Invasive aquatic plants in the Grand Canal have a negative impact on the recreation resource this waterway provides. Many species grow at alarming rates necessitating the use of specialised weed cutting boats and other measures to remove or control them.

The Grand Canal flows from its summit at Lowtown in an easterly direction to the River Liffey (Dublin) and west to the River Shannon (Offaly), covering a distance of 132 km.

Map of the Grand Canal

Map of the Grand Canal

New Zealand Pigmy-weed is found in one stretch of water along the Grand Canal. Extensive operations were conducted with Waterways Ireland in the Grand Canal to control this highly invasive weed. Treatment was focused in a highly infested 2.2 km section of the canal near Lullymore.  The section was dammed at either end and electrofished to capture and re-locate over 35,000 resident fish.  It was then fully dewatered and all the exposed pigmy-weed present was treated with herbicide on two occasions in winter 2011/2012. Subsequently, the top layer of infected spoil was removed to eradicate any remaining viable plant fragments and disposed of in a biosecure manner. In adjacent canal sections which had a low abundance of the weed, manual removal of isolated stands using scuba divers was employed.

New Zealand Pygmy-weed at Lullymore.

All Japanese / Bohemian knotweed stands present in the project area were treated with herbicide.

Nuttall’s pondweed- Control trials using V-blade and jute matting were undertaken in the Grand Canal. The use of V-blades to mechanically cut Nuttall’s pondweed proved highly successful and this weed control technique has been adopted by Waterways Ireland as part of their routine weed management programme in the Grand Canal.

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