Water weeds

Water hyacinth

Hyacinth can block entire river systems

Hyacinth can block entire river systems, shading out the river and starving the water of oxygen
Photo: Greg Calvert

In north Queensland, three floating plants are declared weeds. Water hyacinth, whose pretty blue flowers inspired its introduction into fish ponds and aquariums, quickly found its way into local waterways and spread rapidly. It can block entire river systems, shading out the river and starving the water of oxygen. All the native water plants die in the deep shade and fish can suffocate. Although some people use it for mulch, such harvesting does not appear to make much of a dent. Due to active transpiration of water through the leaves, water hyacinth acts like floating water pumps and contributes greatly to evaporation of water from dams.

Salvinia molesta

The hyacinth shares north Queensland's local waterways with another weed: Salvinia molesta, so named because it molests and destroys water ways.

Salvinia can destroy water ways

Salvinia: the molester and destroyer of water ways
Photo: Greg Calvert

Salvinia is actually a floating fern and, like hyacinth, tends to reproduce vegetatively rather than sexually. It grows extremely fast, doubling in size in just three days.

Once, it formed great mats across Ross River in Townsville, so thickly that people were able to use the weed to walk across the river. Nothing can live beneath such a smothering surface. Fortunately, this weed too has been reined in by the introduction of a particular weevil and it now exists in only small pockets.

Water lettuce

A third invasive water weed is the water lettuce, Pistia stratiotes. For some reason, it has not been so prolific in Queensland. Some say it is due to competition from hyacinth, however, the reduction in hyacinth has not seen an explosion in water lettuce.