SGD chairman Peter Thomas sent a letter to members last week, revealing that bookings for the event on 4 October had been particularly low.
Entitled The Privately Public: the Healing and Learning Garden, the SGD conference was to be held at the Imperial College, London, with a line-up of international speakers including award-winning garden designer Dan Pearson attending the event.
However, Thomas said: "It is no good our just handing out free seats at the end of the day.
"Aside from any profit or loss the SGD might make, it is more important to us that visitors, members and sponsors know we take their money seriously," he explained.
Tickets already sold will be refunded, and the SGD plans to go ahead with a spring conference next year.
In his letter to the society's members Thomas explained: "It seems unlikely that, in the current economic climate, we will approach the near-capacity audience numbers which we would normally expect for this flagship event."