I think it was John Hench (or was it Rolly Crump?) who told the story of Walt coming to visit the WED work room in 1965 where the Disneyland facade for Small World was being worked on, led by Mary Blair, while the ride itself was still operating at the World's Fair. Some lesser Imagineer had put a tray of fake trees on the roof of the model for convenience as they were placing trees and landscaping around the model while Mary Blair fussed over the actual building, and Walt saw it and loved the look. He thought it provided a good sense of depth to have greenery on the roof framing the modern wall of world icons and architecture.
The Imagineers had to pretend they had planned it that way all along! Mature trees were placed in big planter boxes and put onto the south end of the roof for its 1966 opening at the park.
"Glad you liked our tree idea, Walt!"
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