Eleocharis intermedia (Intermediate Spikerush)

Plant Info
Also known as: Matted Spikerush, Mudflat Spike-sedge
Genus:Eleocharis
Family:Cyperaceae (Sedge)
Life cycle:annual
Origin:native
Habitat:part shade, sun; wet sandy or mucky soil; shores, banks, mud flats,
Fruiting season:July - October
Plant height:1 to 10 inches
Wetland Indicator Status:GP: OBL MW: OBL NCNE: OBL
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: indistinct Cluster type: spike

[photo of spikes] A single spike at the top of the stem, lance-elliptic in outline, usually pointed at the tip, 2 to 10 mm (to ~3/8 inch) long, with 5 to 30 florets (usually fewer than 20) spirally arranged, each floret subtended by a single scale. Scales are 1.5 to 2 mm long, blunt to pointed at the tip, reddish-brown to straw-colored to whitish, with a green midrib. Florets have 3 stamens and a 3-parted style. The lowest scale in the spike is similar to the rest, though may be more rounded at the tip, completely wraps around the stem at the base, and may or may not have a flower. The second lowest scale has a flower.

Leaves and stems: Leaf attachment: basal Leaf type: simple

[photo of sheaths] The 2 leaves are bladeless and reduced to sheaths on the lower stem. The upper sheath is straight across to concave on the back, the front pointed at the tip with an obscure tooth at the apex that may extend as much as 1 mm. Sheaths may be firm and persistent or thin and papery, disintegrating with age. Stems are very slender, up to .5mm diameter, grooved, weak and often arching or prostrate, and usually of widely varying lengths. Plants form dense clumps.

Fruit: Fruit type: seed without plume

[photo of spike, scale and acnenes] 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 .9 to 1 mm long (excluding tubercle), olive to dark golden brown with a very fine network pattern or pitting on the surface, nearly round to compressed 3-sided in cross-section with rounded angles, urn-shaped in outline, rounded at the tip end and more abruptly tapered near the base. Tubercles are nearly white to greenish to brown, linear to narrowly pyramidal, 3-sided, .3 to .4 mm long, .1 to .25 mm wide, often twice or more as long as wide and 1/3 or less as wide as the achene. Usually a very short neck/constriction exists between the tip of the achene and the base of the tubercle. Surrounding the achene are 6 or 7 barbed bristles, pale brown to whitish, usually as long as or longer than the achene.

Notes:

Eleocharis intermedia is one of several clump-forming Spikerushes in Minnesota and can be difficult to distinguish from some of the others when achenes are absent. Without achenes, diminutive plants can be essentially indistinguishable from Eleocharis acicularis while more robust plants can strongly resemble Eleocharis ovata or perhaps Eleocharis flavescens, though the latter two typically have more purplish floral scales and achenes more consistently lens-shaped in cross-section. E. acicularis and E. flavescens are also both perennial and colony forming where E. intermedia is annual and lacks rhizomes. All of these species may be found on receding sandy or mucky shorelines.

E. intermedia is distinguished by achenes that are nearly round to 3-sided in cross-section and finely pitted or textured on the surface, and a narrow tubercle longer than wide. It has very slender stems of widely varying lengths, spikes pointed at the tip, red-brown to straw-colored to whitish floral scales, and forms dense clumps.

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More photos

Photos by K. Chayka taken in Ramsey County.

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