Heather Society councillor Richard Canovan said: "Proper pruning does not appear to have been done of late and I understand from the council that some plants are in poor condition."
Perth & Kinross Council (PKC) recently granted permission to convert Cherrybanks's visitor centre into offices.
Canovan added: "I fear there is going to be some damage to about 100 cultivars in five beds that are within the application site, including the historically important Bell cultivar, named after the original owners. Essential facilities for visitors have been lost and little thought appears to have been given to the ongoing maintenance and management of the garden."
Scotland's Garden Trust (SGT), which owns the gardens, plans to sell the office building to help fund the creation of a national garden for Scotland, after the Calyx project planned at Cherrybank failed to win lottery funding.
PKC's Environment Service is maintaining the garden, while negotiations over ownership of the site take place between SGT, PKC and non-profit company Perth & Kinross Leisure (PKL).
PKL chief executive Jim Moyes said: "We're still having discussions with the trust and council and hope to release a joint statement in the not-too-distant future.
"The local authority has made arrangements for the upkeep of the heather garden and we are in contact with the Heather Society and are delighted to have their offer of help. Everybody wants to get the gardens reopened as soon as possible. That's been our position from the outset - the priority is the gardens and the heather collection."
SGT closed the heather garden in March, saying it was no longer "viable" as it received only 16 visitors a day. The Cherrybank site was gifted to SGT by drinks company Diageo for the Calyx project on the condition that if the lottery bid was unsuccessful the garden would transfer to PKL and remain accessible to the public.
The Heather Society has offered to teach local people specialist pruning skills in the spring.