Farmopolis has been made using about 15,000 plants adopted from the RHS show in May which have been planted on a derelict jetty on Greenwich Peninsula.
Those behind the project said they wanted to create a "hub centred around farming" above the Thames.
The garden, which has no entrance fee, will open to the public on 30 July.
It has been developed through a partnership between event organiser Secret Productions, arts and architecture practice Wayward, and landowners Greenwich Peninsula.
Jo Vidler, director of Secret Productions, said part of the difficulty had been keeping flowers that had been grown specially for Chelsea alive, as many are out of season.
"The gardeners are really enjoying the challenge I think," she said.
Organisers also said they hoped to develop the area into a working urban farm.
"We have a lot of ambition around food growing, but we're starting with plants and flowers," Wayward's Heather Ring said.
The garden will be open between 10:00 and 22:00 BST all week.
Tens of thousands of plants and flowers salvaged from this year's Chelsea Flower Show have been used to create a "floating garden" in south-east London.