The council, which lets out a small number of plots through a garden agreement, has written to householders about increasing its fees following a review.
But with claims the potential rise will see an average seven-fold spike in fees, with one resident to be charged 3,000 per cent more, campaigners have raised concerns.
The new rates are “disproportionate” and “unreasonable”, they warn, and could push people out of their license agreements.
Among those to be impacted is an award-winning garden at the Harlow Hill Observation Tower, developed and maintained by local charity Horticap.
“In our case the increase will be £1,000 per annum. That is a massive amount and can’t be justified,” said license holders Neil and Lucy Hind, who let the land and have helped to transform it from unkempt grassland to one which was awarded a wildlife award in 2013.
Mr Hind said he fears land will be returned to the council and could result in “massive environmental loss”, and in his case, lost income to Horticap.
A council review has examined 22 plots ranging in size from 16ft (four metres) to over 13,000 sq ft (1,200m sq) in areas including Harlow Hill, Bilton, and within the Studley Meadows Nature Reserve.
The average fee increase would be 723 per cent, Mr Hind claims, with one resident set to see their rate rise from £15 a year to £468.
The policy change would raise just over £6,000 a year for council coffers.
Coun Graham Swift, the council’s deputy leader and cabinet member for economic development, said many of the garden leases were agreed decades ago on peppercorn rates.
“We put in place a standard policy to ensure all leaseholders were treated equally by paying a consistent, aligned rent that reflects fair market value,” he said.
“Although some tenants saw large percentage increases, in most cases the actual monetary amount was small.
“Clearly, tenants leasing large plots of land were asked to pay more.”
Coun Swift added: “In almost all cases, tenants have accepted the increases as the value that they derive from the use of the land is far greater than the monetary cost.
“No tenant is mandated to pay and if a tenant does not wish to pay the increase, that’s fine too.”
However, Mr Hind claimed the focus on garden rates was disproportionate amid “much more pressing concerns for them that would generate much higher levels of income”, adding: “It seems that the council is determined to extract as much money as possible from residents without any real discussion or justification.”