Thousands of people have already voted in the RHS competition, which pits 12 amateur gardeners' designs against each other.
Six finalists from each age range, 16 and under and over 17, have been shortlisted through to the final round.
Votes can be cast at the Great British Garden competition website.
The winners from each category will then work with the team designing the London 2012 parklands to design a garden that will be in bloom during the Games and remain in legacy.
Olympics Minister Tessa Jowell said: "It's great that the public has got behind the competition and thousands of people have already played their part in deciding how the Olympic Park will look in 2012. The final shortlist reflects the creativity and passion of gardeners across the nation. I would urge anyone who hasn't voted to do so and I can't wait to see what the British public decide."
Olympic Delivery Authority Chairman John Armitt added: "This is a great opportunity for the public to have their say on part of the new park that will form a green backdrop and festival atmosphere for the London 2012 Games and in legacy become the UK's largest new urban park in over a century."
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