He continued his studies at Harvard's School of Landscape Architecture, supplemented with courses at the New Jersey Agricultural College.
Wister trained at the University of Pennsylvania and at Augusta Arsenal before serving in Jonchery, France, with Advance Ordnance Depot 4 during World War I.
He would often send plants back to his friends, the Arthur Hoyt Scotts, noted garden enthusiasts whom he met in 1915.
[3] John Caspar Wister's research in cross-breeding produced hundreds of new hybrid species of common plants and flowers.
The Foundation's 240-acre (0.97 km2) public garden, with its 5,000 species of trees and shrubs adorns the Swarthmore campus, 40 acres (160,000 m2) of which were landscaped by Wister himself.
[5] Absorbed in flowers and plants, Wister did not marry until the age of 73, when he took as his wife Gertrude Smith, a noted horticulturist.
At the time of his death, Wister was director emeritus of both the Arthur Hoyt Scott Horticultural Foundation and the John J. Tyler Arboretum.