Horticulturalists from around the world are displaying their designs at the Royal Hospital in Chelsea, London.
The launch of the show, opening to the public on Tuesday, comes as its organisers warn nearly a quarter of the UK's front gardens are now paved over.
The Royal Horticultural Society (RHS) is urging householders to buck the trend by growing more plants.
Among the gardens being exhibited to special guests in Monday is one created for Sentebale, the charity co-founded by Prince Harry which helps children in Lesotho.
Designer Matt Keightley, who created a southern-African theme for the charity after designing a garden for Help the Heroes at last year's show, said: "It is such a privilege being part of the show.
"It was last year, and nothing has changed in that respect. I'm hoping the Queen will like it - that would be the icing on the cake."
Some 165,000 people are expected to visit the show over five days and for the first time they will be able to view the work of an amateur gardener.
Nurse Sean Murray, from Ashington, Northumberland, who won the BBC's Great Chelsea Garden Challenge, has designed a display for the main avenue at the show.
His front garden incorporates both plants and a parking space, to highlight the RHS's campaign, Greening Grey Britain.
It comes after a survey for the RHS showed that 24% of front gardens are now entirely paved, concreted or gravelled. More than a quarter, 28% of the 1,492 polled, said there was no greenery at all in their front gardens.
RHS principal horticultural advisor Leigh Hunt said: "All gardens are important - vegetation provides so many benefits, preventing flooding, providing homes for wildlife, keeping cities cool in summer, insulating homes in winter."
Joe Swift, BBC presenter of the RHS Chelsea Flower Show, said: "We can all make our streets greener and better places to live and take action against this growing concrete jungle.
"We've made this conversion to grey one garden at a time and now, today, there are 7.24 million front gardens that are mostly paved.
"It's time to get gardening. We can all make a difference: from window boxes to tree planting, let's join the RHS and greening grey Britain."
The Queen is set to be among the first to visit the gardens at this year's Chelsea Flower Show later.