The RHS has been protesting about Highways England planning to improve the road at M25 junction 10, with one option to widen the road on the A3 edge of Wisley garden.
At the launch of its protest the RHS said: "Irreplaceable historic trees that are over 100 years old could be eliminated for a short-sighted road improvement scheme, which would increase air pollution and destroy the habitats of a wide range of wildlife, including birds, moths, badgers, beetles and moles.
"Losing the natural barrier of trees on Wisley’s boundary would be visually devastating and also dramatically increase noise pollution, which would impact negatively on the enjoyment of the garden as a peaceful place to escape, relax and be inspired."
Alan Titchmarsh, RHS ambassador, said: "This potential garden grabbing plan would be another unacceptable example of this government’s poor perception of horticulture and lack of appreciation of the vital role that plants play for the environment, for the nation’s health and well-being and for the UK economy."
Wisley Flower Show (6-10 September) causes traffic to be many times worse, says Highways England.
The other option would be to build on woodland on the other side of the road, which the Woodland Trust say is unacceptable. A decision involving the secretary of state is due in autumn before further public consultation.
Highways England tweeted on Wednesday: "#A3 long delays in both directions due to the flower show at #Wisley. Please allow extra journey time if travelling in the area."
The area also saw a road traffic incident, and children returning to school increased traffic. The agency said the show is a "hotspot" and "pinch-point" for traffic jams.
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.
A RHS Wisley representative told local media: "Our show has not been, and is not causing any backups. Wisley Lane, leading up to our entrance, is clear and our dedicated team is ensuring that traffic can park smoothly."