Basis for Listing

The concern for Eleocharis robbinsii (Robbins’ spikerush) is twofold: it is rare in Minnesota, and it occurs in a habitat type that is especially vulnerable to degradation. The habitat is the zone of native vegetation in shallow water along the margins of certain soft-water lakes. The source of degradation is the accelerating pace of lake shore development and the demand for recreational access to lakes.

The rarity of E. robbinsii has been confirmed by ongoing biological surveys that have targeted potential habitat. In fact, the littoral zone of lakes has been the focus of many botanical surveys for over two decades. Yet with over 2,000 lakes surveyed, fewer than 1 in 100 surveys have returned positive results for this species. 

Wisconsin is the only adjacent state where E. robbinsii has been found, and there it is listed as special concern.  Here in Minnesota, E. robbinsii was listed as a threatened species in 2013.

  Description

Eleocharis robbinsii is a perennial member of the sedge family; the culms, when emergent from the water, are erect, sharply triangular in cross-section, spongy, 20-100 cm (8-40 in.) long, 1.0-2.5 mm (0.04-0.10 in.) wide; if the culms are submerged, they may be filiform and flaccid; the rhizomes are soft, reddish, to 30+ cm (1 ft.) long, and 0.5-2.0 mm (0.02-0.08 in.) thick; the spikelets are acute, 9-33 mm (0.35-1.3 in.) long, 1.5-3.0 mm (0.06-0.12 in) wide, often no wider than the culm and barely distinct, with 4-18 flowers; the achenes are straw-colored or brownish, biconvex or irregularly trigonous, 1.9-2.6 mm (0.07-0.10 in.) long (excluding tubercle), 1.0-1.5 mm (0.04-0.06 in.) wide; the surface has numerous vertical rows of tiny honeycomb-like cells, distinct at 10x; the tubercle of the achene appears distinct from the achene, narrowly pyramidal, 0.5-1.1 mm (0.02-0.04 in.) high, 0.3-0.7 mm (0.01-0.03 in.) wide at the base, the style often persisting. Mature achenes are expected in late summer (Smith et al. 2002).

There are 18 species of the genus Eleocharis known to occur in Minnesota. Among them all, only E. robbinsii has a stem that is distinctly triangular in cross-section; stems of the other species are round or flat. The spikelet is the structure at the top of the stem that produces the flowers and seeds. In most species of Eleocharis, the spikelet is distinct and visible at a distance. You will not notice the spikelet of E. robbinsii from any distance. It is a narrow and pointed structure and appears to be a continuation of the stem, not differing much in color, shape, or texture. Also, many of the stems are sterile; they will not have a spikelet at all. The leaves of E. robbinsii are long, slender and hair-like filaments, and they appear suspended in the water column. They do not emerge from the water or float on the surface of the water, so they are not immediately noticed. They look something like the leaves of Schoenoplectus subterminalis (swaying bulrush) or maybe the flaccid submerged culms of Eleocharis acicularis (least spikerush). When viewed from a distance, all that can be seen of E. robbinsii is a narrow stem sticking out of the water, so it can be very difficult to distinguish it from all the other emergent vegetation.

Technical Description: Plants perennial. Emergent culms erect, triangular in cross-section, spongy, 20--100 cm (8-40 in.) long, 1--2.5 mm (0.04-0.7 in.) wide; sterile tips blunt. Submerged culms filiform and flaccid, sterile, about 0.2 mm (0.008 in.) wide. Rhizomes soft, smooth, to 30+ cm (12+ in.) long, 0.5--2 mm (0.02-0.08 in.) wide. Leaves 2 per culm, reduced to bladeless sheaths; distal sheath thin and membranous. Spikelet acute, 9--33 mm (0.4-1.3 in.) long, 1.5--3 mm (0.06-0.12 in.) wide, often no wider than the culm and barely distinct, with 4--18 flowers. Floral scales predominately green with white or colorless margins. Perianth bristles 6--7, equaling or exceeding the tubercle. Achenes straw-colored or brownish, biconvex or irregularly trigonous, 1.9--2.6 mm (0.07-0.10 in.) long (excluding tubercle), 1--1.5 mm (0.04-0.06 in.) wide; surface with numerous vertical columns of tiny honeycomb-like cells distinct at 10x. Tubercle distinct from achene, narrowly pyramidal, 0.5--1.1 mm (0.02-0.04 in.) high, 0.3--0.7 mm (0.01-0.03 in.) wide; style often persisting. Maturing late summer.

  Habitat

In Minnesota, E. robbinsii has been found growing emergent from shallow water in the littoral zone of a handful of remote soft-water lakes in the Laurentian Mixed Forest Province. In all cases, E. robbinsii has been associated with undisturbed native plant communities, supporting a diversity of submerged floating-leaved and emergent species. Apparently, E. robbinsii is not compatible with dense stands of cattails (Typha spp.), which are likely too aggressive and dominant to allow a population of E. robbinsii to become established. It also does not survive where wave action is too great or where water quality has been compromised. Substrates vary and are usually sandy, gravelly, or rock; though they may be overlain by a layer of fine organic sediments.

It is also possible that E. robbinsii may be found on a sandy beach a short distance from the water’s edge. However, such occurrences may involve only transient individuals and may depend on a stable population in shallow water.

  Biology / Life History

Eleocharis robbinsii is a perennial member of the sedge family. It can form extensive colonies by the lateral growth of rhizomes in soft sediments. It can also reproduce by seeds (achenes). The seeds are small (about 2.5 mm [0.01 in.] long) and have 6-7 perianth bristles, with backward-pointing barbs. The structure of the bristles indicates they could catch in the feathers of water birds or the fur of aquatic mammals and be dispersed in that fashion. It is also likely that some seeds simply fall into the water when they mature in late summer and autumn and are carried on water currents before they sink to the bottom or are washed up on shore.

Seeds are only produced by stems that appear above the surface of the water, though not all emergent stems produce seeds. Observations indicate these emergent stems are sometimes grazed by waterfowl, which is not likely to have a long-term effect on populations of E. robbinsii.

  Conservation / Management

The disturbance of shoreline vegetation in the vicinity of known occurrences of E. robbinsii must be avoided. This would apply to direct removal of vegetation as well as activities in the vicinity of the lake that might have an indirect impact on the shoreline plant community.

A major concern is shoreline erosion, typically in the form of wave action eroding the shoreline, which lakeshore owners often believe erodes their property as well. However, unless the lake level has been artificially raised, this sort of erosion is normal and does no ecological harm. A certain amount of sediment is moved around, but the lakeshore plants are adapted to deal with that. The real concern is sediment being washed into the lake from some distance away, such as from a road cut or excavation for a building. That sort of erosion can blanket lakeshore plants under a considerable amount of sediment and destroy the habitat.

Aquatic invasive plant species are also a matter of great concern in Minnesota. The concern is not only the prospect of native species being crowded out of habitat by a non-native invader but also the herbicides used to control invasive species. In some situations they can kill all the plants in a lake, including the natives. Currently, none of the lakes known to harbor E. robbinsii has been invaded by non-native species.

  Best Time to Search

It may be possible to see and identify E. robbinsii anytime after the middle of the summer, but the best time to search is when the spikelets are fully developed in late summer.

  Authors/Revisions

Welby Smith (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Minnesota Department of Natural Resources. 2003. Field guide to the native plant communities of Minnesota: the Laurentian mixed forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 352 pp.

NatureServe. 2009. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia.

Smith, S. G., J. J. Bruhl, M. S. Gonzalez-Elizondo, and F. J. Menapace. 2002. Eleocharis. Pages 60-120 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 23. Oxford University Press, New York, New York.

Smith, W. R. 2018. Sedges and rushes of Minnesota: the complete guide to species identification. University of Minnesota Press, Minneapolis. 696 pp.