National Trust director of strategy and external affairs Tony Burton says a "gardens link" would "have the benefit of bringing some key organisations together to collaborate more closely".
Burton believes too many disparate bodies are trying to represent gardens and this means issues such as pests and diseases are being ignored at a political level.
The trust has launched an immediate campaign on two plant diseases, Phytophthora ramorum and P. kernoviae, which are devastating the organisation's gardens.
The charity and the National Trust for Scotland sent a joint letter to environment ministers in London and Edinburgh this week, asking for more money to be put into research and to control the spread of the pathogens.
Phytophthora control involves clearing pathogen host Rhododendron ponticum, which has cost the trust £600,000 since 2006.
The call to action comes ahead of a decision from Defra on future management of the diseases, which is expected early next year. A Defra representative said: "Consultation responses are still being considered at the moment."
Phytophthora kernoviae was found for the first time this year on bilberry (Vaccinium myrtillus) on heathland in Cornwall and Scotland.
Meanwhile, 15 National Trust gardens and four National Trust for Scotland gardens have seen outbreaks of both diseases, with historic Rhododendron collections being devastated, particularly in Cornwall.