In his designs, Manning emphasized pre-existing flora through a process of selective pruning to create a "spatial structure and character."

Manning also credited his mother, Lydia, a watercolorist, for his appreciation of the smaller details found in nature by exposing him to the home garden, pointing out "birds, flowers, toads, butterflies and beetles (Karson, 1997).

Manning wrote that he "modeled in sand and mud hills, valleys, tunnels, houses, roads, and gardens with pools" (Karson, 1997).

In 1884, this exposure, coupled with the internship experience at his father's nursery, led to his desire to make "America a finer place in which to live" (Karson, 1997) and to choose landscape design as a profession.

Three years later, he left his father's nursery pursuing his goal, seeking the company of the "most eminent man in the landscape profession" (Stan Hywet Hall and Gardens 2007).

Initially signed on as planting supervisor, his extensive horticultural knowledge quickly expanded his function at the firm, and during his eight-year employment, he participated in 125 projects in 22 states.

While working under Olmsted, Manning gained invaluable experience in planned industrial settings, which would play a key role in his later career.

While working with Charles Eliot, a partner at the firm, on the Boston Metropolitan Park System, Manning was exposed to thorough and extensive research of a site and the method of overlaying sketches of topography, roads, water features, etc.

Aware that Frederick Olmsted Sr. would pass his business on to both of his sons and partner Charles Eliot, Manning decided to start out on his own (Karson, 2000).

This approach was in direct contrast to the then-popular City Beautiful movement which emphasized monumental civic centers and Beaux Arts architecture style public buildings (Karson, 2001).

In 1907, William Gwinn Mather, owner of Cleveland-Cliffs Iron Company, asked Manning, along with Charles A. Platt to design his property outside of Cleveland, Ohio.

Manning also shipped plants in from neighboring states including wildflowers from Michigan and rhododendrons from North Carolina (Karson, 2004).

In 1912, Mather bought another large piece of property adjacent to the Cleveland location, at which Manning designed another "wild garden".

In 1899, three years after leaving the firm of Olmsted Sr., Manning wrote Eliot, seeking help in creating a professional organization for landscape architects.