Ramster in Surrey and Sandringham in Norfolk have opened from the start, said NGS chief executive George Plumptre.
President Mary Berry is filming Secrets from Britain’s Great Houses at Highclere Castle, home of TV’s Downton Abbey, but she said in a video message at the launch of the annual guide: "I’m sorry I’m not able to be woth you to celebrate the 90th anniversary of the NGS. I’m afraid I’m filming all week at country house with the BBC. I love the new branding in bright traditional yellow with bold writing. I think everyone will be able to recognise the NGS."
Berry opened her gardens in Penn, Buckinghamshire for 20 years for the NGS but stopped when interest became too great after she became a TV star in Great British Bake Off.
In Berry’s first BBC series since leaving Bake off in 2016, she explores six stately homes through "the prism of what she’s best known for – food".
More than 90 gardens participated in the second NGS snowdrop festival in February, with Horstead House in Norfolk attracting more than 600 visitors.
Around 3,700 gardens will open for charity through the NGS this year including 500 new or returning.
Plumptre said diversity was important now compared to 1927: "In those days gardens that opened were much of a type, a lot of country houses. Now we’re a more diverse family." He said there were 36 allotments, 15 school gardens and 10 hospice gardens as well as new for 2017 Lamport Hall whose Victorian owner Charles Isham introduced garden gnomes into the UK, and Chawton House with its Jane Austen connections.
In 2016, 3,800 gardens openings raised £3.5m for charity. In 1927, 600 gardens opened raising £8,000 for district nurses.