Managing water weeds

Article by Rhea Phelan, vector and pest plant officer, Sunshine Coast Council

It's time to talk about managing water weeds for healthy waterways and dams!

With all the recent rain during spring and early summer, some areas receiving double the long-term average in late 2021, the water has been filling the catchments and flowing across the land into dams, waterways and eventually into the ocean. So it’s a good time to get out and see if anything new has arrived.

There are 4 main types of aquatic plants; emergent, floating attached, submerged and free floating. While some of these plants play a critical role in maintaining healthy dams and waterways, invasive plants on the other hand degrade them. Before you get started with weed management make sure you get to know the difference between beneficial native plants and invasive ones.

Emergent plants are those that grow at the edge of the waterway. These plants stabilise the bank and filter water flowing into the waterway as well as provide great habitat for wildlife. Emergent invasive plants, however, reduce biodiversity, ecosystem function and overtime water quality.

Floating attached plants include plants like water lilies which are shelter for fish and habitat for lily trotters like water hens and Jacanas. Invasive water lilies can outcompete native ones and reduce biodiversity.

Submerged plants are those that grow beneath the water, help to maintain water quality, oxygenate water and provide habitat for aquatic organisms. Submerged invasive plants will outcompete native ones and reduce diversity of plant life over time, degrading water quality.

Floating aquatic plants play a role in nutrient cycling, providing food and habitat for aquatic organisms and oxygenate the water. Floating invasive plants like water hyacinth, water lettuce and salvinia can severely impact waterways by clogging them up and degrading water quality. To maintain healthy waterways and dams we need to control the plants that degrade them.

Prevention is the key to managing aquatic plants, if you have a waterway or dam on your property that is weed free and has a diversity of native plants then consider yourself fortunate and take steps to keep it healthy. When you bring in watercrafts check them for weeds or weed seeds. Stick to planting native plants in and around the area and keep an eye on what starts growing after a flood event as this is usually when new weeds are introduced. Planting a buffer of native plants where water flows through to the waterway in floods, can act as a barrier to new weed incursions.

There are few herbicides approved for use in and around waterways due to the way chemicals travel in water and the sensitivity of aquatic organisms to them. Because of this it is important to take an integrated weed management approach drawing on different management tools. For example, an integrated weed management approach to alligator weed (Alternanthera philoxeroides) could involve a combination of foliar spraying of different herbicides, lowering water levels to expose the weed to the sun and dry it out, manual removal and replanting with native freshwater plants.

An integrated weed management approach may also involve using the plant for composting or for animal fodder however, the risk of spread from seed would need to be considered. For example, while salvinia may be suitable for composting, water hyacinth is not as there is a risk of spread from seed but it may be possible to use it as fodder for sheep or use the plant for fibre.

Integrated weed management is about thinking creatively about practical long-term sustainable management. Make sure your efforts aren’t wasted because you are too reliant on one management tool that might not work in every situation.

Below are some important things to consider before getting started to help improve outcomes of weed control.

Are there excessive nutrients going into the waterway or dam and can these be managed by creating a buffer of native plants between the surrounding land/farm and the dam or waterway?

What management tools and resources are available to you?

What is the timeframe for seed dormancy of the weed/s? For example, some aquatic weed seeds can stay dormant for up to 10 years or more which means you will need to manage them for this amount of time.

How does the plant reproduce, how often does it set seed? Can you time control work to prevent it from seeding?

Once you have controlled the weedy aquatic plants, are there any plants left to stabilise the banks, oxygenate the water, provide food and shelter for wildlife, cycle nutrients and improve water quality? If not, it may be necessary to put in some native wetland plants and start recreating a healthy aquatic ecosystem.

References

• Chandrasena, N. and Pinto L. (2000) Integrated Management of Alligator Weed (Alternanthera philoxeroides) at Botany Wetlands, Sydney- A Case Study. Asia-Pacific Weed Science Soc. Conf., Colombo, Sri Lanka. (21) 59-64
• Perez, A.E., Trinidad, R.T., Maqueda, S.R., Linares P.J., Vazquez, F.M. Medina, P.L., Moreno, J.L., Gallego, F.L., Cortes, J.G., Guzman, J.S. (2015) Seed germination and risks of using the invasive plant Eichhornia crassipes (water hyacinth) for composting, ovine feeding and biogas production. Acta Botanica Gallica. 162(3) 203-214. DOI: 10.1080/12538078.2015.1056227
• Dorahy, C.J. Pirie, A.D. McMater, I., Muirhead, L., Pengelly, P., Chan, K.Y., Jackson, M., Barchia, M. (2009) Environmental Risk Assessment of Compost Prepared from Salvinia, Egeria densa, and Alligator Weed. Journal of Environmental Quality. 38(4)
• B. Gopal and D. Ghosh (2008) Natural Wetlands. Encyclopaedia of Ecology. 2493-2504.