Sculpture was placed in sacred groves. In Egypt it stood out of doors. In Greece it was usually protected by a temple or grotto. The Romans plundered Greek sanctuaries and took the sculpture to Italy for use in gardens. Large collections were amassed - and they are the origin of modern sculpture gardens.
During the Middle Ages, it appears that sculpture was not used in gardens. It was regarded as pagan and idolatrous.
In the renaissance period and thereafter sculpture was used extensively in gadens - in the Roman manner.
But the abstract art of the twentieth century split sculpture away from architecture and garden design. This was done both in the interets of abstraction and for professional reasons. Indoor sculpture was placed in galleries and outdoor sculpture in gardens.
Installation art is going some way towards re-integrating sculpture with its surroundings and some sculptors are taking an interest in garden design, as well they might.
Photographs of the Hannah Peschar Sculpture Garden (courtesy Paul Hensey)