In the worst-case scenario, some rogue weed control firms take the money but deliver no work - leaving footpaths, roads, parks and sportsgrounds blighted by problem plants including Japanese knotweed, ragwort and giant hogweed.
National Association of Agricultural Contractors (NAAC) chairman Richard Minton said he had enormous concerns about unscrupulous contractors trading in the UK.
He said: "There are some very good contractors out there, but also some unscrupulous ones and I'm in this business to drive them out and raise standards."
Minton advised delegates at the Association for Public Service Excellence parks seminar on 21 February that local authorities could avoid problems by following up on contracts throughout the year, giving longer-term contracts with more incentive to do a good job, and never simply taking the lowest bid.
He added: "There are prices out there that cannot be met. I've seen 10 tenders so far this year and each one says the local council will issue the tender to the lowest bidder. It should be performance related."
The industry is working to eradicate "dodgy" contractors by introducing a quality scheme called Amenity Assured, compiled by BASIS, NAAC and NPTC.
The Amenity Assured contractors adhere to certification that goes beyond legal requirements.
Crop Protection Association amenity sector chairman Mark Phillips said: "Companies that work within the strict herbicide usage legislation are being unfairly beaten on price by less-principled organisations."
Local Government Association representative Richard Stokoe said officers took the issue very seriously. "We will be talking to councils in the coming weeks about their experiences, examining the evidence and looking at ways of spreading best practice from councils to ensure the minimum amount of fraud is possible."