The genus name comes from the Greek sphen, a wedge, and pholis, a scale, and refers to the shape of the second glume, a part of the flower (Fernald 1950).
There is one existing population which is very small and has not been seen since 1991. There are eight historical populations from the early 1900s which need to be resurveyed. There are four populations which no longer exist because their habitat has been destroyed.
The short-term trend is uncertain. A recent visit to the only known population did not find any plants but a few more visits are needed to determine if the plants still exist.
The long-term trend is strongly negative. Populations have declined since the early 1900s from twelve known sites to one.
Succession to more shrubby vegetation may threaten the existence of this plant in the small wetland where it occurs.
Populations need a sufficient natural buffer to protect the hydrology of the site. Invasive exotic wetland species should be removed.
Propagation studies are needed to see if the remaining population can be augmented.
In New York, this species has been found in an open marshy area along a stream in a wet pitch pine oak forest, and in a swampy woods (New York Natural Heritage Program 2012). Springheads, seepage areas, swamps, marshes, and other moist to wet places (FNA 2007).
This grass was most common on Long Island where it is now found in Suffolk County. There are historical records to the west in New York City and a few scattered upstate records in Delaware, Herkimer, Monroe, Niagara, Tompkins, and Ulster counties. There are unconfirmed reports from Dutchess and Yates Counties.
This is a grass of the southeastern and northeastern US extending from Louisiana northeast to Ohio, Pennsylvania, New York, and Massachusetts and south to Florida.
Swamp oats is a clumping perennial grass that grows 3-12 dm tall. The stems are hairless. The leaf sheaths are sometimes hairless or with hairs and the membranous ligules 0.2-1 mm long. The leaf plates are 4-10 centimeters abbreviation long and 2-8 mm wide, flat, smooth or slightly roughened and sometimes hairy. The open panicles are 7-35 cm long and erect to nodding with relatively few, loosely arranged spikelets. The spikelets are 4.5-9.5 mm long with 2-3 flowers each. The glumes fall off with the flowers, the upper glumes 3.6-6.2 mm long. The lemmas of the lowest flowers are 3.5-6 mm long, mostly smooth, with roughened tips that lack awns or have awns to 2.5 mm long. The lemmas of the upper flowers are roughened on the sides and have awns 3-9 mm long that are slightly to obviously bent (FNA 2007).
Sphenopholis pensylvanica is a grass that grows in dense clumps (occasionally as solitary stems) with stems reaching 1.2 meters tall. Its pale panicles are narrow oval to egg-shaped, 7 to 35 cm long, 2 to 10 cm wide, and often lax. Spikelets are 4.5 to 9.5 mm long and do not disarticulate from their stalks (pedicels) at maturity. The second glume is lance to narrowly egg-shaped, broadest above the middle and tapering toward the base (oblanceolate or narrowly obovate). The lemmas are rough (scabrous) and covered with minute projections (papillae). The first lemma is typically awnless, the second has a divergent awn 3.5 to 9 mm long.
This plant can only be identified in flower or fruit, before the spikelets fall from the panicle. Stems with leaves and intact glumes and lemmas are needed.
Sphenopholis obtusata has a more constricted panicle with a more dense arrangement of spikelets. The lemmas of the upper flowers are smooth on the sides and lack awns, in contrast the lemmas of S. pensylvanica's upper flowers are scabrous, and conspicuously awned (FNA 2007).
Mature fruits are present from late May to early July.
The time of year you would expect to find Swamp Oats fruiting in New York.
Sphenopholis pensylvanica (L.) A.S. Hitchc.
Flora of North America Editorial Committee. 2007. Flora of North America, North of Mexico. Volume 24. Magnoliophyta: Commelinidae (in part): Poaceae, part 1. Oxford University Press, New York. 911 pp.
Fernald, M.L. 1950. Gray's manual of botany. 8th edition. D. Van Nostrand, New York. 1632 pp.
Gleason, Henry A. and A. Cronquist. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. The New York Botanical Garden, Bronx, New York. 910 pp.
Holmgren, Noel. 1998. The Illustrated Companion to Gleason and Cronquist's Manual. Illustrations of the Vascular Plants of Northeastern United States and Adjacent Canada. The New York Botanical Garden, Bronx, New York.
Mitchell, Richard S. and Gordon C. Tucker. 1997. Revised Checklist of New York State Plants. Contributions to a Flora of New York State. Checklist IV. Bulletin No. 490. New York State Museum. Albany, NY. 400 pp.
New York Natural Heritage Program. 2010. Biotics database. New York Natural Heritage Program. New York State Department of Environmental Conservation. Albany, NY.
New York Natural Heritage Program. 2022. New York Natural Heritage Program Databases. Albany, NY.
Weldy, T. and D. Werier. 2010. New York flora atlas. [S.M. Landry, K.N. Campbell, and L.D. Mabe (original application development), Florida Center for Community Design and Research http://www.fccdr.usf.edu/. University of South Florida http://www.usf.edu/]. New York Flora Association http://newyork.plantatlas.usf.edu/, Albany, New York
Information for this guide was last updated on: January 7, 2016
Please cite this page as:
New York Natural Heritage Program. 2022. Online Conservation Guide for Sphenopholis pensylvanica. Available from: https://guides.nynhp.org/swamp-oats/. Accessed September 29, 2022.