Cape Pondweed Eradication

The aquatic plant Cape pondweed (Aponogeton distachyos), is a South African species and has been in Waiwhetū Stream for more than 100 years. It was apparently deliberately spread in New Zealand as duck food although it may have been planted as an ornamental aquatic species.

Cape pondweed in Waiwhetū Stream was considered a nuisance weed by Greater Wellington Regional Council’s (GWRC) Flood Protection Group because its abundance impeded water flows.

The local community considered it an environmental weed in that it reduced the aesthetics of the stream, caused extensive surface algal blooms in summer, collected floating domestic rubbish, captured sediment to enhance seedling establishment, and caused sediment to remain suspended in the water column creating muddy water.

Early in 2011, Dr Merilyn Merrett, Senior Lecturer at the Open Polytechnic (OP), Lower Hutt, used her knowledge of plant reproductive biology to undertake research into Cape pondweed. The OP campus borders the stream in the suburb of Waterloo. They supported Merilyn’s research.

Merilyn’s research enabled an understanding of the growth, flowering, seed set, and root structure of cape pondweed; knowledge that led to the development of a strategy to eradicate the weed.

Between September 2011 and May 2014, ‘pondweed’ volunteers from both the OP and Friends of Waiwhetū Stream (FWS) manually dug the weed out of the stream bed. The OP group met at lunch time on Fridays and the FWS group dug fortnightly on Sunday afternoons.

Volunteer effort was supported by both GWRC and Hutt City Council (HCC) by providing waders and gloves and also the removal of piles of pondweed and very large quantities of other rubbish removed from the stream bed.

The total number of volunteer hours taken to remove Cape pondweed is in the vicinity of 1100 and the approximate number of  plants removed around 300000.  A remarkable achievement!

Since the weed has been removed, the stream bed is visible, water flows are much improved, algae growth during summer is much reduced, and there is less sediment deposition resulting in a 50% increase in riffle length.