Retzer Nature Center was John and Florence Retzer’s retirement home. John purchased the original 90 acres from the Federal Land Bank in 1938. The Retzer’s immediately began restoring the degraded land with over 26,000 trees, shrubs, and flowers, many obtained from the Wisconsin Conservation Department. These plantings can be enjoyed today. In 1973, Florence Horn Retzer bequeathed her property to Waukesha County for park purposes. Her vision was "to conserve the scenery, natural life and wildlife, leaving the land unimpaired for the enjoyment of future generations."

Guided by Mrs. Retzer’s wishes, the Waukesha County Park System began developing a nature center in 1974. Significant land acquisitions in 1980 and 1984 expanded the site to 335 acres. The Nature Center focuses on environmental education, natural land management, community restoration, and wildlife habitat improvement. Today restored prairies, a colorful butterfly and rain garden, and an oak opening demonstration area greet visitors and compliment the Retzer’s original homestead. The Retzer Environmental Learning Center and Charles Z. Horwitz Planetarium addition opened in May of 2005.