Eleocharis palustris (Common Spikerush)

Plant Info
Also known as: Common Spike-sedge
Genus:Eleocharis
Family:Cyperaceae (Sedge)
Life cycle:perennial
Origin:native
Habitat:sun; wet; ditches, swales, shores, marshes, wet meadows, fens
Fruiting season:July - September
Plant height:4 to 40 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

Pick an image for a larger view. See the glossary for icon descriptions.

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, 5 to 25 mm (to ~1 inch) long, with 30 to 100 florets spirally arranged, each floret subtended by a single scale. Scales are 3 to 5 mm long, mostly pointed at the tip, reddish or orange-brown to medium brown with a green to straw-colored midrib and translucent edging. Florets have 3 stamens and a 2-parted style.

[close-up of lowest scales] The lowest scale in the spike is more leathery than the rest, mostly green with a band of brown between the green midrib and translucent edging, and is rounded at the tip. The lowest scale wraps less than 75% of the stem at the base, and lacks a flower. The second lowest scale is similar to the lowest but usually somewhat smaller; it and sometimes the third lowest scale also lack flowers.

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

[photo of sheath] The 2 leaves are bladeless and reduced to sheaths on the lower stem. The upper sheath is membranous to papery, persistent or sometimes disintegrating with age, usually red to blackish towards the base and green to reddish towards the tip, often dark reddish-brown to blackish all around the tip edge, straight across to concave on the back and often splitting, the front rounded to blunt at the tip, usually with a few swollen veins on the front but lacking a distinct tooth at the apex. Stems are erect, .5 to 5 mm diameter, round in cross section with 8 to 30 vertical ribs that become conspicuous when stems dry. Plants form colonies from long rhizomes.

Fruit: Fruit type: seed without plume

[photo of scales and achenes] 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 1.1 to 2 mm long, 1 to 1.5 mm wide, yellowish to dark brown, smooth or with a very fine wrinkled texture on the surface, lens-shaped in cross-section, urn-shaped in outline, rounded at the tip end and tapered near the base. Tubercles are whitish to dark brown, pyramidal, .3 to .7 mm long, 1 to 2 times as long as wide. A very short neck/constriction exists between the tip of the achene and the base of the tubercle. Usually surrounding the achene are 4 (rarely 5) barbed bristles, pale brown to whitish, much shorter than to about as long as the achene. Bristles are sometimes absent.

Notes:

Eleocharis palustris, formerly Eleocharis smallii, is one of several common, colony-forming Spikerushes in Minnesota and can be difficult to distinguish from some of the others, even when achenes are present. It is most similar to Eleocharis macrostachya and Eleocharis erythropoda, and all three were at one time considered a single species and are still identified as such in some references (including the national distribution map above). All three may be found in a variety of wet habitats, from road ditches to wet meadows to lake shores, in fresh to brackish waters. All three are also rather variable species, which confounds the situation.

E. palustris is probably the most variable of the three. Flora of North America notes 4 different variants though these do not have official variety or subspecies status at this time. These variants are distinguished by, among other things, stem width (ranging from .5 to 5 mm), whether the sheaths persist or disintegrate, and geographic area. In Minnesota, most often we've encountered populations of fairly robust plants with stout stems (3+ mm wide) in shallow water, mostly in lakes, ditches and other watery channels. Less often we've seen shorter, more slender plants especially when not in standing water.

E. palustris is generally distinguished by: stems round in cross-section, sheaths rounded to nearly straight across at the tip on the front and often with swollen veins but lacking a distinct tooth at the apex, the lowest scale on the spike is leathery and wraps less than 75% of the stem, the second lowest scale (and sometimes the third) is like the first and lacks a flower/fruit, achenes are smooth, brown and lens-shaped in cross-section, usually with 4 bristles varying from much shorter to about as long as the achene, and a whitish to dark brown cone-shaped tubercle 1 to 2 times as tall as wide. The spike is normally lance-elliptic, pointed at the tip with medium brown scales to 3 to 5 mm long that are pointed at the tip. Magnification may be required to see some of these traits.

By comparison, on E. macrostachya, the lowest scale wraps 70-100% of the stem, the next lowest scale may have a flower/fruit, and the sheath often has an obscure to distinct tooth at the apex. On E. erythropoda, the lowest scale wraps 100% of the stem or nearly so, and the second lowest scale has a flower/fruit. To reiterate: these are all variable species so your mileage may vary. A fourth species, Eleocharis mamillata, also resembles this group but usually has 5 or 6 bristles that are distinctly longer than the achene, and has only been recorded 3 times in Minnesota, all in the arrowhead region near the Canadian border.

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More photos

Photos by K. Chayka taken in Anoka, Becker and Itasca counties. Photos courtesy Peter M. Dziuk taken in Anoka, Becker, Lake, and St. Louis counties.

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