The grade II* listed gardens would see the loss of at least 500 individual trees if the road is expanded to the west, but to the east sits "irreplaceable" ancient woodland, which is one of the UKs rarest habitats, covering just 2% of the countryside. The RHS and its patron Alan Titchmarsh are bitterly opposing the "garden grab".
There are two options available to Highways England to widen the A3: one on the east side of the A3 and one on the west. The RHS is calling on the Highways Agency to choose the east option. But Highways England's alternative plan to building a sliproad on the western-side RHS land to make the road next to the 1.2m visitor a year garden safer would damage natural woodland on the other eastern side of the A3, rather than a strip of newer grass and trees near the road on the RHS side.
Wisley Flower Show (6-10 September) has been building numbers for several years since becoming a general flower rather than a specialist show, and brings the highest numbers of visitors to the garden on a single day. On peak visitor days, the inward Wisley garden traffic congestion causes traffic jams on the slip road off the A3, on the A3 itself and on the M25, which the Highways Agency wants to alleviate.
Woodland Trust Director of Conservation Austin Brady said: "The proposed expansion of the A3 is stuck between a rock and a hard place, with both suggestions likely to have a huge environmental impact.
"We are lobbying for stronger protection for ancient woods, trees and other habitats, but until this is implemented we will continue to see cases where the environment is a secondary concern."
The recent Housing White Paper suggested amends to the National Planning Policy Framework (NPPF) which put ancient woods and trees on a par with other designated habitats such as National Parks.
The Government is due to respond to the paper with its recommendations later this year. The Woodland Trust is opposing more than 720 cases where ancient woodland is threatened by development across the UK, over 150 by road or rail infrastructure.
The charity will be writing to Highways England about the A3 proposals and has also contributed to a joint report from an alliance of 17 environmental groups, revealed next week ahead of Highways England publishing its second Road Investment Strategy.