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Riverweed
Podostemum ceratophyllum plants
Photo credits: Stephen M. Young
Scientific Name
Podostemum ceratophyllum
Michx.
Family Name
Podostemaceae
Did you know?
This is the only member of this genus and family
that occurs in North America. The genus is
confined to the New World (Philbrick and Novelo
2004). The genus name Podostemum comes from
the greek words for foot and stamen and is in
reference to the stamens appearing "raised on a
stalk by the side of the ovary" (Fernald 1970). The
specific epithet ceratophyllum means horn-leaf
(Fernald 1970) and may be in reference to the
narrow stiff leaves of this species.
Summary
Protection Threatened in New York State, not listed federally.
This level of state protection means: listed species are those with: 1) 6 to fewer than 20
extant sites, or 2) 1,000 to fewer than 3,000 individuals, or 3) restricted to not less than 4
or more than 7 U.S.G.S. 7 ½ minute topographical maps, or 4) listed as threatened by
U.S. Department of Interior.
Rarity G5, S2
A global rarity rank of G5 means: This species is demonstrably secure globally, though it
may be quite rare in parts of its range, especially at the periphery.
A state rarity rank of S2 means: This plant is threatened/imperiled in New York because of
rarity (typically 6-20 populations or few remaining individuals) or is vulnerable to extirpation
from New York due to biological factors.
Conservation Status in New York
There are 16 known extant populations and 8 populations which have not been seen in
recent years and are considered historical. Many of the populations are clustered along
particular rivers. Habitat is somewhat limited for this species but certainly not rare.
NYNHP Conservation Guide - Riverweed (Podostemum ceratophyllum)
Short-term Trends
Fifteen out of the 16 known extant populations have first been documented in the past 30
years. This would seem to point to an increase in this species but since this species is
often overlooked, resembles algae, and grows in habitat that make it difficult to detect, it is
equally possible that these "new" populations were simply overlooked in the past. Overall,
short term trends are not clear but probably are at least stable.
Long-term Trends
No population of this species is known to have become extirpated. Eight populations have
not been seen in over 25 years. These populations may still be extant, as targeted surveys
at these populations have not been conducted. The habitat for this species may have
decreased some in the past 100+ years as rivers and streams were developed and/or
polluted. Overall long term trends are not clear.
Conservation and Management
Threats
Some of the streams where this species occurs may be used for a water source. This
development could result in a change to the stream that would make the habitat less
suitable for riverweed. Other populations are threatened by water quality deterioration as a
result of development, agriculture, and run off from a sewage treatment plant (New York
Natural Heritage Program 2007). Negative impacts on rivers including siltation, nutrient
pollution, and dam building have been considered threats to this species (Quiroz et al.
1997 in Philbrick and Novelo 2004).
Conservation Strategies and Management Practices
Water quality of the rivers where this species occurs, should be protected.
Research Needs
Surveys to historical populations are needed to determine if these populations are still
extant.
Habitat
Riverweed grows submerged (up to 75+ cm deep) to seasonally exposed on cobbles and bedrock
substrate in fast flowing, relatively large streams or rivers. It often occurs in rapids (New York
Natural Heritage Program 2008). Fast-moving streams, especially rapids (Crow and Hellquist
2000). In rapidly flowing water (Gleason and Cronquist 1991). On rocks in streams (Fernald 1970).
Associated Ecological Communities
Confined River
The aquatic community of relatively large, fast flowing sections of streams with a moderate
to gentle gradient.
Rocky Headwater Stream
NYNHP Conservation Guide - Riverweed (Podostemum ceratophyllum)
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The aquatic community of a small- to moderate-sized perennial rocky stream typically with
a moderate to steep gradient, and cold water that flows over eroded bedrock, boulders, or
cobbles in the area where a stream system originates.
Associated Species
Cladophora sp.
Identification Comments
Riverweed is an olive-green, aquatic plant of firm texture that grows in streams and rivers
attached to rocks. It resembles seaweed, alga, or moss and attaches to rocks by fleshy
disks. The stems are 2-30 cm long and often branched. The leaves are very narrow and
divided into numerous linear lobes. The flowers are small, not showy, and scattered along
the stems (Fernald 1970, Gleason and Cronquist 1991, Philbrick and Novelo 2004).
Best Life Stage for Identifying This Species
It is easiest to identify this plant when it is in flower or fruit although vegetative material,
when not confused with alga or moss, is also relatively easy to identify.
The Best Time to See
Flowers and/or fruits are present from mid-June through early October. Since this species
often grows submerged in rapids and fast moving water it can be difficult to spot.
Therefore, the best time to survey for this species is between mid-June and early October
and when water levels are low.
Vegetative
Flowering
Fruiting
Jan Feb Mar Apr May Jun Jul
Aug Sep Oct Nov Dec
The time of year you would expect to find Riverweed in New York.
Similar Species
This species is perhaps superficially similar to alga and moss but can be distinguished from
both by its true although minute flowers, its firm texture, fleshy disks (which attach to the
substrate), and stipule-like appendages at the bases of the leaves. The latter three
characters will also help separate this species from other aquatic vascular plants.
Conservation Comments
When Michaux (1803) described this genus he used the spelling "Podostemum" in the text
and then in a plate illustrating Podostemum ceratophyllum he used the spelling
"Podostemon". It appears that Michaux's Podostemon was in error (Sprague 1933) and the
family Podostemaceae (based on Podostemum) has been conserved as the correct
NYNHP Conservation Guide - Riverweed (Podostemum ceratophyllum)
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spelling (McNeill et al. 2006). Podostemon is therefore considered an orthographic variant
and is to be replaced by the correct spelling Podostemum.
Taxonomy
Kingdom
Plantae
Phylum
Anthophyta
Class
Dicots (Dicotyledoneae)
Order
Podostemales
Family
Podostemaceae
Additional Common Names
Horn-leaved riverweed
River-moss
Threadfoot
Synonyms
Podostemon ceratophyllum (Michaux)
Additional Resources
Links
New York Flora Atlas
http://www.newyork.plantatlas.usf.edu/Plant.aspx?id=2412
USDA Plants Database
http://plants.usda.gov/java/nameSearch?mode=sciname&keywordquery=
PODOSTEMUM+CERATOPHYLLUM
NatureServe Explorer
http://natureserve.org/explorer/servlet/NatureServe?searchName=PODOSTEMUM+CERA
TOPHYLLUM
Google Images
http://images.google.com/images?q=PODOSTEMUM+CERATOPHYLLUM
References
Crow, Garrett E. and C. Barre Hellquist. 2000. Aquatic and Wetland Plants of Northeastern North
America: A revised and enlarged edition of Norman C. Fassett's a Manual of Aquatic Plants.
Volume One: Pteridophytes, Gymnosperms, and Angiosperms: Dicotyledon
Fernald, M.L. 1950. Gray's manual of botany. 8th edition. D. Van Nostrand, New York. 1632 pp.
Gleason, Henry A. 1952. The New Britton and Brown Illustrated Flora of the Northeastern United
States and Canada.
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.
NYNHP Conservation Guide - Riverweed (Podostemum ceratophyllum)
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Haines, Arthur and Thomas F. Vining. 1998. Flora of Maine. A Manual for Identification of Native
and Naturalized Vascular Plants of Maine.
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.
Keys, Jr.,J.; Carpenter, C.; Hooks, S.; Koenig, F.; McNab, W.H.; Russell, W.;Smith, M.L. 1995.
Ecological units of the eastern United States - first approximation (cd-rom), Atlanta, GA: U.S.
Department of Agriculture, Forest Service. GIS coverage in ARCINFO format, selected imagery,
and map unit tables.
McNeill, J., F.R. Barrie, H.M. Burdet, V. Demoulin, D.J. Hawksworth, K. Marhold, D.H. Nicolson, J.
Prado, P.C. Silva, J.E. Skog, J.H. Wiersema, and N.J. Turland (Editors). 2006. International
Code of Botanical Nomenclature (Vienna Code) adopted by the Sev
Michaux, A. 1803. Flora boreali-americana, sistens caracteres plantarum quas in America
septentrionali collegit et detexit Andreas Michaux. Apud Fratres Levrault, Paris and Strasbourg,
France.
NatureServe. 2005. NatureServe Central Databases. Arlington, Virginia. USA
New York Natural Heritage Program. 2010. Biotics database. New York Natural Heritage Program.
New York State Department of Environmental Conservation. Albany, NY.
Philbrick, C.T. and A. Novelo R. 2004 Monograph of Podostemum (Podostemaceae). Systematic
Botany Monographs 70: 1-106.
Reschke, Carol. 1990. Ecological communities of New York State. New York Natural Heritage
Program, New York State Department of Environmental Conservation. Latham, NY. 96 pp. plus
xi.
Rhoads, Ann F. and Timothy A. Block. 2000. The Plants of Pennsylvania, an Illustrated Manual.
University of Pennsylvania Press, Philadelphia, PA.
Sprague, T.A. 1933. Podostemaceae or Podostemonaceae. Bulletin of Miscellaneous Information
(Royal Gardens, Kew) 46: 46.
Weakley, A. S. 2007. Flora of the Carolinas, Virginia, Georgia, and surrounding areas. Working draft
of 11 January 2007. University of North Carolina Herbarium (NCU), North Carolina Botanical
Garden, University of North Carolina at Chapel Hill. Online. Av
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/
Weldy, Troy W. and David Werier. 2005. New York Flora Atlas. [S.M. Landry, K.N. Campbell, and
L.D. Mabe (original application development), Florida Center for Community Design and
Research. University of South Florida]. New York Flora Association, Albany,
New York Natural Heritage Program
This project is made possible with funding from:
625 Broadway, 5th Floor,
- New York State Department of Environmental Conservation Hudson River
Albany, NY 12233-4757
Estuary Program
Phone: (518) 402-8935
- Division of Lands & Forests, Department of Environmental Conservation
[email protected]
- New York State Office of Parks, Recreation and Historic Preservation
Information for this guide was last updated on Nov 23, 2015
This guide was authored by
NYNHP Conservation Guide - Riverweed (Podostemum ceratophyllum)
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