Basis for Listing

An inconspicuous wetland sedge, Cyperus acuminatus (short-pointed umbrella sedge) occurs widely in the United States but is apparently rare or absent in the Great Basin and many eastern states. There are about a dozen populations recently confirmed extant in Minnesota. In addition, there are at least one "historic" population (which has never been relocated) and one population, in Nicollet County, that was recently destroyed by rock quarrying. Cyperus acuminatus prefers the margins of small prairie ponds and pools, especially small pools that form on outcrops of quartzite bedrock in the prairie region (Harris 2009). This habitat type is quite rare and is largely limited to counties in southwestern Minnesota (Minnesota River Prairie and Inner Coteau subsections). Cyperus acuminatus was originally listed as a special concern species in Minnesota in 1984, but given its extreme rarity, it was reclassified as threatened in 1996.

  Description

Cyperus acuminatus is a small annual sedge that grows in clumps of 2-12 stems (cespitose). Stems are sharply triangular in cross-section, 5-25 cm (2-10 in.) long and have long slender bracts near the top of the stem that might look like a continuation of the stem. The flowering portion of the plant is made up of 1-6 compact heads each made up of 10-50 spikelets, each with 8-25 flowers. The scale that shields each flower has 3 pronounced veins, or nerves, and is curved outward at the tip but lacks a distinct awn. The fruits, or achenes, are greenish and three-sided.

Although C. acuminatus is a small sedge, usually about ankle-high, it is not a true dwarf. In Minnesota, it looks most like the common C. squarrosus (awned umbrella sedge), which is a true dwarf. The two species can be quickly told apart by the floral scales that form the spikelet. Those of C. squarro­sus have tips with curved hook-like awns. The scales of C. acuminatus may have slightly out-turned tips but no awns. The differ­ence is distinctive and visible without magnification.

Technical Description:  Plants annual. Culms 5--25 cm (2-10 in.) long, arising singly or cespitose in clumps of 2--12. Rhizomes absent. Leaves essentially basal, 2--15 cm (0.8-6 in.) long. Involucral bracts 3--5; erect, ascending or nearly horizontal; 3--20 cm (1-8 in.) long, 0.5--2.5 mm (0.02-0.1 in.) wide. Inflorescence with 0--5 stiff ascending rays 0.5--3 cm (0.2-1 in.) long. Spikes or spike-like heads 1--6 per culm; axis shortened to create a dense ovoid or hemispherical head 0.5--2 cm (0.2-0.8 in.) across; heads solitary at tips of rays and/or sessile at base of inflorescence. Spikelets 10--50 per head, 3--10 mm (0.1-0.4 in.) long, 1--2 mm (0.04-0.08 in.) wide, with 8--25 flowers. Floral scales 1.2--1.7 mm (0.05-0.07 in.) long; apex acute, out-turned, not awned, about 0.1 mm (0.004 in.) long; midrib area pale green; flanks pale green, yellowish or whitish. Stigmas 3. Achenes trigonous, greenish, 0.8--0.9 mm (0.03-0.04 in.) long, 0.3--0.4 mm (0.01-0.02 in.) wide. Maturing mid-July to late September (Smith 2018).

  Habitat

Cyperus acuminatus occurs at the edge of shallow rock pools and in the muddy margins of prairie ponds. The shallow rock pools where C. acuminatus has been found are mostly level with shallow depressions where rainwater accumulates. The pools are typically only a few meters across and 5-7 cm (2-3 in.) deep. Plants are typically rooted in a thin layer of organic sediment that accumulates at the bottom. This is a sparsely vegetated, ephemeral habitat that may support only a few scattered plants or several hundred, depending on habitat conditions.

  Biology / Life History

Cyperus acuminatus is an annual species that dies at the end of its first season. The seeds (achenes) are the only part of the plant that survives the winter. Seed production in any given year is dependent on favorable habitat conditions that vary depending on timing and amount of precipitation. To mitigate effects of abnormally dry or wet years, it is likely that a viable population would have a bank of viable seeds in the substrate that germinate over a period of multiple years. In unfavorable years the seed bank would be depleted, but replenished in favorable years. The seeds germinate and grow in moist or wet soil left when the pool loses its standing water to evaporation. It is not aquatic; it will not survive for long in standing water.

  Conservation / Management

Cyperus acuminatus is threatened by habitat loss, as are so many other prairie plants. Prairie remnants are rare, and prairie remnants containing suitable rock outcrops or ponds are rarer still. Native prairies are being lost to agriculture, residential development, and invasion of non-native weeds. Prairie wetlands are being drained, filled, and plowed. A prairie that contains rock outcrops cannot be easily plowed or drained (though both are still attempted), so they are often used to pasture livestock, typically cattle. Cattle grazing is generally destructive to the delicate plant communities associated with rock pools. Herbicide application often accompanies grazing as an attempt to control the non-native weeds that invade following grazing. The effect of herbicides on C. acuminatus, and other native species inhabiting this habitat type, is not known, but it can reasonably be considered a potential threat. A very large and healthy population in Nicollet County was recently destroyed when the quartzite outcrop on which it occurred was quarried for rock to be used in road building. Continued acquisition and protection of prairie remnants is vital for conserving this species.

  Best Time to Search

The best time to search for Cyperus acuminatus is when it has mature spikelets from July through August.

  Conservation Efforts in Minnesota

Currently, eight of the known sites supporting C. acuminatus are in public ownership and broadly managed to preserve the natural qualities of the sites. In addition, in 2007, the Renville and Redwood County Soil and Water Conservation Districts, in association with state and federal agencies, coordinated an initiative to compensate interested landowners for permanently protecting the most ecologically intact rock outcrops and associated wetlands in those two counties. In 2009, the initiative was expanded to encompass five counties in the Upper Minnesota River Valley (Protection of Granite Rock Outcrop Ecosystem). By 2012, approximately 227 ha (560.4 acres) were enrolled into perpetual conservation easements as a result of this initiative, and C. acuminatus is one of the rock outcrop species most likely to benefit from the program. Conservation plans, which outline conservation practices, such as removal of invasive species and livestock exclusion, were developed for each of the easement sites. Habitat maintenance and improvement measures, such as prescribed burning, fencing, rotational grazing, and/or seeding were also included in the plans.

  Authors/Revisions

Welby R. Smith (MNDNR), 2023

(Note: all content ©MNDNR)

  References and Additional Information

Harris, F. 2009. Rock pools on the prairie: ephemeral pools on the prairie form amazing mini-ecosystems. Minnesota Conservation Volunteer 72(423):30-39.

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

Tucker, G. C., B. G. Marks, and J. R. Carter. 2002. Cyperus. Pages 141-191 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 23. Oxford University Press, New York, New York.