Flower:

A single spike at the top of the stem, slightly flattened, lance-elliptic in outline, usually pointed at the tip, 2 to 8 mm (to ~1/3 inch) long, with 4 to 25 florets spirally arranged, each floret subtended by a single scale. Scales are 1.5 to 2.5 mm long, blunt to pointed at the tip, reddish to purplish-brown with a broad green midrib that dries brown, and white, translucent edging; sometimes scales have very little coloring except for the pale green midrib. Florets have 3 stamens and a 3-parted style. The lowest scale in the spike is similar to the rest and has a flower.

Leaves and stems:

The 2 leaves are bladeless and reduced to sheaths on the lower stem. The upper sheath is thin and papery, usually reddish toward the base, slightly expanded and loosely wrapping the stem at the tip, V-shaped to shredded on the back, rounded on the front, and lacks a distinct tooth at the apex. Stems are very slender, up to .5mm diameter, generally round in cross-section, smooth or with 3 to 12 vertical ridges.

Stems may be single but usually form clumps, and frequently form dense mats from slender, white rhizomes. Plants may be completely submersed in water or landlocked. Submersed plants tend to be strictly vegetative, lacking spikes, and the stems can grow significantly longer than terrestrial plants, depending on the water depth; as much as 24 inches by some accounts where terrestrial forms are only a few inches tall.

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 .7 to 1.1 mm long, .3 to .6 mm wide, light brown to yellowish to grayish-white, have 8 to 12 vertical ridges with numerous cross-bars, are nearly round in cross-section, narrowly urn-shaped in outline, rounded at the tip end and tapering at the base. Tubercles are gray to greenish to brown, pyramidal to depressed, .1 to .2 mm long, about as long as wide. A short neck/constriction exists between the tip of the achene and the base of the tubercle. Surrounding the achene may be 2 to 4 barbed bristles, pale brown to whitish, as long as or shorter than the achene, but bristles are usually absent.

Notes:

Eleocharis acicularis is one of the most common Spikerushes both in Minnesota and globally, found across Europe, Asia, North America and into South America. It has both aquatic and terrestrial forms and is a popular plant in aquariums, the dense mats considered good habitat for fish to lay their eggs. It has also been found to absorb heavy metals and may prove beneficial in wetland remediation efforts as well as a process known as phytomining, where these metals are harvested from plants.

Submersed, vegetative plants resemble some other aquatic species, the clumps of non-flowering stems appearing as tufts of leaves, but stems are not more than .5mm wide, generally round in cross-section (not flat), and are either smooth or with vertical ridges but no cross-veins or partitions. The thread-like white rhizomes further distinguish it from some other aquatics. Terrestrial forms of E. acicularis are also distinguished by the short stature, rarely exceeding 3 inches tall, slightly flattened spikes where the lowest scale has a flower, achenes nearly round in cross-section with distinct vertical ridges and cross bars, and are often nearly white. Magnification is required to see many of these traits. Some references note multiple varieties, distinguished by traits such as stem length, number of ridges, and length to width ratio of the achenes, but these distinctions are not currently recognized in Minnesota.

Terrestrial forms are found in a variety of wet places but frequently on sandy or silty shores and banks where water has receded. Without achenes, it may be mistaken for diminutive forms of Eleocharis intermedia, which is an annual not perennial, lacks rhizomes, and the lowest scale on the spike lacks a flower. Also similar is Eleocharis coloradoensis, a rare species of saline soils that forms underground tubers and has achenes with a wrinkled texture but no ridges, and Eleocharis flavescens, another rare species which has more consistently purple scales and achenes that lack ridges. The only other Eleocharis species in Minnesota with achenes like E. acicularis is Eleocharis wolfii, a rare species of rock outcrops that has flat stems often spirally twisted, floral scales 2.5 to 3.5 mm long, and the front of its sheaths are pointed at the tip, not rounded.

Aquatic forms more closely resemble those of Eleocharis robbinsii, which have limp, thread-like stems and reddish rhizomes, where E. acicularis stems are more stiff and rhizomes are white. Also similar is Schoenoplectus subterminalis, which has alternate, bladed leaves (not bladeless sheaths) that are compressed and have distinct cross-partitions. Magnification may be required to see these traits.