In January 2012 staff from IFI, the National Parks and Wildlife Service (NPWS) and the management of Castlegregory Golf Club applied jute matting to a section of a Natterjack toad breeding pond in an effort to control the invasive aquatic weed New Zealand pigmyweed (Crassula helmsii). The jute matting was intended to kill the weed by excluding light, similar to the method used against Curly-leaved waterweed (Lagarosiphon major) in Lough Corrib (http://www.fisheriesireland.ie/lagarosiphon-major/lagarosiphon-publications-and-technical-reports.html).

Crassula

On the 7th of June the site was revisited to assess the effectiveness of the matting as a control method. The water level in the pond had fallen following a period of dry weather and, as a consequence, much of the jute matting around the pond margin was exposed. Small clumps of Crassula were found growing through the still intact matting and probably occupied less than 15% of the surface area. An examination of the vegetation beneath the matting found healthy stands of the weed that were clearly receiving sunlight and would likely eventually penetrate the matting.

Crassula

Where the matting was still submerged, relatively little vegetation was present as a result of a thin layer of sediment reducing the amount of light permeating to the pond bed. The vegetation that was present was due to disturbance of the matting by wind-induced wave action and water movement.

A very different picture emerged from the section of pond that was left untreated. Here, dense domes of Crassula were present along the exposed pond margin and grew up to the water’s edge, where they encountered extremely dense stands of Chara hispida.

Crassula

These results suggest that, if the jute matting is firmly secured to the pond bed, Crassula could be effectively excluded from the submerged portion of the pond. However, as the majority of Crassula was found along the exposed and non-matted section of the pond, physical removal combined with chemical treatment could control or eradicate the weed.

1) New Zealand pigmyweed and Marsh pennywort (Hydrocotyle vulgaris) growing through the jute matting.

2) The exposed pond margin showing the treated and untreated sections of the pond.

3) A dense stand of New Zealand Pigmyweed from the untreated section of the pond margin.

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