Clarke's garden for Tourism Authority Thailand followed a silver gilt winning garden for the same client in 2009.
Clarke works for Elevations Exhibition design and met the Thais through his role there.
James Wong was a big fan of the garden, which contrasted with the TV gardener's Malaysia tourism gold-winning Chelsea design with its naturalistic planting.
Clarke said: "I can't believe we won best in show. You can grow a fair bit of the plants used here in the UK. The more exotic spices can be used to make Thai green curry paste. Thai food is so popular now."
He said he was surprised at landscape architect Peter Wilder's comment at the Greenwich degree show (see above). Clarke hopes to be able to collaborate with Tourism Thailand again.
RHS shows director Stephen Bennett said the Thais, Trinidad and Tobago, Taiwan and some other nationalities were considering exhibiting on Hampton Court Long Water after the feature was unfilled this year.