The inaugural plaque commemorating Cardiff-born Ralph Hancock (1893-1950) was unveiled on 30 January at the Roof Gardens in Kensington, west London.
Hancock previously designed the roof gardens at New York's Rockefeller Centre. But little was known about him before research work carried by Neath Port Talbot College senior horticulture lecturer Bob Priddle.
"We don't recognise gardeners enough even though we think of ourselves as a great gardening nation," said Priddle. "I'd like to see plaques for Percy Thrower, William Robinson, Gertrude Jekyll, Humphry Repton, Caxton, Forrest and EH Wilson."
Roof Gardens owner Sir Richard Branson also backed the scheme.