured across the surface (magnification required), lens-shaped in cross-section, broadly urn-shaped in outline, rounded at the tip end and narrowed at the base. Tubercles are pale, .3 to .7 mm long, .2 to .4 mm wide, less than 1/3 as wide as the achene, generally triangular but swollen or expanded some at the base. Usually a very short neck/constriction exists between the tip of the achene and the base of the tubercle. Surrounding the achene are 5 to 8 barbed bristles (usually 7, sometimes absent), pale brown to whitish, 1 to 2 times as long as the achene, rarely shorter.

Notes:

Eleocharis flavescens is one of the rare Spikerushes in Minnesota, typically found in sandy or peaty shores, mudflats, and on floating mats. The first collections were from mucky lake shores in Todd and Clearwater counties in the 1930s and it was not reported again for more than 40 years. As of this writing, only 9 locations are identified in the herbarium collections and, according to the DNR, one of those populations has been potentially destroyed after a beaver dam flooded the area, though its fate is unknown. Due to its rarity it was listed as a Threatened species in 1984. There are 2 recognized varieties of E. flavescens: var. flavescens with reddish to dark brown achenes and bristles as long as or shorter than the achene, found in the southern and western US and into South America, and var. olivacea (formerly Eleocharis olivacea), with green to dark brown achenes and bristles as long as or longer than the achene, found in the eastern US into Canada and the species found in Minnesota, where it reaches the western edge of its range.

E. flavescens superficially resembles some of the other clump-forming Spikerushes, but is distinguished by the clump-forming and colony forming growth, fragile sheaths that are loose around the stem and easily disintegrate, reddish-brown to purple floral scales, lens-shaped achenes that are minutely textured, bristles 1 to 2 times as long as the achene, and triangular tubercle enlarged at the base (when the swollen part is compressed the whole thing reminds me of a sombrero). Somewhat similar are Eleocharis ovata and Eleocharis intermedia, which are annuals and do not form colonies, and have firm sheaths that are pointed at the tip and more tightly wrap the stem. Eleocharis acicularis is colony forming and has fragile sheaths like E. flavescens, but has proportionately narrower achenes with distinct longitudinal ridges and cross-bars, though magnification is necessary to see them clearly, and stems are even more hair-like, usually less than .5mm diameter. It is also very common, found across the state.