Growing Gardens

with Julia Hancock

Pond Weeds

Ponds that have become overgrown in the summer can have their plants thinned out now, but more important is the eradication of any weeds that may have proliferated during the summer months.

There are currently several serious aquatic weeds causing major problems in waterways throughout Australia, some of which can also be found in backyard pools and ponds.

In the Richmond River catchment area there are three main offenders to look out for: cabomba (Cabomba caroliniana), salvinia (Salvinia molesta) and water hyacinth (Eichhornia crassipes). All three of these species originally came from South America and were most likely imported by aquarium retailers whose customers inadvertently tossed out fragments of these weeds when they cleaned out their tanks.

Cabomba's fan-shaped and feathery leaves trail just beneath the surface of still or gently flowing water on stems up to 10m long. Pale yellow flowers appear for most of the year and are held on stalks above the surface. Severe infestations smother out any native aquatic plants growing nearby.

Salvinia is a spongy, mat-like plant with the ability to double in size within a week - even the smallest fragment will reproduce ad infinitum. The good news is that successful research has been undertaken using the salvinia weevil as a biological control. And salvinia plants scooped out of the pond and onto the compost heap break down quickly and add valuable nutrients to the soil.

Water hyacinth has an entrancing violet blue flower, which is why some people initially chose to cultivate it in their home water features. The downside is that it is phenomenally successful at reproducing itself and if it escapes into waterways it can do irreparable damage. Again, hopes of eradication rest on biological control.

If you suspect you may be harbouring some of these undesirables but need help identifying your pond weeds, Lismore City Council provides useful information.