Individual study: Herbicide treatment to create an open channel in a stretch of river choked with emergent branched bur-reed Sparganium erectum & common club-rush Schoenoplectus lacustris, the River Evenlode, Oxfordshire, England
Barrett P. (1999) Localised control of aquatic weeds in rivers - 2 years on, in
In 1996, a trial was set up along a section of the River Evenlode in southwest England, to assess the long-term effectiveness of localised herbicide control of dense growths of emergent water plants. The objectives were to part-clear channels in order to re-instate increased water flow and in so doing improve habitat quality for species reliant upon higher water velocities and more open river channels, and also enhance amenity value.
Study site: A short section of the River Evenlode, Oxfordshire, approximately 40 m long by 5 m wide was selected for herbicide treatment. This section of the river contained a dense growth of emergent branched bur-reed Sparganium erectum and common club-rush Schoenoplectus lacustris. During low-flow conditions the water in much of this area was less than 10 cm deep and, during the summer of 1997, some of the channel dried out completely.
Herbicide application: in the summer of 1996, the bur-reed and club-rush were carefully sprayed with glyphosate (at the manufacturers recommended concentration) using a knapsack spayer.
Plant recolonisation: By the autumn of 1996, the glyphosate treatment had effectively killed the area of Sparganium and Schoenoplectus. However, low water levels in 1997 allowed initial recolonisation by annual broad-leaved weeds, and by the summer of 1998, Sparganium and Schoenoplectus had also started to reinvade the sprayed area. This rapid recovery of emergent plant growth slowed the velocity of the water and re-stabilised the river bed sediment. It would have been possible to re-spray the area but this was not considered worthwhile because of the very shallow water depth along this section which would have encouraged further rapid plant colonisation of the site, especially if low flow conditions persisted. In addition, this stretch of the river was too shallow for angling and there did not appear to be any other environmental advantage in attempting to keep it clear of emergent vegetation. It was therefore decided not to re-spray the area and thus simply observe how long plants would take to recolonise.
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