Lagarosiphon major (Ridl.) Moss

Family: Hydrocharitaceae

Common names:  oxygen weed

Disseminule:  stem fragments

Brief description

A perennial, submersed, dioecious aquatic herb with branched stems to 5 m long rooted in the soil; stems curve downwards near the water surface. Leaves spirally alternate, recurved. Tiny white female flowers reach above the water surface on long tubes; male flowers form in axils, break off and rise to the surface, where they bump into and pollinate female flowers. Fruit a capsule with ca. 9 seeds.

Identification remarks

Was mistakenly thought to be Elodea canadensis Mitchx. in New Zealand and is often called Elodea crispa in the aquarium trade. Recurved leaves, spiral leaf arrangement and downward-curving stems towards apex help to distinguish Lagarosiphon major from Egeria, Elodea and Hydrilla.

Distribution

South Africa, Zimbabwe, New Zealand, and in Europe: Austria, France, Germany, Italy, Portugal, Spain, Switzerland, United Kingdom.

Native to southern Africa.

Habitat

Temperate; cool quiet waters; slow-moving streams, ponds, lakes.

General information

Only the female plants of Lagarosiphon major grow outside of its native range, so reproduction where naturalized is only by vegetative stem fragments. These fragments are carried by flowing water and produce adventitious roots when they sink to the bottom. The curving, interwoven, spreading stems form dense subsurface mats that can be free-floating. These mats block light and may totally fill the volume of shallow lakes. Lagarosiphon major was introduced to New Zealand in 1950; it has now infested many of this country's lakes, obstructing navigation and water use, outcompeting native species, and once actually caused the shutdown of a hydroelectric station.

Photo: V. Ramey © University of Florida

Photo: V. Ramey © University of Florida

Photo: John Clayton © National Institute of Water & Atmospheric Research, New Zealand (NIWA)