Swamp Oats

Sphenopholis pensylvanica (L.) A.S. Hitchc.

Sphenopholis pensylvanica drawing from USDA-NRCS PLANTS
Britton and Brown 1913

Monocotyledoneae (Monocots)
Poaceae (Grass Family)
State Protection
Listed as Endangered by New York State: in imminent danger of extirpation in New York. For animals, taking, importation, transportation, or possession is prohibited, except under license or permit. For plants, removal or damage without the consent of the landowner is prohibited.
Federal Protection
Not Listed
State Conservation Status Rank
Critically Imperiled in New York - Especially vulnerable to disappearing from New York due to extreme rarity or other factors; typically 5 or fewer populations or locations in New York, very few individuals, very restricted range, very few remaining acres (or miles of stream), and/or very steep declines.
Global Conservation Status Rank
Apparently Secure globally - Uncommon in the world but not rare; usually widespread, but may be rare in some parts of its range; possibly some cause for long-term concern due to declines or other factors.


Did you know?

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).

State Ranking Justification

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.

Short-term Trends

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.

Long-term Trends

The long-term trend is strongly negative. Populations have declined since the early 1900s from twelve known sites to one.

Conservation and Management


Succession to more shrubby vegetation may threaten the existence of this plant in the small wetland where it occurs.

Conservation Strategies and Management Practices

Populations need a sufficient natural buffer to protect the hydrology of the site. Invasive exotic wetland species should be removed.

Research Needs

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).

Associated Ecological Communities

  • Pitch pine-oak forest (guide)
    A mixed forest that typically occurs on well-drained, sandy soils of glacial outwash plains or moraines; it also occurs on thin, rocky soils of ridgetops. The dominant trees are pitch pine mixed with one or more of the following oaks: scarlet oak, white oak, red oak, or black oak.
  • Red maple-blackgum swamp* (guide)
    A maritime, coastal, or inland hardwood swamp that occurs in poorly drained depressions, sometimes in a narrow band between a stream and upland. Red maple and blackgum are often codominant or blackgum may be the dominant tree. Pitch pine may occur on drier hummock islands in pine barrens settings.

* probable association but not confirmed.

Associated Species

  • Acer rubrum
  • Carex collinsii (Collins's sedge)
  • Carex intumescens (bladder sedge)
  • Carex sp.
  • Carex stricta (tussock sedge)
  • Galium sp.
  • Maianthemum canadense (Canada mayflower)
  • Mikania scandens (climbing hempweed, climbing boneset)
  • Nyssa sylvatica (black-gum, sour-gum)
  • Onoclea sensibilis (sensitive fern)
  • Osmunda cinnamomea
  • Parthenocissus quinquefolia (Virginia-creeper)
  • Symplocarpus foetidus (skunk-cabbage)
  • Toxicodendron radicans
  • Trientalis borealis
  • Vaccinium corymbosum (highbush blueberry)


New York State Distribution

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.

Global Distribution

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.

Identification Comments

General Description

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).

Identifying Characteristics

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.

Best Life Stage for Proper Identification

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.

Similar Species

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).

Best Time to See

Mature fruits are present from late May to early July.

  • Fruiting

The time of year you would expect to find Swamp Oats fruiting in New York.

Swamp Oats Images


Swamp Oats
Sphenopholis pensylvanica (L.) A.S. Hitchc.

  • Kingdom Plantae
    • Phylum Anthophyta
      • Class Monocotyledoneae (Monocots)
        • Order Cyperales
          • Family Poaceae (Grass Family)


  • Trisetum pensylvanicum (L.) Beauv. ex Roemer & J.A. Schultes

Additional Resources

Best Identification Reference

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.

Other References

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. 2024. 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


About This Guide

Information for this guide was last updated on: January 7, 2016

Please cite this page as:
New York Natural Heritage Program. 2024. Online Conservation Guide for Sphenopholis pensylvanica. Available from: https://guides.nynhp.org/swamp-oats/. Accessed April 16, 2024.