“Plant Trees” was the Morton's family motto. Arbor Lodge, the family home in Nebraska City, Nebraska, is now a state park. Joy's mother, Caroline Morton (nee Joy, 1834–1881), loved gardening and Joy inherited both of his parents' interests in trees and horticulture.
The work of transforming Joy's Thornhill estate in Lisle, Illinois, into an Arboretum began early in 1921 when Joy was 65. In starting the Arboretum, he sought the advice of Dr. Charles Sprague Sargent (1841–1927), Director of Harvard's Arnold Arboretum for 50 years.
Sargent visited the Arboretum several times and became a significant influence in shaping The Morton Arboretum. He suggested important European gardens for the Mortons to visit, hired and trained key personnel, recommended landscape architect O.C. Simonds (1855–1931), promoted early establishment of an herbarium and library, donated herbarium specimens, books, and living plants, edited the statement of purpose, and offered general advice and encouragement.
When he was planning the Arboretum, Joy involved his son, Sterling, and his daughter, Jean, from the start. Joy chose to appoint seven family members and two Morton Salt executives to life terms on the board of trustees, assuring long-term continuity of leadership. In 1921, he named Lake Marmo in honor of his wife Margaret Morton.
When Joy died in 1934, the Arboretum consisted of 735 acres. It had a general landscape plan, many planting and nurseries, an extensive system of roads and paths, and a small but excellent staff. It was operated out of renovated farm buildings except for the library wing (now the Founder's Room), which was added to the Thornhill mansion in 1923.