Bid To Control River Weeds

Latest efforts to eradicate weeds in the lower reaches of the Avon River have been partially successful.

Preliminary results showed that the herbicide diquat had controlled egeria and elodea but had a limited effect on native potamogetons.

The Council is particularly keen to limit the spread of egeria which can crowd out other species and raise water levels because of the volume of plant material produced. Originally from South America, it looks quite similar to common oxygen weed but it is much larger.

Initial attempts to stamp it out in the Avon River included suction dredging and the use of carpet to keep the weeds in the dark and thwart photosynthesis. When these methods failed the Council opted to try diquat. Although the herbicide has controlled egeria, three different species of potamogetons - red pondweed or manahi, curled pondweed and blunt pondweed - continue to thrive and cause problems for rowers. Their impact has been particularly noticeable because the harvester has not been used during the herbicide trials.

Dense potamogetons growth also caused problems during the second trial on March 19. The boat used while applying the diquat had to be launched at high tide rather than the optimum low tide when there is smaller water volume.

Follow-up tests showed mixed results in the control of the target weeds. “Preliminary results from NIWA show that diquat has controlled egeria and elodea, but only had an effect on the potamogetons directly where the diquat was applied,” said Council botanist Kate McCombs.

Water analysis indicated that the diquat dispersed quite quickly, although levels did increase again briefly on the incoming tide. Results from the monitoring of invertebrates and eels have shown no adverse effects so far.

A full review of the trials will be discussed by interested groups in early May.