Horticultural Expos or not, flowers were always there during the World Expos since the first.
A good example, here at the Montréal Expo'67, again a beautiful carpet of flowers, with behind, the famous and well known pavilion of USA, now named "Biosphere", this pavilion already exists today.
The Biosphère is a museum in Montreal dedicated to the environment. It is located at Parc Jean-Drapeau, on Île Sainte-Hélène in the former pavilion of the United States for the 1967 World Fair Expo 67.
Biosphère Environment Museum
In August, 1990, Environment Canada purchased the site for $17.5 million to turn it into an interactive museum showcasing and exploring the water ecosystems of the Great Lakes-Saint Lawrence River regions. The museum was inaugurated in 1995 as a water museum, and is a set of enclosed buildings designed by Éric Gauthier, inside the original steel skeleton. The Biosphère changed its name in 2007 to become an environment museum. It offers interactive activities and presents exhibitions about the major environmental issues related to water, climate change, air, ecotechnologies and sustainable development.