Horses mane weed, lakeweed
Vascular – Native
Herbs - Monocots
2n = 18
Current conservation status
The threat classification status of all known New Zealand vascular plant taxa at the rank of species and below were reassessed in 2017 using the New Zealand Threat Classification System (NZTCS) – more information about this can be found on the NZTCS website This report includes a statistical summary and brief notes on changes since 2012 and replaces all previous NZTCS lists for vascular plants. Authors: By Peter J. de Lange, Jeremy R. Rolfe, John W. Barkla, Shannel P. Courtney, Paul D. Champion, Leon R. Perrie, Sarah M. Beadel, Kerry A. Ford, Ilse Breitwieser, Ines Schönberger, Rowan Hindmarsh-Walls, Peter B. Heenan and Kate Ladley. Please note, threat classifications are often suggested by authors when publications fall between NZTCS assessment periods – a suggested threat classification status has not been assessed by the NZTCS panel.
2017 | Not Threatened | Qualifiers: SO
Previous conservation statuses
2012 | Not Threatened
2009 | Not Threatened
2004 | Not Threatened
Indigenous. North, South and Chatham Islands. Also in Australia
Saline ponds, lagoons, brackish streams, slow flowing fresh water streams and fresh water lakes from sea level to 700 m a.s.l.
Wetland plant indicator status rating
Information derived from the revised national wetland plant list prepared to assist councils in delineating and monitoring wetlands (Clarkson et al., 2021 Manaaki Whenua – Landcare Research Contract Report LC3975 for Hawke’s Bay Regional Council). The national plant list categorises plants by the extent to which they are found in wetlands and not ‘drylands’. The indicator status ratings are OBL (obligate wetland), FACW (facultative wetland), FAC (facultative), FACU (facultative upland), and UPL (obligate upland).
OBL: Obligate Wetland
Almost always is a hydrophyte, rarely in uplands (non-wetlands).
Rhizome prominent, and either much branched and congested giving rise to short stems and long leaves in dense grass-like mats or less closely branched with longer, sparsely branched stems. Leaves c.50–300 × 0.3–0.4 mm, dark green, almost filiform and only slightly flattened; apex obtuse, often most minutely denticulate; sheath (10-)25–35–50 mm long, auricles narrow. Peduncles to 0.5 m long, usually conspicuously thicker just below flowers, becoming spirally coiled as fruit matures. Flowers protandrous; carpels (5-)8(-16). Podogynes slender. Achenes (1.7-)2(-2.7) mm long, asymmetric, brown; mesocarp thin; endocarp smooth, black, with 2 longitudinal slits opposite to and about on level of top of operculum; beak 0.2–0.3 mm long.
Ruppia polycarpa differs from R. megacarpa by its curved to straight rather than zig-zagged branching pattern, obtuse rather than bidentate leaf apices; carpels usually 4 per flower (rather than (5-)8(-16)), and fruits which are 4-5 mm long rather than < 3 mm long. Stuckenia pectinata which sometimes grows with both species of Ruppia is superficially similar. It is easily distinguished from both Ruppia species by the ligulate acute-tipped leaves, tuberous stems, and spicate inflorescences.
October - February
October - May
Uncertain. Probably easily grown in suitable conditions but Ruppia is unlikely to be widely cultivated (if at all). Most people regard it as a pest and are only interested in eradicating it.
ruppia: Named after Heinrich Bernhard Ruppius (1689-1719), an 18th century German botanist
Where To Buy
Not commercially available
Fact sheet prepared for NZPCN by P.J. de Lange 2 March 2011. Description adapted from Moore & Edgar (1970).
References and further reading
Moore, L.B.; Edgar, E. 1970: Flora of New Zealand Vol. II. Wellington, Government Printer.
NZPCN Fact Sheet citation
Please cite as: de Lange, P.J. (Year at time of access): Ruppia polycarpa Fact Sheet (content continuously updated). New Zealand Plant Conservation Network. https://www.nzpcn.org.nz/flora/species/ruppia-polycarpa/ (Date website was queried)