HTA director general David Gwyther said he was extremely disappointed with the RHS statement, which proposes the worldwide horticulture industry develops systems to manage risks "that growing and trading of ornamental plants are now creating", similar to food assurance schemes driven by supermarkets.
"The attack on the industry is wholly unwarranted," said Gwyther. "The industry is very sensitive about plant health, not least because when they get stock written off it hits straight in the pocket."
Alien pests and diseases inadvertently imported on exotic plants are threatening garden plants, said the report, Non-native diseases and the future of UK gardens.
A voluntary code of conduct could sit alongside the UK plant inspection programme and offer further reassurances to the plant buying public, the report said.
RHS director of science and learning Dr Simon Thornton Wood criticised the current inspection scheme for "only applying to plants known to host diseases and (not providing) the opportunity to capture evidence of new diseases' symptoms throughout the plant production, transportation and selling process".
But Gwyther insisted there is already enough regulation in the form of plant passports and inspections on nurseries and garden centres. "If there are any concerns about plant health risks in the future the place to look is continental Europe, not the UK," he added.
Gwyther said an HTA representative had been sitting on the working group but the RHS statement had not been agreed with members of the group before it was issued.
He will meet with RHS director general Inga Grimsey next week.