Eleocharis robbinsii (Robbins' Spikerush)
|Also known as:
|sun; sandy or peaty soil; shallow bays in lakes and ponds
|July - August
|6 to 30 inches
|Wetland Indicator Status:
|GP: none MW: OBL NCNE: OBL
|MN county distribution (click map to enlarge):
|National distribution (click map to enlarge):
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A single spike at the top of the stem, linear in outline and not any wider than the stem (or barely so), pointed at the tip, 9 to 33 mm (to 1+ inch) long, with 4 to 18 florets spirally arranged, each floret subtended by a single scale. Scales are 5 to 7.8 mm long, blunt to pointed at the tip, nearly all colorless to straw-colored to brown-tinged, often red-dotted, with a green midrib and translucent edging. Florets have 3 stamens and a 3-parted style. The lowest scale in the spike appears to be a continuation of the stem, is blunt to rounded at the tip with translucent edging, and has a flower.
Leaves and stems:
The 2 leaves are bladeless and reduced to sheaths on the lower stem. On flowering stems, the upper sheath is thin and papery, persisting or disintegrating, rounded to pointed at the tip on the front and lacks a distinct tooth at the apex. Stems are sharply 3-sided in cross-section, 1 to 2 mm wide, erect and stiff but spongy, single or multiple from the base forming small clumps. Stems of vegetative shoots are limp and thread-like, floating on the surface or submersed, usually in denser clumps. Plants form colonies from reddish rhizomes up to 2 mm thick, and may also form underground tubers.
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. Scales leave a conspicuous wing persisting on the stalk. Achenes are 1.9 to 2.6 mm long, 1 to 1.4 mm wide, straw-colored to brown, have many vertical vertical rows of rectangular cells, are compressed 3-sided to lens-shaped in cross-section, urn-shaped in outline, rounded at the tip end and more abruptly tapering near the base. Tubercles are straw-colored to dark brown, narrowly triangular, .5 to 1.1 mm long, longer than wide. A distinct neck slightly wider than the tubercle base exists between the tip of the achene and the base of the tubercle. Surrounding the achene are 6 or 7 barbed, yellowish to reddish-brown bristles, all about the same length and usually rather longer than the achene.
Eleocharis robbinsii is one of the rare Spikerushes in Minnesota, where it reaches the western edge of its range. While it may be found on receding shores, in Minnesota it is typically found in the shallow waters and protected bays of soft-water lakes, ponds and streams. According to the DNR, it was first recorded in 1995 but subsequent biological surveys of over 1500 lakes only located 15 additional populations, all of which are small. Its preferred habitat is at risk from a host of threats from human activity, most notably recreation and shoreline development. It was listed as a MN Threatened species in 2013 and is currently a Special Concern species in Wisconsin.
Flowering stems of E. robbinsii should not be confused with other colony-forming Minnesota Spikerushes. The shallow water habitat, sharply 3-sided stem, spike that is not any wider than the stem, relatively large achenes with a corrugated texture, and bristles rather longer than the achene, are a unique combination. While Eleocharis palustris also forms colonies in shallow waters, its flowering stem is round in cross-section, the spike is noticeably wider than the stem, and achenes have a smooth surface or only faint wrinkling. Vegetative plants of E. robbinsii can resemble some other aquatics, in particular vegetative forms of Eleocharis acicularis, very common in the state, which has stiffer vegetative stems and all white rhizomes, and 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.
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- Eleocharis robbinsii plants
- Eleocharis robbinsii habitat
- mature spike
- comparison of Minnesota's Eleocharis achenes
Photos by K. Chayka taken in Pine County.
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