Drainage reserve smothered by thick azolla bloom

FLOATING plants have invaded a waterway on the edge of Tea Gardens, believed to be another side-effect of the year of rain experienced in 2025.

Azolla, a free-floating mini-fern plant, has covered Dunshea Lake, an artificially enlarged drainage reserve on the western side of the town that receives run-off from approximately 13 hectares and dozens of houses in Dunshea Estate.

Most of the drainage lake is now covered in a thick vegetative carpet of the native fern, which nearby residents fear will lead to a kill-off of the local mullet that can be found within the 12,000sqm body of water.

“Azolla can quickly spread to cover open areas of water and may build up in stationary water bodies, particularly if nutrients levels are adequate, and azolla produces spores and can spread by fragments,” the NSW Department of Primary Industries states.

“Azolla is not declared as a weed in Australia.”

According to Agriculture Victoria, the occurrence of a vigorously growing population of azolla generally indicates high nutrient levels in the water, namely phosphates and nitrates, which are commonly found in garden fertiliser, among other sources.

“Excess growth of the plant can cause problems including blocked pump inlets and filters, impeded flow where plants bank up at structures, reduced sunlight penetration of waters, and when azolla dies off it can reduce oxygen levels in the dam,” Agriculture Victoria states.

Residents say this is the largest azolla blooming seen in the area in many years.

Concerns are held that the ducks and black swans feeding off the azolla infestation may be taking on unnatural chemical compounds that could prove potentially lethal for them, while the fish below could be deprived of oxygen should the plant continue to grow unabated.

The plant could also cause blockages of stormwater infrastructure, which could lead to dirty water back-flooding onto the streets that the lake was originally designed to keep drained and clear.

By Thomas O’KEEFE