The floating rosettes of this perennial are found in shallow, calcareous, mesotrophic or meso-eutrophic water in the sheltered bays of lakes or in ponds, canals and ditches. Reproduction is primarily vegetative; viable seed is sometimes set, but seedlings are probably rare. Lowland.
This species has declined in Britain, having suffered from the conversion of grazing marshes to arable, and from eutrophication. It is more stable in Ireland, where it is more frequent in lakes. It is increasingly planted or discarded in the wild.
Eurosiberian Temperate element; widely naturalised outside its native range.
Light (Ellenberg): 7
Moisture (Ellenberg): 11
Reaction (Ellenberg): 7
Nitrogen (Ellenberg): 7
Salt Tolerance (Ellenberg): 0
January Mean Temperature (Celsius): 3.9
July Mean Temperature (Celsius): 15.8
Annual Precipitation (mm): 772
Perennation - primary
Life Form - primary
Clonality - primary
Count of 10km squares in Great Britain: 323
Count of 10km squares in Ireland: 84
Count of 10km squares in the Channel Isles: 2
Atlas Change Index: -0.89
Scarce Atlas Account
Hydrocharis morsus-ranae L.
Status: not scarce
H. morsus-ranae is a free-floating species which typically occurs as a dominant of grazing marsh ditches, although it is also widespread in farm ponds and canals. Its preferred habitat is still (or very slow-flowing) water which is neutral to base-rich with moderate levels of nitrogen and other nutrients. Characteristic associates include Lemna gibba, L. trisulca, Spirodela polyrhiza and, locally Stratiotes aloides. H. morsus-ranae is intolerant of shade from tall emergents such as Phragmites australis and competes poorly with water-lilies and other large floating-leaved species. It is much commoner in sheltered sites protected from the wind, and on grazing marshes is generally associated with narrow field ditches rather than with broader arterial drains. H. morsus-ranae is most common where marginal grazing or occasional mechanised clearing of ditches suppresses the growth of emergents. On grazing marshes, therefore, it rarely persists in ditches adjoining fields which have been converted to arable. It grows on both alluvial and peaty soils and is rarely found above 50 metres, although it occasionally occurs in ponds at higher altitudes and reaches 240 metres above Westbury-sub-Mendip.
H. morsus-ranae reproduces almost entirely vegetatively. It is a stoloniferous species, with daughter plants produced at the end of the stolons, and becoming detached. In autumn turions are produced which sink to the bottom of the water and in the spring float to the surface, producing a new plant. This species is a partly monoecious perennial, but fruits are rarely produced in Britain.
H. morsus-ranae has declined markedly in the last century except in those coastal flatlands (e.g. the Somerset Levels and Moors) which remain mostly under pasture. It has suffered from the conversion of grazing marsh to arable, the elimination of ditches and changes in their management. This rapid decrease has also been experienced by its relative and associate, Stratiotes aloides.
H. morsus-ranae is found throughout temperate Eurasia, but it is generally local and is probably declining throughout its range. It is rare in the Mediterranean region. It is naturalised in North America.
J. O. Mountford