Around £20,000 worth of the children’s plastic toy blocks will be used at the Hampton Court Palace Flower Show garden, which opens on 6 July.
RHS shows development director Bob Sweet admits the Legoland garden will be difficult to mark and that it brings into question what a garden really is.
He said: "The Lego garden will be difficult to judge. Our judges have no experience in how you would incorporate Lego into a garden."
The family garden, which includes pirate ships, a rollercoaster log flume car made from 10,000 bricks and a curved wall made from 80,000 bricks, as well as a 12,000-brick sculpture. The garden takes its inspiration from the park’s latest attraction, Pirates Landing which opened earlier this year. Lego flowers, plants, birds, butterflies and dragonflies are hidden within the planting and a contemporary curved white wall, multicoloured planters and a 3D sculpture have been used to illustrate the versatility of Lego bricks.
Sweet added: "There are so many new materials. At Chelsea this year we saw the first artificial grass and at BBC Gardeners’ World in Birmingham last month the best small garden used purple weatherproof carpet as a patio.
"And at Chelsea in 2009 James May made a plasticine garden. We judged him but because 30 per cent of judging is on plants, James scored zero so did not win a medal."
Sweet said: "RHS judging criteria considers plants are essential to make a garden. But judges need to be flexible in their views and attitudes to what they are looking at. If they only judge traditionally they are not probably going to take us forward very quickly.
"There is a school of thought that it is possible to make a garden without plants. The debate centres around what might be architecture and where the boundaries of architecture and landscape design are and what role the RHS plays in that and what we are going to do in our shows.
"The judges will have to take a view how can you have a plants that associates well with Lego."
The RHS approached Legoland with the idea. The landscape architect responsible for the design, Paula Young, is a former trainee of RHS chief judge Andrew Wilson at Hadlow College in Kent.
Young, who has been Legoland Windsor landscape designer since 2002 and has been planning the garden since November 2009, said: "The garden could be compared to James May's plasticine garden but we have actually used some real plants. Hopefully the judges will like it-it's completely different. We would be pleased to get any medal, being first-time exhibitors.
"I wanted it to be more of a garden but I had to bring more Lego into it. It’s a fun garden - more contemporary.
The Legoland garden will be rebuilt at Legoland after the show.