Alan Baines has spent the past 18 months collecting litter, cutting down overgrowth and removing debris from a brook at Rayner Park in Hindley.
But he has now been sent a letter by Wigan Council telling him to stop cutting down the vegetation.
The letter - signed by Brendan Whitworth, the council’s assistant director of legal services - says if Mr Baines continues his work, he will be reported to the police for criminal damage and the council will seek an injunction banning him from the park.
Mr Baines, who lives in Hindley, said: “I was disgusted, disappointed, amazed. I have just probably done six months of solid work that would have cost tens and tens and tens of thousands for the council to do.”
He said the park had vastly improved over the past 18 months and he has the support of more than 400 people in a Facebook group.
The 62-year-old visits the park every day and is a familiar face to visitors, which he claims has helped to tackle anti-social behaviour.
Mr Baines said the council was aware of his work, having used the ReportIt app to ask them to remove cuttings and meeting with a council officer at the park in August.
Mr Baines’ efforts included clearing Japanese knotweed and balsam, which is the focus of the council’s concerns.
He says he has now stopped cutting back the vegetation but continues to litter pick.
But he fears all his hard work will now go to waste and the plants will grow back.
Mr Baines, who works as a caretaker in the NHS, said: “I have cleared all the banking and if they leave it 12 months, they will have exactly the same problem with brambles and nettles. I think it’s tragic.”
A council spokesman said: “We have been made aware of unauthorised habitat works taking place within Rayner Park without our knowledge or consent. The works include the cutting back of trees and vegetation including Japanese knotweed and Himalayan balsam.
“Not only is it a criminal offence to interfere with Japanese knotweed in this way, but these particular plants can damage the environment and should be disposed of by a registered waste carrier. In addition, all works to our open spaces should be carried out with the proper safety measures in place.
“We are committed to working with the local community to ensure our parks and green spaces are enjoyable for all to use, however, we do have procedures in place to protect the habitat and individuals operating machinery on site.
“For the safety of the local community and to ensure no further environmental issues arise, we have instructed for any interference with these plants and all other works carried out to stop.
“The remaining knotweed will be treated as part of the council’s ongoing invasive weed programme.
“Should residents have any concerns about the vegetation in the park, they can report the issues to us using our ReportIt app.”