Salvinia molesta

This South American perennial free floating aquatic fern with slender stems, floating leaves and a root-like structure forms dense mats with multi-branched horizontal stems. Individual plants are 5-30cm long and invade still or slow-moving water bodies.

Salvinia has the potential to spread to much of Australia and is regarded as a very serious threat to waterways and irrigation areas because it: 

  • disrupts aquatic ecosystems, seriously affecting native animals and plant life;
  • decreases the quality of water by causing odours, accumulation of organic matter and stagnation of streams;
  • degrades the aesthetic value of waterways;
  • reduces or prevents the use of waterways for recreation and transport;
  • interferes with the functioning of river control structures, especially during flooding.

The main infestations of salvinia are found in coastal streams from Cairns in North Queensland to the South coast of New South Wales. Although infestations have also been recorded near Perth, Darwin, Melbourne and Adelaide. To date, only isolated infestations have been recorded on inland waterways. In NSW, salvinia is common in the Tweed, Richmond, Clarence and Macleay catchments, the central coast and metropolitan areas. 

Salvinia is still found in aquarium and rockery ponds as an illegally-propagated aquatic plant. This Noxious Weed has been spread in Australia mostly by humans and despite it being labelled as being noxious it is still being traded as an aquarium plant. 
Salvinia is dispersed by fragments in floodwater, with human activity, animals and water birds.  Salvinia is capable of vegetative spread in mainly two ways:
  1. by breaking into daughter plants; and
  2. by the separation of young growth through death or damage of the parent material connecting these sections.
A number of branches can develop at each node and give rise to individual plants following the separation from the parent plant. Damaged or dead plant material stimulates the development of buds into branches, and the whole process sets off once again.

Salvinia is a weed of still and slow flowing fresh water and it is very adaptable and will survive in many climates, although low temperatures will reduce its growth somewhat. It can withstand the odd frost but persistently low temperatures will kill the exposed portions of the plants. Where a thick mat of weed is present and there is enough protection from frosts regeneration can occur successfully. 

Salvinia seems to thrive in warm, nutrient rich water. Under these circumstances it can double its mass in just two days! It will not, however survive in sea water.

An ant-sized weevil is playing a vital role in helping control salvinia in our part of the world – the north coast of NSW. The salvinia weevil (Cyrtobagous alviniae) is originally from south-eastern Brazil and was introduced into Australia by the CSIRO in 1980 to combat this weed. These weevils were first released into Lake Moondarra, near Mount Isa and in less than 12 months destroyed an estimated 30,000 tonnes of weed to clear the 800 hectare lake!

In warmer tropical regions of Queensland and in Papua New Guinea it has been an extremely effective biological control agent. On the NSW North Coast there has been mixed results with the weevil – some instances it has been very effective killing off infestations within 12 months while in other locations it has taken 2-3 years. The two main factors influencing the salvinia weevil’s success are temperature and nitrogen levels in salvinia plants. When temperatures drop below 19 degrees C, the weevil lays fewer eggs. The ideal temperature for their development is about 30 degrees C, which enables a life cycle to be completed in about six weeks. This life cycle will take longer if temperatures are reduced.
It would seem that we all need to be vigilant in not allowing this aquatic noxious weed to escape from aquariums and ponds. Scoop it out, bag it and place in the Council Red bin. So if you have it in your pond or aquarium it’s time to get rid of salvinia before there is even the remotest chance of it escaping into waterways. 
Information for this post was taken from ‘Noxious Weeds (Weed Control) Order 2014’ published in the NSW Government Gazette.

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