Q. When did The Philadelphia Flower Show start showcasing Show Gardens by garden designers?
Having never been asked this question, I wasn't sure of an answer, so I had to do some digging. Here's what we know, by searching old newspapers and our own history:
June 1864, PHS was a participant in the Great Central Fair, held on Logan Square in Philadelphia, Pa. This was a fundraising effort for the benefit of the United States Sanitary Commission and the relief of wounded soldiers. According to a newspaper article describing the Great Central Fair, "There was a beautiful arrangement of flowers and plants, with an artificial lake, islands and rustic bridges, under the superintendence of the Pennsylvania Horticultural Society." see attached article on past exhibitions in Philadelphia, Sept 14, 1884 Philadelphia Times and this stereo view from the Library of Congress. The Horticultural Department at the Great Central Fair was chaired by PHS president D. Rodney King, and many of the specimen plants on display were owned by PHS members.
Read Thomas Meehan's detailed description of the horticultural and floral exhibits at the Great Central Fair.
1868 Show: Newspaper article describes a waterfall, and a fountain feature
In September, 1884, the Pennsylvania State Fair was held in Philadelphia was produced by several partners including PHS. At that fair, there was "a design for a garden was beautifully laid out with walks, rockwork, etc., and was the centre of attraction for the crowd" (Boyd. History of the Pennsylvania Horticultural Society, p. 240. )
There very well may have been other designed gardens in the 19th century exhibitions. Unfortuneately, we don't have detailed visual records of the shows that took place at that time, and our minute books are not as detailed as we would like. We do know that nurseries and seed houses exhibited, but don't know exactly what the exhibits looked like. And from descriptions I've read of the florists' exhibits at the shows -- some of them seemed to be quite elaborate.
Certainly, by the 1920s the Shows had designed landscapes, which we know about through photographs. Hope this helps.