September 2011 Azolla growth in farm dams

The presence of azolla is evident in many farm dams Is azolla a problem? across Victoria. Azolla is a native aquatic plant found in still or slow-moving water bodies. Azolla appears green Azolla is not harmful to stock that drink the water or consume the plant. Nor does azolla have a harmful effect or red depending on exposure to the sun. In shaded on domestic house and garden water. conditions the leaves are usually green, whilst in direct sunlight they become reddish. However, excess growth of the plant can cause the following problems: Azolla and how it grows • Blocked pump inlets and filters • Impeded flow where plants bank up at structures Azolla is a common free floating fern up to 10 to 30 millimetres in diameter with roots hanging down to about 40 millimetres • Restricted stock access to drinking water if plant density below the water surface. Azolla fronds float on the surface of is sufficient the water individually or as large mats. • Reduced sunlight penetration of dam waters Two species of Azolla are commonly found in Australia, including Azolla pinnata and Azolla filiculoides . The plant When azolla dies off it can also reduce oxygen levels in supports nitrogen fixing bacterium, which allows it to use the dam. nitrogen from the water and air for its own growth. The need to control excessive growths of azolla due to the The occurrence of a vigorously growing population of azolla in problems listed above should be carefully evaluated. a farm dam generally indicates high nutrient levels in the water. Azolla can survive within a water pH range of 3.5 to 10, Reconfiguration or relocation of pump suction hoses, inlets but optimum growth occurs in the pH range of 4.5 to 7 and and filters and the establishment of off dam watering temperature range of 18 oC to 26 oC. troughs may be adequate to address the common problems associated with azolla. Under optimum conditions, azolla’s growth spreads across the dam surface until it covers the surface of the water in a dense Azolla in farm dams may well be vital in the ecology of the cover. Azolla can double its leaf area in seven days if dam and if controlled the benefits of azolla could be lost, conditions of high nutrient levels and water temperatures including reduced bank erosion, provision of habitat for persist. aquatic life and reduced water evaporation rates and temperatures.

In addition, azolla takes up the nutrients in the water to enable it to grow, and therefore restricts the nutrients available for the growth of other nuisance plants and potentially toxic blue green algal blooms.

Photo: Azolla plants covering 95% of a dam’s surface as a thick vegetative mat Azolla growth in farm dams

What should I do? Prevention tips

If azolla growth becomes a problem and control is deemed • Limit the influx of nutrients to the dam by preventing runoff necessary, it is best achieved by mechanical or manual carrying fertilisers and sediment from entering the dam. removal, such as scraping it off the top of the dam with a scoop net. • Establish buffer strips of native grass, shrubs and trees Some chemical controls are available to help control azolla, above the dam and along drainage lines into the dam to please contact your local chemical reseller about these. intercept some nutrients and soil particles from pastures Please note that dead azolla in your dam can reduce the and cropping areas. oxygen in the water for a period of time. • Minimise sunlight on the dam by planting native vegetation If azolla is left to dry for a few hours it dies off. This method on the north and west sides of the dam. Do not plant trees has the advantage of removing nutrients in the plants rather on the embankment. than returning them to the water as the plants break-down. • Limit stock access to the dam by providing off dam Excessive growth is most easily prevented by limiting the watering troughs to stop stock defecating and urinating in amount of available nutrients in the dam which make the water. conditions favourable for azolla.

Photo: Close up of azolla. The amount of sunlight influences whether Azolla appears green or red.

Authored by Tracey Walker in 2002 and reviewed by Rachael Campbell from the Victorian Department of Primary Industries in 2011.

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