Azolla growth in farm dams

The presence of azolla is evident in many farm dams across Victoria. Azolla is a native aquatic plant found in still or slow-moving water bodies. Azolla appears green or red depending on exposure to the sun. In shaded conditions the leaves are usually green, while in direct sunlight they become reddish.

Azolla and how it grows

Azolla is a common free-floating fern up to 10 to 30 millimetres in diameter with roots hanging down to about 40 millimetres below the water surface. Azolla fronds float on the surface of the water individually or as large mats. Two species of azolla are commonly found in Australia, including Azolla pinnata and Azolla filiculoides. The plant supports nitrogen fixing bacterium, which allows it to use nitrogen from the water and air for its own growth. The occurrence of a vigorously growing population of azolla in a farm dam generally indicates high nutrient levels in the water. Azolla can survive within a water pH range of 3.5 to 10, but optimum growth occurs in the pH range of 4.5 to 7 and temperature range of 18°C to 26°C. Under optimum conditions, azolla’s growth spreads across the dam surface until it covers the surface of the water in a dense cover. Azolla can double its leaf area in seven days if conditions of high nutrient levels and water temperatures persist.

Is azolla a problem?

Azolla is not harmful to stock that drink the water or consume the plant. Azolla does not have a harmful effect on domestic house and garden water. However, excess growth of the plant can cause the following problems:

• blocked pump inlets and filters
• impeded flow where plants bank up at structures
• restricted stock access to drinking water if plant density is sufficient
• reduced sunlight penetration of dam waters
• when azolla dies off it can reduce oxygen levels in the dam.

The need to control excessive growths of azolla due to these problems should be carefully evaluated. Reconfiguration or relocation of pump suction hoses, inlets and filters and the establishment of off-dam watering troughs may be adequate to address the common problems associated with azolla. Azolla in farm dams may be vital in the ecology of the dam and if controlled the benefits of azolla could be lost, including reduced bank erosion, provision of habitat for aquatic life and reduced water evaporation rates and temperatures. 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.

What should I do?

If azolla growth becomes a problem and control is deemed necessary, it is best achieved by mechanical or manual removal, such as scraping it off the top of the dam with a scoop net. Some chemical controls are available to help control azolla; please contact your local chemical reseller about these. Dead azolla in your dam can reduce the oxygen in the water for a period of time. If azolla is left to dry for a few hours, it dies off. This method has the advantage of removing nutrients in the plants rather than returning them to the water as the plants break down. Excessive growth is most easily prevented by limiting the amount of available nutrients in the dam which make conditions favourable for azolla.

Prevention tips:

1. Limit the influx of nutrients to the dam by preventing runoff carrying fertilisers and sediment from entering the dam.
2. Establish buffer strips of native grass, shrubs and trees above the dam and along drainage lines into the dam to intercept some nutrients and soil particles from pastures and cropping areas.
3. Minimise sunlight on the dam by planting native vegetation on the north and west sides of the dam. Do not plant trees on the embankment.
4. Limit stock access to the dam by providing off-dam watering troughs to stop stock defecating and urinating in the water.