Scottish Native Woods (SNW) regional manager John Parrott said: "We were putting signs up in Urquhart Bay Wood explaining the problem, and thought, why not enlist the public's help? They really got into it, and areas along the footpaths were clear in days. Fortunately, Himalayan balsam doesn't take much effort to pull up."
Described by owner Woodland Trust as "one of the best examples of surviving ancient wet woodland in Europe", the 20ha wood on the shores of Loch Ness is a Site of Special Scientific Interest but is threatened with several invasive species including white butterbur (Petasites albus).
SNW had traced the infestation of Himalayan balsam to a private garden 13km away. "Someone probably bought a packet of seeds for 20p 25 years ago, and now there's a bill of around £10,000 to clean it up," Parrott said.