Originally these weeds were introduced as ornamental plants for ponds because of their attractive flowers or foliage, or their hardiness and ability to grow quickly. The intentional and unintentional dumping of pond plants, water and unwanted fish has led to infestations of natural waterways. The spread of these weeds has also occurred through the sharing of cuttings or division of existing plants by friends and family. Birds often transport plant material through feeding and nesting, while boats can also spread the weeds when moving between different waterways.
Many of these aquatic weeds are now banned from sale in nurseries however they continue to be mistakenly sold at weekend markets, backyard sales and online websites and purchased by people unaware of the invasive nature of the plant. This article provides information on the most common aquatic plants which have become serious weeds in our waterways, and details ways in which you can help to reduce their spread.
Types of aquatic weeds
Aquatic weeds are typically categorised into four main groups depending on their growth habit. These are: emergent, free floating, floating leaf and submerged weeds.
Emergent weeds have both the stems and leaves above the waterline and are often growing on the fringes of ponds and waterways. Free floating weeds are not attached to the soil in any way but can still have root systems. Floating leaf weeds are rooted into the soil with long stems that stretch to the water surface where the leaves float. Submerged weeds are rooted into the soil and all parts of the plant are completely submerged under the water.
The following aquatic plants are declared weeds in WA and plants must be reported to the Department of Agriculture and Food, Western Australia (DAFWA) for eradication:
- Leafy elodea (Egeria densa)
- Hydrocotyl (Hydrocotyle ranunculoides)
- Parrot's feather (Myriophyllum aquaticum)
- Sagittaria (Sagittaria platyphylla)
- Salvinia (Salvinia molesta)
- Senegal tea (Gymnocoronis spilanthoides)
- Water hyacinth (Eichhornia crassipes)
- Water lettuce (Pistia stratiotes).
Controlling aquatic weed infestations poses major challenges. Waterways are environmentally sensitive systems and particular attention is needed when utilising chemicals for control. Consideration for the ecology of the infested area, including the wildlife and fish living in and around the water and other native plant life, will all have an effect on what chemicals you can use. Emergent weeds are often controlled with the use of glyphosate at the prescribed label rates. When treating emergent water weeds the application method should reduce the potential for chemicals to enter the water. Application by “painting” the herbicide directly onto the target weed, with a brush or herbicide blanket, will ensure that the chemical is applied directly to the target weed. This reduces the potential for spray droplets to enter the water.
For larger infestations of free floating and floating leaf aquatic weeds, a range of chemicals can be used, including glyphosate, diquat or 2, 4-D, when the plant is actively growing. Submerged weeds are often the hardest to control. The volume of water needs to be calculated accurately and chemicals applied uniformly to achieve the desired outcome. If the infestation is particularly large consider temporarily removing the water by drainage or pumping. The weed can then be removed manually or treated with a herbicide. After treatment the water can be returned and the waterway restored to its pre-weed status. In a home garden situation the best control available is to manually remove all parts of the plant from the pond or aquarium and burn or freeze them or allow them to dry out completely before putting them into plastic bags and into the bin.
What to do
- Do not buy aquatic weeds for your garden pond or aquariums. Always ask for native pond species which will also encourage frogs.
- Dispose of unwanted pond and aquarium plants by drying or freezing them for at least 24 hours before placing in the household bin. Do not compost aquatic weeds as many seeds can withstand drying and freezing. The plants can also be burned or solarised.
- Avoid buying aquatic weeds from markets and websites where invasive species are possibly being sold. Buy from reputable nurseries and aquarium stores only.
- Do not take plant cuttings or replants of invasive and declared plants from friends or family, as this will spread weeds further.
- Never dump aquatic plants, rocks, water or fish from your pond or aquarium into drains or waterways.
- Control any weed infestations before they spread and become a major problem.
- Educate yourself and others about aquatic weeds and their impact.
Report banned species to the Pest and Disease Information Service (PaDIS) on 1800 084 881. Photographs can be emailed to firstname.lastname@example.org for identification.
Please read the sending specimens for identification web article before sending, or taking, samples to the Pest and Disease Information Service, 3 Baron-Hay Court, South Perth, 6151, WA.