As director of horticulture, Tim Johnson oversees and directs operations in the Horticulture division, which includes 27 display gardens, plant production, plant healthcare, and general grounds.

Johnson graduated with a bachelor of science degree in horticulture from the University of Nebraska-Lincoln in 1984. He worked as a foreman for Campbell's Garden Center, installing and maintaining landscapes. During summer breaks from college, he completed internships at the Beatrice Developmental Center in Beatrice, Nebraska; Callaway Gardens in Pine Mountain, Georgia; and the Colonial Williamsburg Foundation in Williamsburg, Virginia. Johnson has been with the Garden since January 1985 in a  variety of positions—including assistant horticulturist, horticulturist, grounds supervisor, and grounds manager—before he became the director of horticulture in 1996.

Johnson is an instructor at the Joseph Regenstein, Jr. School of the Chicago Botanic Garden. He writes a weekly gardening column for the Chicago Tribune and a weekly gardening tips column for the Daily Herald, and responds to other media request. He is a regular speaker at the American Public Garden Association's annual meetings.

Johnson's interest in gardening began in grade school, when he had a large collection of houseplants. He also planted and transplanted flowers around the family home, and helped his grandfather maintain two large vegetable gardens each summer. Johnson's garden at home is low maintenance and consists primarily of mixed shrub and herbaceous borders.