New chairman Dr James Bartos said it was felt both bodies would be much stronger working together, especially on the conservation of historic designed gardens and landscapes.
The trust will assume the Garden History Society's role as statutory consultee for planning applications that affect historic parks and gardens registered with Historic England - a role in which it has already been successful.
In a recent example, the society's advice helped avert potential damage around the grade I-registered landscape at Kedleston, Derbyshire. A 400-home development was refused consent on the grounds that it would harm the settings of heritage assets, contrary to planning policy.
The merger will give the Gardens Trust wider expertise to call on in its conservation work and could play a role in future policy-level decisions, said Bartos. The merger should also make running costs more efficient, although projections show it will still run at a loss. More members are welcome to join.