Born in Berlin, Germany, Jaenicke attended Berlin University, where he studied landscape gardening and horticulture. He immigrated to the U.S. in 1893 after being recruited for his seed-growing expertise by W. Atlee Burpee. He worked with Burpee in Philadelphia, and also served as Manager of the Propagating Department at J. L. Childs Nursery in Floral Park, New York. Around 1912 he became foreman of the Missouri Botanical Gardens in St. Louis. On a return trip to Philadelphia in 1917, Jaenicke encountered Col. David N. Foster of Fort Wayne, Indiana, who offered him a job as the Superintendent of Parks. He spent the remainder of his career in that position, and played a transformative role in beautifying the city. Fort Wayne had only two parks when he arrived but had 39 in 1948, when Jaenicke passed away.
Two of Jaenicke’s most revered designs in the city’s park system are the Rose Gardens in Lakeside Park, with 23,000 plants and 500 rose varieties; and the Jaenicke Gardens (originally called the Japanese Gardens) in Swinney Park, which were transformed out of a slaughterhouse cesspool. Jaenicke is also credited with planting 24,000 tree trees in Fort Wayne and for organizing the Children’s Flower Growing Association.